1 @node Sockets, Low-Level Terminal Interface, Pipes and FIFOs, Top
2 @c %MENU% A more complicated IPC mechanism, with networking support
5 This chapter describes the GNU facilities for interprocess
6 communication using sockets.
9 @cindex interprocess communication, with sockets
10 A @dfn{socket} is a generalized interprocess communication channel.
11 Like a pipe, a socket is represented as a file descriptor. Unlike pipes
12 sockets support communication between unrelated processes, and even
13 between processes running on different machines that communicate over a
14 network. Sockets are the primary means of communicating with other
15 machines; @code{telnet}, @code{rlogin}, @code{ftp}, @code{talk} and the
16 other familiar network programs use sockets.
18 Not all operating systems support sockets. In @theglibc{}, the
19 header file @file{sys/socket.h} exists regardless of the operating
20 system, and the socket functions always exist, but if the system does
21 not really support sockets these functions always fail.
23 @strong{Incomplete:} We do not currently document the facilities for
24 broadcast messages or for configuring Internet interfaces. The
25 reentrant functions and some newer functions that are related to IPv6
26 aren't documented either so far.
29 * Socket Concepts:: Basic concepts you need to know about.
30 * Communication Styles::Stream communication, datagrams and other styles.
31 * Socket Addresses:: How socket names (``addresses'') work.
32 * Interface Naming:: Identifying specific network interfaces.
33 * Local Namespace:: Details about the local namespace.
34 * Internet Namespace:: Details about the Internet namespace.
35 * Misc Namespaces:: Other namespaces not documented fully here.
36 * Open/Close Sockets:: Creating sockets and destroying them.
37 * Connections:: Operations on sockets with connection state.
38 * Datagrams:: Operations on datagram sockets.
39 * Inetd:: Inetd is a daemon that starts servers on request.
40 The most convenient way to write a server
41 is to make it work with Inetd.
42 * Socket Options:: Miscellaneous low-level socket options.
43 * Networks Database:: Accessing the database of network names.
47 @section Socket Concepts
49 @cindex communication style (of a socket)
50 @cindex style of communication (of a socket)
51 When you create a socket, you must specify the style of communication
52 you want to use and the type of protocol that should implement it.
53 The @dfn{communication style} of a socket defines the user-level
54 semantics of sending and receiving data on the socket. Choosing a
55 communication style specifies the answers to questions such as these:
61 @cindex stream (sockets)
62 @strong{What are the units of data transmission?} Some communication
63 styles regard the data as a sequence of bytes with no larger
64 structure; others group the bytes into records (which are known in
65 this context as @dfn{packets}).
68 @cindex loss of data on sockets
69 @cindex data loss on sockets
70 @strong{Can data be lost during normal operation?} Some communication
71 styles guarantee that all the data sent arrives in the order it was
72 sent (barring system or network crashes); other styles occasionally
73 lose data as a normal part of operation, and may sometimes deliver
74 packets more than once or in the wrong order.
76 Designing a program to use unreliable communication styles usually
77 involves taking precautions to detect lost or misordered packets and
78 to retransmit data as needed.
81 @strong{Is communication entirely with one partner?} Some
82 communication styles are like a telephone call---you make a
83 @dfn{connection} with one remote socket and then exchange data
84 freely. Other styles are like mailing letters---you specify a
85 destination address for each message you send.
88 @cindex namespace (of socket)
89 @cindex domain (of socket)
90 @cindex socket namespace
92 You must also choose a @dfn{namespace} for naming the socket. A socket
93 name (``address'') is meaningful only in the context of a particular
94 namespace. In fact, even the data type to use for a socket name may
95 depend on the namespace. Namespaces are also called ``domains'', but we
96 avoid that word as it can be confused with other usage of the same
97 term. Each namespace has a symbolic name that starts with @samp{PF_}.
98 A corresponding symbolic name starting with @samp{AF_} designates the
99 address format for that namespace.
101 @cindex network protocol
102 @cindex protocol (of socket)
103 @cindex socket protocol
104 @cindex protocol family
105 Finally you must choose the @dfn{protocol} to carry out the
106 communication. The protocol determines what low-level mechanism is used
107 to transmit and receive data. Each protocol is valid for a particular
108 namespace and communication style; a namespace is sometimes called a
109 @dfn{protocol family} because of this, which is why the namespace names
110 start with @samp{PF_}.
112 The rules of a protocol apply to the data passing between two programs,
113 perhaps on different computers; most of these rules are handled by the
114 operating system and you need not know about them. What you do need to
115 know about protocols is this:
119 In order to have communication between two sockets, they must specify
120 the @emph{same} protocol.
123 Each protocol is meaningful with particular style/namespace
124 combinations and cannot be used with inappropriate combinations. For
125 example, the TCP protocol fits only the byte stream style of
126 communication and the Internet namespace.
129 For each combination of style and namespace there is a @dfn{default
130 protocol}, which you can request by specifying 0 as the protocol
131 number. And that's what you should normally do---use the default.
134 Throughout the following description at various places
135 variables/parameters to denote sizes are required. And here the trouble
136 starts. In the first implementations the type of these variables was
137 simply @code{int}. On most machines at that time an @code{int} was 32
138 bits wide, which created a @emph{de facto} standard requiring 32-bit
139 variables. This is important since references to variables of this type
140 are passed to the kernel.
142 Then the POSIX people came and unified the interface with the words "all
143 size values are of type @code{size_t}". On 64-bit machines
144 @code{size_t} is 64 bits wide, so pointers to variables were no longer
147 The Unix98 specification provides a solution by introducing a type
148 @code{socklen_t}. This type is used in all of the cases that POSIX
149 changed to use @code{size_t}. The only requirement of this type is that
150 it be an unsigned type of at least 32 bits. Therefore, implementations
151 which require that references to 32-bit variables be passed can be as
152 happy as implementations which use 64-bit values.
155 @node Communication Styles
156 @section Communication Styles
158 @Theglibc{} includes support for several different kinds of sockets,
159 each with different characteristics. This section describes the
160 supported socket types. The symbolic constants listed here are
161 defined in @file{sys/socket.h}.
164 @comment sys/socket.h
166 @deftypevr Macro int SOCK_STREAM
167 The @code{SOCK_STREAM} style is like a pipe (@pxref{Pipes and FIFOs}).
168 It operates over a connection with a particular remote socket and
169 transmits data reliably as a stream of bytes.
171 Use of this style is covered in detail in @ref{Connections}.
174 @comment sys/socket.h
176 @deftypevr Macro int SOCK_DGRAM
177 The @code{SOCK_DGRAM} style is used for sending
178 individually-addressed packets unreliably.
179 It is the diametrical opposite of @code{SOCK_STREAM}.
181 Each time you write data to a socket of this kind, that data becomes
182 one packet. Since @code{SOCK_DGRAM} sockets do not have connections,
183 you must specify the recipient address with each packet.
185 The only guarantee that the system makes about your requests to
186 transmit data is that it will try its best to deliver each packet you
187 send. It may succeed with the sixth packet after failing with the
188 fourth and fifth packets; the seventh packet may arrive before the
189 sixth, and may arrive a second time after the sixth.
191 The typical use for @code{SOCK_DGRAM} is in situations where it is
192 acceptable to simply re-send a packet if no response is seen in a
193 reasonable amount of time.
195 @xref{Datagrams}, for detailed information about how to use datagram
200 @c This appears to be only for the NS domain, which we aren't
201 @c discussing and probably won't support either.
202 @comment sys/socket.h
204 @deftypevr Macro int SOCK_SEQPACKET
205 This style is like @code{SOCK_STREAM} except that the data are
206 structured into packets.
208 A program that receives data over a @code{SOCK_SEQPACKET} socket
209 should be prepared to read the entire message packet in a single call
210 to @code{read}; if it only reads part of the message, the remainder of
211 the message is simply discarded instead of being available for
212 subsequent calls to @code{read}.
214 Many protocols do not support this communication style.
219 @comment sys/socket.h
221 @deftypevr Macro int SOCK_RDM
222 This style is a reliable version of @code{SOCK_DGRAM}: it sends
223 individually addressed packets, but guarantees that each packet sent
224 arrives exactly once.
226 @strong{Warning:} It is not clear this is actually supported
227 by any operating system.
231 @comment sys/socket.h
233 @deftypevr Macro int SOCK_RAW
234 This style provides access to low-level network protocols and
235 interfaces. Ordinary user programs usually have no need to use this
239 @node Socket Addresses
240 @section Socket Addresses
242 @cindex address of socket
243 @cindex name of socket
244 @cindex binding a socket address
245 @cindex socket address (name) binding
246 The name of a socket is normally called an @dfn{address}. The
247 functions and symbols for dealing with socket addresses were named
248 inconsistently, sometimes using the term ``name'' and sometimes using
249 ``address''. You can regard these terms as synonymous where sockets
252 A socket newly created with the @code{socket} function has no
253 address. Other processes can find it for communication only if you
254 give it an address. We call this @dfn{binding} the address to the
255 socket, and the way to do it is with the @code{bind} function.
257 You need be concerned with the address of a socket if other processes
258 are to find it and start communicating with it. You can specify an
259 address for other sockets, but this is usually pointless; the first time
260 you send data from a socket, or use it to initiate a connection, the
261 system assigns an address automatically if you have not specified one.
263 Occasionally a client needs to specify an address because the server
264 discriminates based on address; for example, the rsh and rlogin
265 protocols look at the client's socket address and only bypass password
266 checking if it is less than @code{IPPORT_RESERVED} (@pxref{Ports}).
268 The details of socket addresses vary depending on what namespace you are
269 using. @xref{Local Namespace}, or @ref{Internet Namespace}, for specific
272 Regardless of the namespace, you use the same functions @code{bind} and
273 @code{getsockname} to set and examine a socket's address. These
274 functions use a phony data type, @code{struct sockaddr *}, to accept the
275 address. In practice, the address lives in a structure of some other
276 data type appropriate to the address format you are using, but you cast
277 its address to @code{struct sockaddr *} when you pass it to
281 * Address Formats:: About @code{struct sockaddr}.
282 * Setting Address:: Binding an address to a socket.
283 * Reading Address:: Reading the address of a socket.
286 @node Address Formats
287 @subsection Address Formats
289 The functions @code{bind} and @code{getsockname} use the generic data
290 type @code{struct sockaddr *} to represent a pointer to a socket
291 address. You can't use this data type effectively to interpret an
292 address or construct one; for that, you must use the proper data type
293 for the socket's namespace.
295 Thus, the usual practice is to construct an address of the proper
296 namespace-specific type, then cast a pointer to @code{struct sockaddr *}
297 when you call @code{bind} or @code{getsockname}.
299 The one piece of information that you can get from the @code{struct
300 sockaddr} data type is the @dfn{address format designator}. This tells
301 you which data type to use to understand the address fully.
304 The symbols in this section are defined in the header file
307 @comment sys/socket.h
309 @deftp {Data Type} {struct sockaddr}
310 The @code{struct sockaddr} type itself has the following members:
313 @item short int sa_family
314 This is the code for the address format of this address. It
315 identifies the format of the data which follows.
317 @item char sa_data[14]
318 This is the actual socket address data, which is format-dependent. Its
319 length also depends on the format, and may well be more than 14. The
320 length 14 of @code{sa_data} is essentially arbitrary.
324 Each address format has a symbolic name which starts with @samp{AF_}.
325 Each of them corresponds to a @samp{PF_} symbol which designates the
326 corresponding namespace. Here is a list of address format names:
329 @comment sys/socket.h
333 This designates the address format that goes with the local namespace.
334 (@code{PF_LOCAL} is the name of that namespace.) @xref{Local Namespace
335 Details}, for information about this address format.
337 @comment sys/socket.h
341 This is a synonym for @code{AF_LOCAL}. Although @code{AF_LOCAL} is
342 mandated by POSIX.1g, @code{AF_UNIX} is portable to more systems.
343 @code{AF_UNIX} was the traditional name stemming from BSD, so even most
344 POSIX systems support it. It is also the name of choice in the Unix98
345 specification. (The same is true for @code{PF_UNIX}
346 vs. @code{PF_LOCAL}).
348 @comment sys/socket.h
352 This is another synonym for @code{AF_LOCAL}, for compatibility.
353 (@code{PF_FILE} is likewise a synonym for @code{PF_LOCAL}.)
355 @comment sys/socket.h
359 This designates the address format that goes with the Internet
360 namespace. (@code{PF_INET} is the name of that namespace.)
361 @xref{Internet Address Formats}.
363 @comment sys/socket.h
364 @comment IPv6 Basic API
366 This is similar to @code{AF_INET}, but refers to the IPv6 protocol.
367 (@code{PF_INET6} is the name of the corresponding namespace.)
369 @comment sys/socket.h
373 This designates no particular address format. It is used only in rare
374 cases, such as to clear out the default destination address of a
375 ``connected'' datagram socket. @xref{Sending Datagrams}.
377 The corresponding namespace designator symbol @code{PF_UNSPEC} exists
378 for completeness, but there is no reason to use it in a program.
381 @file{sys/socket.h} defines symbols starting with @samp{AF_} for many
382 different kinds of networks, most or all of which are not actually
383 implemented. We will document those that really work as we receive
384 information about how to use them.
386 @node Setting Address
387 @subsection Setting the Address of a Socket
390 Use the @code{bind} function to assign an address to a socket. The
391 prototype for @code{bind} is in the header file @file{sys/socket.h}.
392 For examples of use, see @ref{Local Socket Example}, or see @ref{Inet Example}.
394 @comment sys/socket.h
396 @deftypefun int bind (int @var{socket}, struct sockaddr *@var{addr}, socklen_t @var{length})
397 The @code{bind} function assigns an address to the socket
398 @var{socket}. The @var{addr} and @var{length} arguments specify the
399 address; the detailed format of the address depends on the namespace.
400 The first part of the address is always the format designator, which
401 specifies a namespace, and says that the address is in the format of
404 The return value is @code{0} on success and @code{-1} on failure. The
405 following @code{errno} error conditions are defined for this function:
409 The @var{socket} argument is not a valid file descriptor.
412 The descriptor @var{socket} is not a socket.
415 The specified address is not available on this machine.
418 Some other socket is already using the specified address.
421 The socket @var{socket} already has an address.
424 You do not have permission to access the requested address. (In the
425 Internet domain, only the super-user is allowed to specify a port number
426 in the range 0 through @code{IPPORT_RESERVED} minus one; see
430 Additional conditions may be possible depending on the particular namespace
434 @node Reading Address
435 @subsection Reading the Address of a Socket
438 Use the function @code{getsockname} to examine the address of an
439 Internet socket. The prototype for this function is in the header file
442 @comment sys/socket.h
444 @deftypefun int getsockname (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr})
445 The @code{getsockname} function returns information about the
446 address of the socket @var{socket} in the locations specified by the
447 @var{addr} and @var{length-ptr} arguments. Note that the
448 @var{length-ptr} is a pointer; you should initialize it to be the
449 allocation size of @var{addr}, and on return it contains the actual
450 size of the address data.
452 The format of the address data depends on the socket namespace. The
453 length of the information is usually fixed for a given namespace, so
454 normally you can know exactly how much space is needed and can provide
455 that much. The usual practice is to allocate a place for the value
456 using the proper data type for the socket's namespace, then cast its
457 address to @code{struct sockaddr *} to pass it to @code{getsockname}.
459 The return value is @code{0} on success and @code{-1} on error. The
460 following @code{errno} error conditions are defined for this function:
464 The @var{socket} argument is not a valid file descriptor.
467 The descriptor @var{socket} is not a socket.
470 There are not enough internal buffers available for the operation.
474 You can't read the address of a socket in the file namespace. This is
475 consistent with the rest of the system; in general, there's no way to
476 find a file's name from a descriptor for that file.
478 @node Interface Naming
479 @section Interface Naming
481 Each network interface has a name. This usually consists of a few
482 letters that relate to the type of interface, which may be followed by a
483 number if there is more than one interface of that type. Examples
484 might be @code{lo} (the loopback interface) and @code{eth0} (the first
487 Although such names are convenient for humans, it would be clumsy to
488 have to use them whenever a program needs to refer to an interface. In
489 such situations an interface is referred to by its @dfn{index}, which is
490 an arbitrarily-assigned small positive integer.
492 The following functions, constants and data types are declared in the
493 header file @file{net/if.h}.
496 @deftypevr Constant size_t IFNAMSIZ
497 This constant defines the maximum buffer size needed to hold an
498 interface name, including its terminating zero byte.
502 @comment IPv6 basic API
503 @deftypefun {unsigned int} if_nametoindex (const char *@var{ifname})
504 This function yields the interface index corresponding to a particular
505 name. If no interface exists with the name given, it returns 0.
509 @comment IPv6 basic API
510 @deftypefun {char *} if_indextoname (unsigned int @var{ifindex}, char *@var{ifname})
511 This function maps an interface index to its corresponding name. The
512 returned name is placed in the buffer pointed to by @code{ifname}, which
513 must be at least @code{IFNAMSIZ} bytes in length. If the index was
514 invalid, the function's return value is a null pointer, otherwise it is
519 @comment IPv6 basic API
520 @deftp {Data Type} {struct if_nameindex}
521 This data type is used to hold the information about a single
522 interface. It has the following members:
525 @item unsigned int if_index;
526 This is the interface index.
529 This is the null-terminated index name.
535 @comment IPv6 basic API
536 @deftypefun {struct if_nameindex *} if_nameindex (void)
537 This function returns an array of @code{if_nameindex} structures, one
538 for every interface that is present. The end of the list is indicated
539 by a structure with an interface of 0 and a null name pointer. If an
540 error occurs, this function returns a null pointer.
542 The returned structure must be freed with @code{if_freenameindex} after
547 @comment IPv6 basic API
548 @deftypefun void if_freenameindex (struct if_nameindex *@var{ptr})
549 This function frees the structure returned by an earlier call to
553 @node Local Namespace
554 @section The Local Namespace
555 @cindex local namespace, for sockets
557 This section describes the details of the local namespace, whose
558 symbolic name (required when you create a socket) is @code{PF_LOCAL}.
559 The local namespace is also known as ``Unix domain sockets''. Another
560 name is file namespace since socket addresses are normally implemented
564 * Concepts: Local Namespace Concepts. What you need to understand.
565 * Details: Local Namespace Details. Address format, symbolic names, etc.
566 * Example: Local Socket Example. Example of creating a socket.
569 @node Local Namespace Concepts
570 @subsection Local Namespace Concepts
572 In the local namespace socket addresses are file names. You can specify
573 any file name you want as the address of the socket, but you must have
574 write permission on the directory containing it.
575 @c XXX The following was said to be wrong.
576 @c In order to connect to a socket you must have read permission for it.
577 It's common to put these files in the @file{/tmp} directory.
579 One peculiarity of the local namespace is that the name is only used
580 when opening the connection; once open the address is not meaningful and
583 Another peculiarity is that you cannot connect to such a socket from
584 another machine--not even if the other machine shares the file system
585 which contains the name of the socket. You can see the socket in a
586 directory listing, but connecting to it never succeeds. Some programs
587 take advantage of this, such as by asking the client to send its own
588 process ID, and using the process IDs to distinguish between clients.
589 However, we recommend you not use this method in protocols you design,
590 as we might someday permit connections from other machines that mount
591 the same file systems. Instead, send each new client an identifying
592 number if you want it to have one.
594 After you close a socket in the local namespace, you should delete the
595 file name from the file system. Use @code{unlink} or @code{remove} to
596 do this; see @ref{Deleting Files}.
598 The local namespace supports just one protocol for any communication
599 style; it is protocol number @code{0}.
601 @node Local Namespace Details
602 @subsection Details of Local Namespace
605 To create a socket in the local namespace, use the constant
606 @code{PF_LOCAL} as the @var{namespace} argument to @code{socket} or
607 @code{socketpair}. This constant is defined in @file{sys/socket.h}.
609 @comment sys/socket.h
611 @deftypevr Macro int PF_LOCAL
612 This designates the local namespace, in which socket addresses are local
613 names, and its associated family of protocols. @code{PF_Local} is the
614 macro used by Posix.1g.
617 @comment sys/socket.h
619 @deftypevr Macro int PF_UNIX
620 This is a synonym for @code{PF_LOCAL}, for compatibility's sake.
623 @comment sys/socket.h
625 @deftypevr Macro int PF_FILE
626 This is a synonym for @code{PF_LOCAL}, for compatibility's sake.
629 The structure for specifying socket names in the local namespace is
630 defined in the header file @file{sys/un.h}:
635 @deftp {Data Type} {struct sockaddr_un}
636 This structure is used to specify local namespace socket addresses. It has
637 the following members:
640 @item short int sun_family
641 This identifies the address family or format of the socket address.
642 You should store the value @code{AF_LOCAL} to designate the local
643 namespace. @xref{Socket Addresses}.
645 @item char sun_path[108]
646 This is the file name to use.
648 @strong{Incomplete:} Why is 108 a magic number? RMS suggests making
649 this a zero-length array and tweaking the following example to use
650 @code{alloca} to allocate an appropriate amount of storage based on
651 the length of the filename.
655 You should compute the @var{length} parameter for a socket address in
656 the local namespace as the sum of the size of the @code{sun_family}
657 component and the string length (@emph{not} the allocation size!) of
658 the file name string. This can be done using the macro @code{SUN_LEN}:
662 @deftypefn {Macro} int SUN_LEN (@emph{struct sockaddr_un *} @var{ptr})
663 The macro computes the length of socket address in the local namespace.
666 @node Local Socket Example
667 @subsection Example of Local-Namespace Sockets
669 Here is an example showing how to create and name a socket in the local
673 @include mkfsock.c.texi
676 @node Internet Namespace
677 @section The Internet Namespace
678 @cindex Internet namespace, for sockets
680 This section describes the details of the protocols and socket naming
681 conventions used in the Internet namespace.
683 Originally the Internet namespace used only IP version 4 (IPv4). With
684 the growing number of hosts on the Internet, a new protocol with a
685 larger address space was necessary: IP version 6 (IPv6). IPv6
686 introduces 128-bit addresses (IPv4 has 32-bit addresses) and other
687 features, and will eventually replace IPv4.
689 To create a socket in the IPv4 Internet namespace, use the symbolic name
690 @code{PF_INET} of this namespace as the @var{namespace} argument to
691 @code{socket} or @code{socketpair}. For IPv6 addresses you need the
692 macro @code{PF_INET6}. These macros are defined in @file{sys/socket.h}.
695 @comment sys/socket.h
697 @deftypevr Macro int PF_INET
698 This designates the IPv4 Internet namespace and associated family of
702 @comment sys/socket.h
704 @deftypevr Macro int PF_INET6
705 This designates the IPv6 Internet namespace and associated family of
709 A socket address for the Internet namespace includes the following components:
713 The address of the machine you want to connect to. Internet addresses
714 can be specified in several ways; these are discussed in @ref{Internet
715 Address Formats}, @ref{Host Addresses} and @ref{Host Names}.
718 A port number for that machine. @xref{Ports}.
721 You must ensure that the address and port number are represented in a
722 canonical format called @dfn{network byte order}. @xref{Byte Order},
723 for information about this.
726 * Internet Address Formats:: How socket addresses are specified in the
728 * Host Addresses:: All about host addresses of Internet host.
729 * Ports:: Internet port numbers.
730 * Services Database:: Ports may have symbolic names.
731 * Byte Order:: Different hosts may use different byte
732 ordering conventions; you need to
733 canonicalize host address and port number.
734 * Protocols Database:: Referring to protocols by name.
735 * Inet Example:: Putting it all together.
738 @node Internet Address Formats
739 @subsection Internet Socket Address Formats
741 In the Internet namespace, for both IPv4 (@code{AF_INET}) and IPv6
742 (@code{AF_INET6}), a socket address consists of a host address
743 and a port on that host. In addition, the protocol you choose serves
744 effectively as a part of the address because local port numbers are
745 meaningful only within a particular protocol.
747 The data types for representing socket addresses in the Internet namespace
748 are defined in the header file @file{netinet/in.h}.
751 @comment netinet/in.h
753 @deftp {Data Type} {struct sockaddr_in}
754 This is the data type used to represent socket addresses in the
755 Internet namespace. It has the following members:
758 @item sa_family_t sin_family
759 This identifies the address family or format of the socket address.
760 You should store the value @code{AF_INET} in this member.
761 @xref{Socket Addresses}.
763 @item struct in_addr sin_addr
764 This is the Internet address of the host machine. @xref{Host
765 Addresses}, and @ref{Host Names}, for how to get a value to store
768 @item unsigned short int sin_port
769 This is the port number. @xref{Ports}.
773 When you call @code{bind} or @code{getsockname}, you should specify
774 @code{sizeof (struct sockaddr_in)} as the @var{length} parameter if
775 you are using an IPv4 Internet namespace socket address.
777 @deftp {Data Type} {struct sockaddr_in6}
778 This is the data type used to represent socket addresses in the IPv6
779 namespace. It has the following members:
782 @item sa_family_t sin6_family
783 This identifies the address family or format of the socket address.
784 You should store the value of @code{AF_INET6} in this member.
785 @xref{Socket Addresses}.
787 @item struct in6_addr sin6_addr
788 This is the IPv6 address of the host machine. @xref{Host
789 Addresses}, and @ref{Host Names}, for how to get a value to store
792 @item uint32_t sin6_flowinfo
793 This is a currently unimplemented field.
795 @item uint16_t sin6_port
796 This is the port number. @xref{Ports}.
802 @subsection Host Addresses
804 Each computer on the Internet has one or more @dfn{Internet addresses},
805 numbers which identify that computer among all those on the Internet.
806 Users typically write IPv4 numeric host addresses as sequences of four
807 numbers, separated by periods, as in @samp{128.52.46.32}, and IPv6
808 numeric host addresses as sequences of up to eight numbers separated by
809 colons, as in @samp{5f03:1200:836f:c100::1}.
811 Each computer also has one or more @dfn{host names}, which are strings
812 of words separated by periods, as in @samp{www.gnu.org}.
814 Programs that let the user specify a host typically accept both numeric
815 addresses and host names. To open a connection a program needs a
816 numeric address, and so must convert a host name to the numeric address
820 * Abstract Host Addresses:: What a host number consists of.
821 * Data type: Host Address Data Type. Data type for a host number.
822 * Functions: Host Address Functions. Functions to operate on them.
823 * Names: Host Names. Translating host names to host numbers.
826 @node Abstract Host Addresses
827 @subsubsection Internet Host Addresses
828 @cindex host address, Internet
829 @cindex Internet host address
832 Each computer on the Internet has one or more Internet addresses,
833 numbers which identify that computer among all those on the Internet.
836 @cindex network number
837 @cindex local network address number
838 An IPv4 Internet host address is a number containing four bytes of data.
839 Historically these are divided into two parts, a @dfn{network number} and a
840 @dfn{local network address number} within that network. In the
841 mid-1990s classless addresses were introduced which changed this
842 behavior. Since some functions implicitly expect the old definitions,
843 we first describe the class-based network and will then describe
844 classless addresses. IPv6 uses only classless addresses and therefore
845 the following paragraphs don't apply.
847 The class-based IPv4 network number consists of the first one, two or
848 three bytes; the rest of the bytes are the local address.
850 IPv4 network numbers are registered with the Network Information Center
851 (NIC), and are divided into three classes---A, B and C. The local
852 network address numbers of individual machines are registered with the
853 administrator of the particular network.
855 Class A networks have single-byte numbers in the range 0 to 127. There
856 are only a small number of Class A networks, but they can each support a
857 very large number of hosts. Medium-sized Class B networks have two-byte
858 network numbers, with the first byte in the range 128 to 191. Class C
859 networks are the smallest; they have three-byte network numbers, with
860 the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes
861 of an Internet address specify a network. The remaining bytes of the
862 Internet address specify the address within that network.
864 The Class A network 0 is reserved for broadcast to all networks. In
865 addition, the host number 0 within each network is reserved for broadcast
866 to all hosts in that network. These uses are obsolete now but for
867 compatibility reasons you shouldn't use network 0 and host number 0.
869 The Class A network 127 is reserved for loopback; you can always use
870 the Internet address @samp{127.0.0.1} to refer to the host machine.
872 Since a single machine can be a member of multiple networks, it can
873 have multiple Internet host addresses. However, there is never
874 supposed to be more than one machine with the same host address.
876 @c !!! this section could document the IN_CLASS* macros in <netinet/in.h>.
877 @c No, it shouldn't since they're obsolete.
879 @cindex standard dot notation, for Internet addresses
880 @cindex dot notation, for Internet addresses
881 There are four forms of the @dfn{standard numbers-and-dots notation}
882 for Internet addresses:
885 @item @var{a}.@var{b}.@var{c}.@var{d}
886 This specifies all four bytes of the address individually and is the
887 commonly used representation.
889 @item @var{a}.@var{b}.@var{c}
890 The last part of the address, @var{c}, is interpreted as a 2-byte quantity.
891 This is useful for specifying host addresses in a Class B network with
892 network address number @code{@var{a}.@var{b}}.
894 @item @var{a}.@var{b}
895 The last part of the address, @var{b}, is interpreted as a 3-byte quantity.
896 This is useful for specifying host addresses in a Class A network with
897 network address number @var{a}.
900 If only one part is given, this corresponds directly to the host address
904 Within each part of the address, the usual C conventions for specifying
905 the radix apply. In other words, a leading @samp{0x} or @samp{0X} implies
906 hexadecimal radix; a leading @samp{0} implies octal; and otherwise decimal
909 @subsubheading Classless Addresses
911 IPv4 addresses (and IPv6 addresses also) are now considered classless;
912 the distinction between classes A, B and C can be ignored. Instead an
913 IPv4 host address consists of a 32-bit address and a 32-bit mask. The
914 mask contains set bits for the network part and cleared bits for the
915 host part. The network part is contiguous from the left, with the
916 remaining bits representing the host. As a consequence, the netmask can
917 simply be specified as the number of set bits. Classes A, B and C are
918 just special cases of this general rule. For example, class A addresses
919 have a netmask of @samp{255.0.0.0} or a prefix length of 8.
921 Classless IPv4 network addresses are written in numbers-and-dots
922 notation with the prefix length appended and a slash as separator. For
923 example the class A network 10 is written as @samp{10.0.0.0/8}.
925 @subsubheading IPv6 Addresses
927 IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host
928 address is usually written as eight 16-bit hexadecimal numbers that are
929 separated by colons. Two colons are used to abbreviate strings of
930 consecutive zeros. For example, the IPv6 loopback address
931 @samp{0:0:0:0:0:0:0:1} can just be written as @samp{::1}.
933 @node Host Address Data Type
934 @subsubsection Host Address Data Type
936 IPv4 Internet host addresses are represented in some contexts as integers
937 (type @code{uint32_t}). In other contexts, the integer is
938 packaged inside a structure of type @code{struct in_addr}. It would
939 be better if the usage were made consistent, but it is not hard to extract
940 the integer from the structure or put the integer into a structure.
942 You will find older code that uses @code{unsigned long int} for
943 IPv4 Internet host addresses instead of @code{uint32_t} or @code{struct
944 in_addr}. Historically @code{unsigned long int} was a 32-bit number but
945 with 64-bit machines this has changed. Using @code{unsigned long int}
946 might break the code if it is used on machines where this type doesn't
947 have 32 bits. @code{uint32_t} is specified by Unix98 and guaranteed to have
950 IPv6 Internet host addresses have 128 bits and are packaged inside a
951 structure of type @code{struct in6_addr}.
953 The following basic definitions for Internet addresses are declared in
954 the header file @file{netinet/in.h}:
957 @comment netinet/in.h
959 @deftp {Data Type} {struct in_addr}
960 This data type is used in certain contexts to contain an IPv4 Internet
961 host address. It has just one field, named @code{s_addr}, which records
962 the host address number as an @code{uint32_t}.
965 @comment netinet/in.h
967 @deftypevr Macro {uint32_t} INADDR_LOOPBACK
968 You can use this constant to stand for ``the address of this machine,''
969 instead of finding its actual address. It is the IPv4 Internet address
970 @samp{127.0.0.1}, which is usually called @samp{localhost}. This
971 special constant saves you the trouble of looking up the address of your
972 own machine. Also, the system usually implements @code{INADDR_LOOPBACK}
973 specially, avoiding any network traffic for the case of one machine
977 @comment netinet/in.h
979 @deftypevr Macro {uint32_t} INADDR_ANY
980 You can use this constant to stand for ``any incoming address'' when
981 binding to an address. @xref{Setting Address}. This is the usual
982 address to give in the @code{sin_addr} member of @w{@code{struct
983 sockaddr_in}} when you want to accept Internet connections.
986 @comment netinet/in.h
988 @deftypevr Macro {uint32_t} INADDR_BROADCAST
989 This constant is the address you use to send a broadcast message.
990 @c !!! broadcast needs further documented
993 @comment netinet/in.h
995 @deftypevr Macro {uint32_t} INADDR_NONE
996 This constant is returned by some functions to indicate an error.
999 @comment netinet/in.h
1000 @comment IPv6 basic API
1001 @deftp {Data Type} {struct in6_addr}
1002 This data type is used to store an IPv6 address. It stores 128 bits of
1003 data, which can be accessed (via a union) in a variety of ways.
1006 @comment netinet/in.h
1007 @comment IPv6 basic API
1008 @deftypevr Constant {struct in6_addr} in6addr_loopback
1009 This constant is the IPv6 address @samp{::1}, the loopback address. See
1010 above for a description of what this means. The macro
1011 @code{IN6ADDR_LOOPBACK_INIT} is provided to allow you to initialize your
1012 own variables to this value.
1015 @comment netinet/in.h
1016 @comment IPv6 basic API
1017 @deftypevr Constant {struct in6_addr} in6addr_any
1018 This constant is the IPv6 address @samp{::}, the unspecified address. See
1019 above for a description of what this means. The macro
1020 @code{IN6ADDR_ANY_INIT} is provided to allow you to initialize your
1021 own variables to this value.
1024 @node Host Address Functions
1025 @subsubsection Host Address Functions
1029 These additional functions for manipulating Internet addresses are
1030 declared in the header file @file{arpa/inet.h}. They represent Internet
1031 addresses in network byte order, and network numbers and
1032 local-address-within-network numbers in host byte order. @xref{Byte
1033 Order}, for an explanation of network and host byte order.
1035 @comment arpa/inet.h
1037 @deftypefun int inet_aton (const char *@var{name}, struct in_addr *@var{addr})
1038 This function converts the IPv4 Internet host address @var{name}
1039 from the standard numbers-and-dots notation into binary data and stores
1040 it in the @code{struct in_addr} that @var{addr} points to.
1041 @code{inet_aton} returns nonzero if the address is valid, zero if not.
1044 @comment arpa/inet.h
1046 @deftypefun {uint32_t} inet_addr (const char *@var{name})
1047 This function converts the IPv4 Internet host address @var{name} from the
1048 standard numbers-and-dots notation into binary data. If the input is
1049 not valid, @code{inet_addr} returns @code{INADDR_NONE}. This is an
1050 obsolete interface to @code{inet_aton}, described immediately above. It
1051 is obsolete because @code{INADDR_NONE} is a valid address
1052 (255.255.255.255), and @code{inet_aton} provides a cleaner way to
1053 indicate error return.
1056 @comment arpa/inet.h
1058 @deftypefun {uint32_t} inet_network (const char *@var{name})
1059 This function extracts the network number from the address @var{name},
1060 given in the standard numbers-and-dots notation. The returned address is
1061 in host order. If the input is not valid, @code{inet_network} returns
1064 The function works only with traditional IPv4 class A, B and C network
1065 types. It doesn't work with classless addresses and shouldn't be used
1069 @comment arpa/inet.h
1071 @deftypefun {char *} inet_ntoa (struct in_addr @var{addr})
1072 This function converts the IPv4 Internet host address @var{addr} to a
1073 string in the standard numbers-and-dots notation. The return value is
1074 a pointer into a statically-allocated buffer. Subsequent calls will
1075 overwrite the same buffer, so you should copy the string if you need
1078 In multi-threaded programs each thread has an own statically-allocated
1079 buffer. But still subsequent calls of @code{inet_ntoa} in the same
1080 thread will overwrite the result of the last call.
1082 Instead of @code{inet_ntoa} the newer function @code{inet_ntop} which is
1083 described below should be used since it handles both IPv4 and IPv6
1087 @comment arpa/inet.h
1089 @deftypefun {struct in_addr} inet_makeaddr (uint32_t @var{net}, uint32_t @var{local})
1090 This function makes an IPv4 Internet host address by combining the network
1091 number @var{net} with the local-address-within-network number
1095 @comment arpa/inet.h
1097 @deftypefun uint32_t inet_lnaof (struct in_addr @var{addr})
1098 This function returns the local-address-within-network part of the
1099 Internet host address @var{addr}.
1101 The function works only with traditional IPv4 class A, B and C network
1102 types. It doesn't work with classless addresses and shouldn't be used
1106 @comment arpa/inet.h
1108 @deftypefun uint32_t inet_netof (struct in_addr @var{addr})
1109 This function returns the network number part of the Internet host
1112 The function works only with traditional IPv4 class A, B and C network
1113 types. It doesn't work with classless addresses and shouldn't be used
1117 @comment arpa/inet.h
1118 @comment IPv6 basic API
1119 @deftypefun int inet_pton (int @var{af}, const char *@var{cp}, void *@var{buf})
1120 This function converts an Internet address (either IPv4 or IPv6) from
1121 presentation (textual) to network (binary) format. @var{af} should be
1122 either @code{AF_INET} or @code{AF_INET6}, as appropriate for the type of
1123 address being converted. @var{cp} is a pointer to the input string, and
1124 @var{buf} is a pointer to a buffer for the result. It is the caller's
1125 responsibility to make sure the buffer is large enough.
1128 @comment arpa/inet.h
1129 @comment IPv6 basic API
1130 @deftypefun {const char *} inet_ntop (int @var{af}, const void *@var{cp}, char *@var{buf}, socklen_t @var{len})
1131 This function converts an Internet address (either IPv4 or IPv6) from
1132 network (binary) to presentation (textual) form. @var{af} should be
1133 either @code{AF_INET} or @code{AF_INET6}, as appropriate. @var{cp} is a
1134 pointer to the address to be converted. @var{buf} should be a pointer
1135 to a buffer to hold the result, and @var{len} is the length of this
1136 buffer. The return value from the function will be this buffer address.
1140 @subsubsection Host Names
1141 @cindex hosts database
1142 @cindex converting host name to address
1143 @cindex converting host address to name
1145 Besides the standard numbers-and-dots notation for Internet addresses,
1146 you can also refer to a host by a symbolic name. The advantage of a
1147 symbolic name is that it is usually easier to remember. For example,
1148 the machine with Internet address @samp{158.121.106.19} is also known as
1149 @samp{alpha.gnu.org}; and other machines in the @samp{gnu.org}
1150 domain can refer to it simply as @samp{alpha}.
1154 Internally, the system uses a database to keep track of the mapping
1155 between host names and host numbers. This database is usually either
1156 the file @file{/etc/hosts} or an equivalent provided by a name server.
1157 The functions and other symbols for accessing this database are declared
1158 in @file{netdb.h}. They are BSD features, defined unconditionally if
1159 you include @file{netdb.h}.
1163 @deftp {Data Type} {struct hostent}
1164 This data type is used to represent an entry in the hosts database. It
1165 has the following members:
1169 This is the ``official'' name of the host.
1171 @item char **h_aliases
1172 These are alternative names for the host, represented as a null-terminated
1175 @item int h_addrtype
1176 This is the host address type; in practice, its value is always either
1177 @code{AF_INET} or @code{AF_INET6}, with the latter being used for IPv6
1178 hosts. In principle other kinds of addresses could be represented in
1179 the database as well as Internet addresses; if this were done, you
1180 might find a value in this field other than @code{AF_INET} or
1181 @code{AF_INET6}. @xref{Socket Addresses}.
1184 This is the length, in bytes, of each address.
1186 @item char **h_addr_list
1187 This is the vector of addresses for the host. (Recall that the host
1188 might be connected to multiple networks and have different addresses on
1189 each one.) The vector is terminated by a null pointer.
1192 This is a synonym for @code{h_addr_list[0]}; in other words, it is the
1197 As far as the host database is concerned, each address is just a block
1198 of memory @code{h_length} bytes long. But in other contexts there is an
1199 implicit assumption that you can convert IPv4 addresses to a
1200 @code{struct in_addr} or an @code{uint32_t}. Host addresses in
1201 a @code{struct hostent} structure are always given in network byte
1202 order; see @ref{Byte Order}.
1204 You can use @code{gethostbyname}, @code{gethostbyname2} or
1205 @code{gethostbyaddr} to search the hosts database for information about
1206 a particular host. The information is returned in a
1207 statically-allocated structure; you must copy the information if you
1208 need to save it across calls. You can also use @code{getaddrinfo} and
1209 @code{getnameinfo} to obtain this information.
1213 @deftypefun {struct hostent *} gethostbyname (const char *@var{name})
1214 The @code{gethostbyname} function returns information about the host
1215 named @var{name}. If the lookup fails, it returns a null pointer.
1219 @comment IPv6 Basic API
1220 @deftypefun {struct hostent *} gethostbyname2 (const char *@var{name}, int @var{af})
1221 The @code{gethostbyname2} function is like @code{gethostbyname}, but
1222 allows the caller to specify the desired address family (e.g.@:
1223 @code{AF_INET} or @code{AF_INET6}) of the result.
1228 @deftypefun {struct hostent *} gethostbyaddr (const void *@var{addr}, socklen_t @var{length}, int @var{format})
1229 The @code{gethostbyaddr} function returns information about the host
1230 with Internet address @var{addr}. The parameter @var{addr} is not
1231 really a pointer to char - it can be a pointer to an IPv4 or an IPv6
1232 address. The @var{length} argument is the size (in bytes) of the address
1233 at @var{addr}. @var{format} specifies the address format; for an IPv4
1234 Internet address, specify a value of @code{AF_INET}; for an IPv6
1235 Internet address, use @code{AF_INET6}.
1237 If the lookup fails, @code{gethostbyaddr} returns a null pointer.
1241 If the name lookup by @code{gethostbyname} or @code{gethostbyaddr}
1242 fails, you can find out the reason by looking at the value of the
1243 variable @code{h_errno}. (It would be cleaner design for these
1244 functions to set @code{errno}, but use of @code{h_errno} is compatible
1245 with other systems.)
1247 Here are the error codes that you may find in @code{h_errno}:
1252 @item HOST_NOT_FOUND
1253 @vindex HOST_NOT_FOUND
1254 No such host is known in the database.
1260 This condition happens when the name server could not be contacted. If
1261 you try again later, you may succeed then.
1267 A non-recoverable error occurred.
1273 The host database contains an entry for the name, but it doesn't have an
1274 associated Internet address.
1277 The lookup functions above all have one in common: they are not
1278 reentrant and therefore unusable in multi-threaded applications.
1279 Therefore provides @theglibc{} a new set of functions which can be
1280 used in this context.
1284 @deftypefun int gethostbyname_r (const char *restrict @var{name}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop})
1285 The @code{gethostbyname_r} function returns information about the host
1286 named @var{name}. The caller must pass a pointer to an object of type
1287 @code{struct hostent} in the @var{result_buf} parameter. In addition
1288 the function may need extra buffer space and the caller must pass an
1289 pointer and the size of the buffer in the @var{buf} and @var{buflen}
1292 A pointer to the buffer, in which the result is stored, is available in
1293 @code{*@var{result}} after the function call successfully returned. The
1294 buffer passed as the @var{buf} parameter can be freed only once the caller
1295 has finished with the result hostent struct, or has copied it including all
1296 the other memory that it points to. If an error occurs or if no entry is
1297 found, the pointer @code{*@var{result}} is a null pointer. Success is
1298 signalled by a zero return value. If the function failed the return value
1299 is an error number. In addition to the errors defined for
1300 @code{gethostbyname} it can also be @code{ERANGE}. In this case the call
1301 should be repeated with a larger buffer. Additional error information is
1302 not stored in the global variable @code{h_errno} but instead in the object
1303 pointed to by @var{h_errnop}.
1305 Here's a small example:
1308 gethostname (char *host)
1310 struct hostent *hostbuf, *hp;
1316 hostbuf = malloc (sizeof (struct hostent));
1318 tmphstbuf = malloc (hstbuflen);
1320 while ((res = gethostbyname_r (host, hostbuf, tmphstbuf, hstbuflen,
1321 &hp, &herr)) == ERANGE)
1323 /* Enlarge the buffer. */
1325 tmphstbuf = realloc (tmphstbuf, hstbuflen);
1329 /* Check for errors. */
1330 if (res || hp == NULL)
1339 @deftypefun int gethostbyname2_r (const char *@var{name}, int @var{af}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop})
1340 The @code{gethostbyname2_r} function is like @code{gethostbyname_r}, but
1341 allows the caller to specify the desired address family (e.g.@:
1342 @code{AF_INET} or @code{AF_INET6}) for the result.
1347 @deftypefun int gethostbyaddr_r (const void *@var{addr}, socklen_t @var{length}, int @var{format}, struct hostent *restrict @var{result_buf}, char *restrict @var{buf}, size_t @var{buflen}, struct hostent **restrict @var{result}, int *restrict @var{h_errnop})
1348 The @code{gethostbyaddr_r} function returns information about the host
1349 with Internet address @var{addr}. The parameter @var{addr} is not
1350 really a pointer to char - it can be a pointer to an IPv4 or an IPv6
1351 address. The @var{length} argument is the size (in bytes) of the address
1352 at @var{addr}. @var{format} specifies the address format; for an IPv4
1353 Internet address, specify a value of @code{AF_INET}; for an IPv6
1354 Internet address, use @code{AF_INET6}.
1356 Similar to the @code{gethostbyname_r} function, the caller must provide
1357 buffers for the result and memory used internally. In case of success
1358 the function returns zero. Otherwise the value is an error number where
1359 @code{ERANGE} has the special meaning that the caller-provided buffer is
1363 You can also scan the entire hosts database one entry at a time using
1364 @code{sethostent}, @code{gethostent} and @code{endhostent}. Be careful
1365 when using these functions because they are not reentrant.
1369 @deftypefun void sethostent (int @var{stayopen})
1370 This function opens the hosts database to begin scanning it. You can
1371 then call @code{gethostent} to read the entries.
1373 @c There was a rumor that this flag has different meaning if using the DNS,
1374 @c but it appears this description is accurate in that case also.
1375 If the @var{stayopen} argument is nonzero, this sets a flag so that
1376 subsequent calls to @code{gethostbyname} or @code{gethostbyaddr} will
1377 not close the database (as they usually would). This makes for more
1378 efficiency if you call those functions several times, by avoiding
1379 reopening the database for each call.
1384 @deftypefun {struct hostent *} gethostent (void)
1385 This function returns the next entry in the hosts database. It
1386 returns a null pointer if there are no more entries.
1391 @deftypefun void endhostent (void)
1392 This function closes the hosts database.
1396 @subsection Internet Ports
1399 A socket address in the Internet namespace consists of a machine's
1400 Internet address plus a @dfn{port number} which distinguishes the
1401 sockets on a given machine (for a given protocol). Port numbers range
1404 Port numbers less than @code{IPPORT_RESERVED} are reserved for standard
1405 servers, such as @code{finger} and @code{telnet}. There is a database
1406 that keeps track of these, and you can use the @code{getservbyname}
1407 function to map a service name onto a port number; see @ref{Services
1410 If you write a server that is not one of the standard ones defined in
1411 the database, you must choose a port number for it. Use a number
1412 greater than @code{IPPORT_USERRESERVED}; such numbers are reserved for
1413 servers and won't ever be generated automatically by the system.
1414 Avoiding conflicts with servers being run by other users is up to you.
1416 When you use a socket without specifying its address, the system
1417 generates a port number for it. This number is between
1418 @code{IPPORT_RESERVED} and @code{IPPORT_USERRESERVED}.
1420 On the Internet, it is actually legitimate to have two different
1421 sockets with the same port number, as long as they never both try to
1422 communicate with the same socket address (host address plus port
1423 number). You shouldn't duplicate a port number except in special
1424 circumstances where a higher-level protocol requires it. Normally,
1425 the system won't let you do it; @code{bind} normally insists on
1426 distinct port numbers. To reuse a port number, you must set the
1427 socket option @code{SO_REUSEADDR}. @xref{Socket-Level Options}.
1429 @pindex netinet/in.h
1430 These macros are defined in the header file @file{netinet/in.h}.
1432 @comment netinet/in.h
1434 @deftypevr Macro int IPPORT_RESERVED
1435 Port numbers less than @code{IPPORT_RESERVED} are reserved for
1439 @comment netinet/in.h
1441 @deftypevr Macro int IPPORT_USERRESERVED
1442 Port numbers greater than or equal to @code{IPPORT_USERRESERVED} are
1443 reserved for explicit use; they will never be allocated automatically.
1446 @node Services Database
1447 @subsection The Services Database
1448 @cindex services database
1449 @cindex converting service name to port number
1450 @cindex converting port number to service name
1452 @pindex /etc/services
1453 The database that keeps track of ``well-known'' services is usually
1454 either the file @file{/etc/services} or an equivalent from a name server.
1455 You can use these utilities, declared in @file{netdb.h}, to access
1456 the services database.
1461 @deftp {Data Type} {struct servent}
1462 This data type holds information about entries from the services database.
1463 It has the following members:
1467 This is the ``official'' name of the service.
1469 @item char **s_aliases
1470 These are alternate names for the service, represented as an array of
1471 strings. A null pointer terminates the array.
1474 This is the port number for the service. Port numbers are given in
1475 network byte order; see @ref{Byte Order}.
1478 This is the name of the protocol to use with this service.
1479 @xref{Protocols Database}.
1483 To get information about a particular service, use the
1484 @code{getservbyname} or @code{getservbyport} functions. The information
1485 is returned in a statically-allocated structure; you must copy the
1486 information if you need to save it across calls.
1490 @deftypefun {struct servent *} getservbyname (const char *@var{name}, const char *@var{proto})
1491 The @code{getservbyname} function returns information about the
1492 service named @var{name} using protocol @var{proto}. If it can't find
1493 such a service, it returns a null pointer.
1495 This function is useful for servers as well as for clients; servers
1496 use it to determine which port they should listen on (@pxref{Listening}).
1501 @deftypefun {struct servent *} getservbyport (int @var{port}, const char *@var{proto})
1502 The @code{getservbyport} function returns information about the
1503 service at port @var{port} using protocol @var{proto}. If it can't
1504 find such a service, it returns a null pointer.
1508 You can also scan the services database using @code{setservent},
1509 @code{getservent} and @code{endservent}. Be careful when using these
1510 functions because they are not reentrant.
1514 @deftypefun void setservent (int @var{stayopen})
1515 This function opens the services database to begin scanning it.
1517 If the @var{stayopen} argument is nonzero, this sets a flag so that
1518 subsequent calls to @code{getservbyname} or @code{getservbyport} will
1519 not close the database (as they usually would). This makes for more
1520 efficiency if you call those functions several times, by avoiding
1521 reopening the database for each call.
1526 @deftypefun {struct servent *} getservent (void)
1527 This function returns the next entry in the services database. If
1528 there are no more entries, it returns a null pointer.
1533 @deftypefun void endservent (void)
1534 This function closes the services database.
1538 @subsection Byte Order Conversion
1539 @cindex byte order conversion, for socket
1540 @cindex converting byte order
1543 @cindex little-endian
1544 Different kinds of computers use different conventions for the
1545 ordering of bytes within a word. Some computers put the most
1546 significant byte within a word first (this is called ``big-endian''
1547 order), and others put it last (``little-endian'' order).
1549 @cindex network byte order
1550 So that machines with different byte order conventions can
1551 communicate, the Internet protocols specify a canonical byte order
1552 convention for data transmitted over the network. This is known
1553 as @dfn{network byte order}.
1555 When establishing an Internet socket connection, you must make sure that
1556 the data in the @code{sin_port} and @code{sin_addr} members of the
1557 @code{sockaddr_in} structure are represented in network byte order.
1558 If you are encoding integer data in the messages sent through the
1559 socket, you should convert this to network byte order too. If you don't
1560 do this, your program may fail when running on or talking to other kinds
1563 If you use @code{getservbyname} and @code{gethostbyname} or
1564 @code{inet_addr} to get the port number and host address, the values are
1565 already in network byte order, and you can copy them directly into
1566 the @code{sockaddr_in} structure.
1568 Otherwise, you have to convert the values explicitly. Use @code{htons}
1569 and @code{ntohs} to convert values for the @code{sin_port} member. Use
1570 @code{htonl} and @code{ntohl} to convert IPv4 addresses for the
1571 @code{sin_addr} member. (Remember, @code{struct in_addr} is equivalent
1572 to @code{uint32_t}.) These functions are declared in
1573 @file{netinet/in.h}.
1574 @pindex netinet/in.h
1576 @comment netinet/in.h
1578 @deftypefun {uint16_t} htons (uint16_t @var{hostshort})
1579 This function converts the @code{uint16_t} integer @var{hostshort} from
1580 host byte order to network byte order.
1583 @comment netinet/in.h
1585 @deftypefun {uint16_t} ntohs (uint16_t @var{netshort})
1586 This function converts the @code{uint16_t} integer @var{netshort} from
1587 network byte order to host byte order.
1590 @comment netinet/in.h
1592 @deftypefun {uint32_t} htonl (uint32_t @var{hostlong})
1593 This function converts the @code{uint32_t} integer @var{hostlong} from
1594 host byte order to network byte order.
1596 This is used for IPv4 Internet addresses.
1599 @comment netinet/in.h
1601 @deftypefun {uint32_t} ntohl (uint32_t @var{netlong})
1602 This function converts the @code{uint32_t} integer @var{netlong} from
1603 network byte order to host byte order.
1605 This is used for IPv4 Internet addresses.
1608 @node Protocols Database
1609 @subsection Protocols Database
1610 @cindex protocols database
1612 The communications protocol used with a socket controls low-level
1613 details of how data are exchanged. For example, the protocol implements
1614 things like checksums to detect errors in transmissions, and routing
1615 instructions for messages. Normal user programs have little reason to
1616 mess with these details directly.
1618 @cindex TCP (Internet protocol)
1619 The default communications protocol for the Internet namespace depends on
1620 the communication style. For stream communication, the default is TCP
1621 (``transmission control protocol''). For datagram communication, the
1622 default is UDP (``user datagram protocol''). For reliable datagram
1623 communication, the default is RDP (``reliable datagram protocol'').
1624 You should nearly always use the default.
1626 @pindex /etc/protocols
1627 Internet protocols are generally specified by a name instead of a
1628 number. The network protocols that a host knows about are stored in a
1629 database. This is usually either derived from the file
1630 @file{/etc/protocols}, or it may be an equivalent provided by a name
1631 server. You look up the protocol number associated with a named
1632 protocol in the database using the @code{getprotobyname} function.
1634 Here are detailed descriptions of the utilities for accessing the
1635 protocols database. These are declared in @file{netdb.h}.
1640 @deftp {Data Type} {struct protoent}
1641 This data type is used to represent entries in the network protocols
1642 database. It has the following members:
1646 This is the official name of the protocol.
1648 @item char **p_aliases
1649 These are alternate names for the protocol, specified as an array of
1650 strings. The last element of the array is a null pointer.
1653 This is the protocol number (in host byte order); use this member as the
1654 @var{protocol} argument to @code{socket}.
1658 You can use @code{getprotobyname} and @code{getprotobynumber} to search
1659 the protocols database for a specific protocol. The information is
1660 returned in a statically-allocated structure; you must copy the
1661 information if you need to save it across calls.
1665 @deftypefun {struct protoent *} getprotobyname (const char *@var{name})
1666 The @code{getprotobyname} function returns information about the
1667 network protocol named @var{name}. If there is no such protocol, it
1668 returns a null pointer.
1673 @deftypefun {struct protoent *} getprotobynumber (int @var{protocol})
1674 The @code{getprotobynumber} function returns information about the
1675 network protocol with number @var{protocol}. If there is no such
1676 protocol, it returns a null pointer.
1679 You can also scan the whole protocols database one protocol at a time by
1680 using @code{setprotoent}, @code{getprotoent} and @code{endprotoent}.
1681 Be careful when using these functions because they are not reentrant.
1685 @deftypefun void setprotoent (int @var{stayopen})
1686 This function opens the protocols database to begin scanning it.
1688 If the @var{stayopen} argument is nonzero, this sets a flag so that
1689 subsequent calls to @code{getprotobyname} or @code{getprotobynumber} will
1690 not close the database (as they usually would). This makes for more
1691 efficiency if you call those functions several times, by avoiding
1692 reopening the database for each call.
1697 @deftypefun {struct protoent *} getprotoent (void)
1698 This function returns the next entry in the protocols database. It
1699 returns a null pointer if there are no more entries.
1704 @deftypefun void endprotoent (void)
1705 This function closes the protocols database.
1709 @subsection Internet Socket Example
1711 Here is an example showing how to create and name a socket in the
1712 Internet namespace. The newly created socket exists on the machine that
1713 the program is running on. Rather than finding and using the machine's
1714 Internet address, this example specifies @code{INADDR_ANY} as the host
1715 address; the system replaces that with the machine's actual address.
1718 @include mkisock.c.texi
1721 Here is another example, showing how you can fill in a @code{sockaddr_in}
1722 structure, given a host name string and a port number:
1725 @include isockad.c.texi
1728 @node Misc Namespaces
1729 @section Other Namespaces
1736 Certain other namespaces and associated protocol families are supported
1737 but not documented yet because they are not often used. @code{PF_NS}
1738 refers to the Xerox Network Software protocols. @code{PF_ISO} stands
1739 for Open Systems Interconnect. @code{PF_CCITT} refers to protocols from
1740 CCITT. @file{socket.h} defines these symbols and others naming protocols
1741 not actually implemented.
1743 @code{PF_IMPLINK} is used for communicating between hosts and Internet
1744 Message Processors. For information on this and @code{PF_ROUTE}, an
1745 occasionally-used local area routing protocol, see the GNU Hurd Manual
1746 (to appear in the future).
1748 @node Open/Close Sockets
1749 @section Opening and Closing Sockets
1751 This section describes the actual library functions for opening and
1752 closing sockets. The same functions work for all namespaces and
1756 * Creating a Socket:: How to open a socket.
1757 * Closing a Socket:: How to close a socket.
1758 * Socket Pairs:: These are created like pipes.
1761 @node Creating a Socket
1762 @subsection Creating a Socket
1763 @cindex creating a socket
1764 @cindex socket, creating
1765 @cindex opening a socket
1767 The primitive for creating a socket is the @code{socket} function,
1768 declared in @file{sys/socket.h}.
1769 @pindex sys/socket.h
1771 @comment sys/socket.h
1773 @deftypefun int socket (int @var{namespace}, int @var{style}, int @var{protocol})
1774 This function creates a socket and specifies communication style
1775 @var{style}, which should be one of the socket styles listed in
1776 @ref{Communication Styles}. The @var{namespace} argument specifies
1777 the namespace; it must be @code{PF_LOCAL} (@pxref{Local Namespace}) or
1778 @code{PF_INET} (@pxref{Internet Namespace}). @var{protocol}
1779 designates the specific protocol (@pxref{Socket Concepts}); zero is
1780 usually right for @var{protocol}.
1782 The return value from @code{socket} is the file descriptor for the new
1783 socket, or @code{-1} in case of error. The following @code{errno} error
1784 conditions are defined for this function:
1787 @item EPROTONOSUPPORT
1788 The @var{protocol} or @var{style} is not supported by the
1789 @var{namespace} specified.
1792 The process already has too many file descriptors open.
1795 The system already has too many file descriptors open.
1798 The process does not have the privilege to create a socket of the specified
1799 @var{style} or @var{protocol}.
1802 The system ran out of internal buffer space.
1805 The file descriptor returned by the @code{socket} function supports both
1806 read and write operations. However, like pipes, sockets do not support file
1807 positioning operations.
1810 For examples of how to call the @code{socket} function,
1811 see @ref{Local Socket Example}, or @ref{Inet Example}.
1814 @node Closing a Socket
1815 @subsection Closing a Socket
1816 @cindex socket, closing
1817 @cindex closing a socket
1818 @cindex shutting down a socket
1819 @cindex socket shutdown
1821 When you have finished using a socket, you can simply close its
1822 file descriptor with @code{close}; see @ref{Opening and Closing Files}.
1823 If there is still data waiting to be transmitted over the connection,
1824 normally @code{close} tries to complete this transmission. You
1825 can control this behavior using the @code{SO_LINGER} socket option to
1826 specify a timeout period; see @ref{Socket Options}.
1828 @pindex sys/socket.h
1829 You can also shut down only reception or transmission on a
1830 connection by calling @code{shutdown}, which is declared in
1831 @file{sys/socket.h}.
1833 @comment sys/socket.h
1835 @deftypefun int shutdown (int @var{socket}, int @var{how})
1836 The @code{shutdown} function shuts down the connection of socket
1837 @var{socket}. The argument @var{how} specifies what action to
1842 Stop receiving data for this socket. If further data arrives,
1846 Stop trying to transmit data from this socket. Discard any data
1847 waiting to be sent. Stop looking for acknowledgement of data already
1848 sent; don't retransmit it if it is lost.
1851 Stop both reception and transmission.
1854 The return value is @code{0} on success and @code{-1} on failure. The
1855 following @code{errno} error conditions are defined for this function:
1859 @var{socket} is not a valid file descriptor.
1862 @var{socket} is not a socket.
1865 @var{socket} is not connected.
1870 @subsection Socket Pairs
1871 @cindex creating a socket pair
1873 @cindex opening a socket pair
1875 @pindex sys/socket.h
1876 A @dfn{socket pair} consists of a pair of connected (but unnamed)
1877 sockets. It is very similar to a pipe and is used in much the same
1878 way. Socket pairs are created with the @code{socketpair} function,
1879 declared in @file{sys/socket.h}. A socket pair is much like a pipe; the
1880 main difference is that the socket pair is bidirectional, whereas the
1881 pipe has one input-only end and one output-only end (@pxref{Pipes and
1884 @comment sys/socket.h
1886 @deftypefun int socketpair (int @var{namespace}, int @var{style}, int @var{protocol}, int @var{filedes}@t{[2]})
1887 This function creates a socket pair, returning the file descriptors in
1888 @code{@var{filedes}[0]} and @code{@var{filedes}[1]}. The socket pair
1889 is a full-duplex communications channel, so that both reading and writing
1890 may be performed at either end.
1892 The @var{namespace}, @var{style} and @var{protocol} arguments are
1893 interpreted as for the @code{socket} function. @var{style} should be
1894 one of the communication styles listed in @ref{Communication Styles}.
1895 The @var{namespace} argument specifies the namespace, which must be
1896 @code{AF_LOCAL} (@pxref{Local Namespace}); @var{protocol} specifies the
1897 communications protocol, but zero is the only meaningful value.
1899 If @var{style} specifies a connectionless communication style, then
1900 the two sockets you get are not @emph{connected}, strictly speaking,
1901 but each of them knows the other as the default destination address,
1902 so they can send packets to each other.
1904 The @code{socketpair} function returns @code{0} on success and @code{-1}
1905 on failure. The following @code{errno} error conditions are defined
1910 The process has too many file descriptors open.
1913 The specified namespace is not supported.
1915 @item EPROTONOSUPPORT
1916 The specified protocol is not supported.
1919 The specified protocol does not support the creation of socket pairs.
1924 @section Using Sockets with Connections
1929 The most common communication styles involve making a connection to a
1930 particular other socket, and then exchanging data with that socket
1931 over and over. Making a connection is asymmetric; one side (the
1932 @dfn{client}) acts to request a connection, while the other side (the
1933 @dfn{server}) makes a socket and waits for the connection request.
1938 @ref{Connecting}, describes what the client program must do to
1939 initiate a connection with a server.
1942 @ref{Listening} and @ref{Accepting Connections} describe what the
1943 server program must do to wait for and act upon connection requests
1947 @ref{Transferring Data}, describes how data are transferred through the
1953 * Connecting:: What the client program must do.
1954 * Listening:: How a server program waits for requests.
1955 * Accepting Connections:: What the server does when it gets a request.
1956 * Who is Connected:: Getting the address of the
1957 other side of a connection.
1958 * Transferring Data:: How to send and receive data.
1959 * Byte Stream Example:: An example program: a client for communicating
1960 over a byte stream socket in the Internet namespace.
1961 * Server Example:: A corresponding server program.
1962 * Out-of-Band Data:: This is an advanced feature.
1966 @subsection Making a Connection
1967 @cindex connecting a socket
1968 @cindex socket, connecting
1969 @cindex socket, initiating a connection
1970 @cindex socket, client actions
1972 In making a connection, the client makes a connection while the server
1973 waits for and accepts the connection. Here we discuss what the client
1974 program must do with the @code{connect} function, which is declared in
1975 @file{sys/socket.h}.
1977 @comment sys/socket.h
1979 @deftypefun int connect (int @var{socket}, struct sockaddr *@var{addr}, socklen_t @var{length})
1980 The @code{connect} function initiates a connection from the socket
1981 with file descriptor @var{socket} to the socket whose address is
1982 specified by the @var{addr} and @var{length} arguments. (This socket
1983 is typically on another machine, and it must be already set up as a
1984 server.) @xref{Socket Addresses}, for information about how these
1985 arguments are interpreted.
1987 Normally, @code{connect} waits until the server responds to the request
1988 before it returns. You can set nonblocking mode on the socket
1989 @var{socket} to make @code{connect} return immediately without waiting
1990 for the response. @xref{File Status Flags}, for information about
1992 @c !!! how do you tell when it has finished connecting? I suspect the
1993 @c way you do it is select for writing.
1995 The normal return value from @code{connect} is @code{0}. If an error
1996 occurs, @code{connect} returns @code{-1}. The following @code{errno}
1997 error conditions are defined for this function:
2001 The socket @var{socket} is not a valid file descriptor.
2004 File descriptor @var{socket} is not a socket.
2007 The specified address is not available on the remote machine.
2010 The namespace of the @var{addr} is not supported by this socket.
2013 The socket @var{socket} is already connected.
2016 The attempt to establish the connection timed out.
2019 The server has actively refused to establish the connection.
2022 The network of the given @var{addr} isn't reachable from this host.
2025 The socket address of the given @var{addr} is already in use.
2028 The socket @var{socket} is non-blocking and the connection could not be
2029 established immediately. You can determine when the connection is
2030 completely established with @code{select}; @pxref{Waiting for I/O}.
2031 Another @code{connect} call on the same socket, before the connection is
2032 completely established, will fail with @code{EALREADY}.
2035 The socket @var{socket} is non-blocking and already has a pending
2036 connection in progress (see @code{EINPROGRESS} above).
2039 This function is defined as a cancellation point in multi-threaded
2040 programs, so one has to be prepared for this and make sure that
2041 allocated resources (like memory, files descriptors, semaphores or
2042 whatever) are freed even if the thread is canceled.
2043 @c @xref{pthread_cleanup_push}, for a method how to do this.
2047 @subsection Listening for Connections
2048 @cindex listening (sockets)
2049 @cindex sockets, server actions
2050 @cindex sockets, listening
2052 Now let us consider what the server process must do to accept
2053 connections on a socket. First it must use the @code{listen} function
2054 to enable connection requests on the socket, and then accept each
2055 incoming connection with a call to @code{accept} (@pxref{Accepting
2056 Connections}). Once connection requests are enabled on a server socket,
2057 the @code{select} function reports when the socket has a connection
2058 ready to be accepted (@pxref{Waiting for I/O}).
2060 The @code{listen} function is not allowed for sockets using
2061 connectionless communication styles.
2063 You can write a network server that does not even start running until a
2064 connection to it is requested. @xref{Inetd Servers}.
2066 In the Internet namespace, there are no special protection mechanisms
2067 for controlling access to a port; any process on any machine
2068 can make a connection to your server. If you want to restrict access to
2069 your server, make it examine the addresses associated with connection
2070 requests or implement some other handshaking or identification
2073 In the local namespace, the ordinary file protection bits control who has
2074 access to connect to the socket.
2076 @comment sys/socket.h
2078 @deftypefun int listen (int @var{socket}, int @var{n})
2079 The @code{listen} function enables the socket @var{socket} to accept
2080 connections, thus making it a server socket.
2082 The argument @var{n} specifies the length of the queue for pending
2083 connections. When the queue fills, new clients attempting to connect
2084 fail with @code{ECONNREFUSED} until the server calls @code{accept} to
2085 accept a connection from the queue.
2087 The @code{listen} function returns @code{0} on success and @code{-1}
2088 on failure. The following @code{errno} error conditions are defined
2093 The argument @var{socket} is not a valid file descriptor.
2096 The argument @var{socket} is not a socket.
2099 The socket @var{socket} does not support this operation.
2103 @node Accepting Connections
2104 @subsection Accepting Connections
2105 @cindex sockets, accepting connections
2106 @cindex accepting connections
2108 When a server receives a connection request, it can complete the
2109 connection by accepting the request. Use the function @code{accept}
2112 A socket that has been established as a server can accept connection
2113 requests from multiple clients. The server's original socket
2114 @emph{does not become part of the connection}; instead, @code{accept}
2115 makes a new socket which participates in the connection.
2116 @code{accept} returns the descriptor for this socket. The server's
2117 original socket remains available for listening for further connection
2120 The number of pending connection requests on a server socket is finite.
2121 If connection requests arrive from clients faster than the server can
2122 act upon them, the queue can fill up and additional requests are refused
2123 with an @code{ECONNREFUSED} error. You can specify the maximum length of
2124 this queue as an argument to the @code{listen} function, although the
2125 system may also impose its own internal limit on the length of this
2128 @comment sys/socket.h
2130 @deftypefun int accept (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length_ptr})
2131 This function is used to accept a connection request on the server
2132 socket @var{socket}.
2134 The @code{accept} function waits if there are no connections pending,
2135 unless the socket @var{socket} has nonblocking mode set. (You can use
2136 @code{select} to wait for a pending connection, with a nonblocking
2137 socket.) @xref{File Status Flags}, for information about nonblocking
2140 The @var{addr} and @var{length-ptr} arguments are used to return
2141 information about the name of the client socket that initiated the
2142 connection. @xref{Socket Addresses}, for information about the format
2145 Accepting a connection does not make @var{socket} part of the
2146 connection. Instead, it creates a new socket which becomes
2147 connected. The normal return value of @code{accept} is the file
2148 descriptor for the new socket.
2150 After @code{accept}, the original socket @var{socket} remains open and
2151 unconnected, and continues listening until you close it. You can
2152 accept further connections with @var{socket} by calling @code{accept}
2155 If an error occurs, @code{accept} returns @code{-1}. The following
2156 @code{errno} error conditions are defined for this function:
2160 The @var{socket} argument is not a valid file descriptor.
2163 The descriptor @var{socket} argument is not a socket.
2166 The descriptor @var{socket} does not support this operation.
2169 @var{socket} has nonblocking mode set, and there are no pending
2170 connections immediately available.
2173 This function is defined as a cancellation point in multi-threaded
2174 programs, so one has to be prepared for this and make sure that
2175 allocated resources (like memory, files descriptors, semaphores or
2176 whatever) are freed even if the thread is canceled.
2177 @c @xref{pthread_cleanup_push}, for a method how to do this.
2180 The @code{accept} function is not allowed for sockets using
2181 connectionless communication styles.
2183 @node Who is Connected
2184 @subsection Who is Connected to Me?
2186 @comment sys/socket.h
2188 @deftypefun int getpeername (int @var{socket}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr})
2189 The @code{getpeername} function returns the address of the socket that
2190 @var{socket} is connected to; it stores the address in the memory space
2191 specified by @var{addr} and @var{length-ptr}. It stores the length of
2192 the address in @code{*@var{length-ptr}}.
2194 @xref{Socket Addresses}, for information about the format of the
2195 address. In some operating systems, @code{getpeername} works only for
2196 sockets in the Internet domain.
2198 The return value is @code{0} on success and @code{-1} on error. The
2199 following @code{errno} error conditions are defined for this function:
2203 The argument @var{socket} is not a valid file descriptor.
2206 The descriptor @var{socket} is not a socket.
2209 The socket @var{socket} is not connected.
2212 There are not enough internal buffers available.
2217 @node Transferring Data
2218 @subsection Transferring Data
2219 @cindex reading from a socket
2220 @cindex writing to a socket
2222 Once a socket has been connected to a peer, you can use the ordinary
2223 @code{read} and @code{write} operations (@pxref{I/O Primitives}) to
2224 transfer data. A socket is a two-way communications channel, so read
2225 and write operations can be performed at either end.
2227 There are also some I/O modes that are specific to socket operations.
2228 In order to specify these modes, you must use the @code{recv} and
2229 @code{send} functions instead of the more generic @code{read} and
2230 @code{write} functions. The @code{recv} and @code{send} functions take
2231 an additional argument which you can use to specify various flags to
2232 control special I/O modes. For example, you can specify the
2233 @code{MSG_OOB} flag to read or write out-of-band data, the
2234 @code{MSG_PEEK} flag to peek at input, or the @code{MSG_DONTROUTE} flag
2235 to control inclusion of routing information on output.
2238 * Sending Data:: Sending data with @code{send}.
2239 * Receiving Data:: Reading data with @code{recv}.
2240 * Socket Data Options:: Using @code{send} and @code{recv}.
2244 @subsubsection Sending Data
2246 @pindex sys/socket.h
2247 The @code{send} function is declared in the header file
2248 @file{sys/socket.h}. If your @var{flags} argument is zero, you can just
2249 as well use @code{write} instead of @code{send}; see @ref{I/O
2250 Primitives}. If the socket was connected but the connection has broken,
2251 you get a @code{SIGPIPE} signal for any use of @code{send} or
2252 @code{write} (@pxref{Miscellaneous Signals}).
2254 @comment sys/socket.h
2256 @deftypefun ssize_t send (int @var{socket}, const void *@var{buffer}, size_t @var{size}, int @var{flags})
2257 The @code{send} function is like @code{write}, but with the additional
2258 flags @var{flags}. The possible values of @var{flags} are described
2259 in @ref{Socket Data Options}.
2261 This function returns the number of bytes transmitted, or @code{-1} on
2262 failure. If the socket is nonblocking, then @code{send} (like
2263 @code{write}) can return after sending just part of the data.
2264 @xref{File Status Flags}, for information about nonblocking mode.
2266 Note, however, that a successful return value merely indicates that
2267 the message has been sent without error, not necessarily that it has
2268 been received without error.
2270 The following @code{errno} error conditions are defined for this function:
2274 The @var{socket} argument is not a valid file descriptor.
2277 The operation was interrupted by a signal before any data was sent.
2278 @xref{Interrupted Primitives}.
2281 The descriptor @var{socket} is not a socket.
2284 The socket type requires that the message be sent atomically, but the
2285 message is too large for this to be possible.
2288 Nonblocking mode has been set on the socket, and the write operation
2289 would block. (Normally @code{send} blocks until the operation can be
2293 There is not enough internal buffer space available.
2296 You never connected this socket.
2299 This socket was connected but the connection is now broken. In this
2300 case, @code{send} generates a @code{SIGPIPE} signal first; if that
2301 signal is ignored or blocked, or if its handler returns, then
2302 @code{send} fails with @code{EPIPE}.
2305 This function is defined as a cancellation point in multi-threaded
2306 programs, so one has to be prepared for this and make sure that
2307 allocated resources (like memory, files descriptors, semaphores or
2308 whatever) are freed even if the thread is canceled.
2309 @c @xref{pthread_cleanup_push}, for a method how to do this.
2312 @node Receiving Data
2313 @subsubsection Receiving Data
2315 @pindex sys/socket.h
2316 The @code{recv} function is declared in the header file
2317 @file{sys/socket.h}. If your @var{flags} argument is zero, you can
2318 just as well use @code{read} instead of @code{recv}; see @ref{I/O
2321 @comment sys/socket.h
2323 @deftypefun ssize_t recv (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags})
2324 The @code{recv} function is like @code{read}, but with the additional
2325 flags @var{flags}. The possible values of @var{flags} are described
2326 in @ref{Socket Data Options}.
2328 If nonblocking mode is set for @var{socket}, and no data are available to
2329 be read, @code{recv} fails immediately rather than waiting. @xref{File
2330 Status Flags}, for information about nonblocking mode.
2332 This function returns the number of bytes received, or @code{-1} on failure.
2333 The following @code{errno} error conditions are defined for this function:
2337 The @var{socket} argument is not a valid file descriptor.
2340 The descriptor @var{socket} is not a socket.
2343 Nonblocking mode has been set on the socket, and the read operation
2344 would block. (Normally, @code{recv} blocks until there is input
2345 available to be read.)
2348 The operation was interrupted by a signal before any data was read.
2349 @xref{Interrupted Primitives}.
2352 You never connected this socket.
2355 This function is defined as a cancellation point in multi-threaded
2356 programs, so one has to be prepared for this and make sure that
2357 allocated resources (like memory, files descriptors, semaphores or
2358 whatever) are freed even if the thread is canceled.
2359 @c @xref{pthread_cleanup_push}, for a method how to do this.
2362 @node Socket Data Options
2363 @subsubsection Socket Data Options
2365 @pindex sys/socket.h
2366 The @var{flags} argument to @code{send} and @code{recv} is a bit
2367 mask. You can bitwise-OR the values of the following macros together
2368 to obtain a value for this argument. All are defined in the header
2369 file @file{sys/socket.h}.
2371 @comment sys/socket.h
2373 @deftypevr Macro int MSG_OOB
2374 Send or receive out-of-band data. @xref{Out-of-Band Data}.
2377 @comment sys/socket.h
2379 @deftypevr Macro int MSG_PEEK
2380 Look at the data but don't remove it from the input queue. This is
2381 only meaningful with input functions such as @code{recv}, not with
2385 @comment sys/socket.h
2387 @deftypevr Macro int MSG_DONTROUTE
2388 Don't include routing information in the message. This is only
2389 meaningful with output operations, and is usually only of interest for
2390 diagnostic or routing programs. We don't try to explain it here.
2393 @node Byte Stream Example
2394 @subsection Byte Stream Socket Example
2396 Here is an example client program that makes a connection for a byte
2397 stream socket in the Internet namespace. It doesn't do anything
2398 particularly interesting once it has connected to the server; it just
2399 sends a text string to the server and exits.
2401 This program uses @code{init_sockaddr} to set up the socket address; see
2405 @include inetcli.c.texi
2408 @node Server Example
2409 @subsection Byte Stream Connection Server Example
2411 The server end is much more complicated. Since we want to allow
2412 multiple clients to be connected to the server at the same time, it
2413 would be incorrect to wait for input from a single client by simply
2414 calling @code{read} or @code{recv}. Instead, the right thing to do is
2415 to use @code{select} (@pxref{Waiting for I/O}) to wait for input on
2416 all of the open sockets. This also allows the server to deal with
2417 additional connection requests.
2419 This particular server doesn't do anything interesting once it has
2420 gotten a message from a client. It does close the socket for that
2421 client when it detects an end-of-file condition (resulting from the
2422 client shutting down its end of the connection).
2424 This program uses @code{make_socket} to set up the socket address; see
2428 @include inetsrv.c.texi
2431 @node Out-of-Band Data
2432 @subsection Out-of-Band Data
2434 @cindex out-of-band data
2435 @cindex high-priority data
2436 Streams with connections permit @dfn{out-of-band} data that is
2437 delivered with higher priority than ordinary data. Typically the
2438 reason for sending out-of-band data is to send notice of an
2439 exceptional condition. To send out-of-band data use
2440 @code{send}, specifying the flag @code{MSG_OOB} (@pxref{Sending
2443 Out-of-band data are received with higher priority because the
2444 receiving process need not read it in sequence; to read the next
2445 available out-of-band data, use @code{recv} with the @code{MSG_OOB}
2446 flag (@pxref{Receiving Data}). Ordinary read operations do not read
2447 out-of-band data; they read only ordinary data.
2449 @cindex urgent socket condition
2450 When a socket finds that out-of-band data are on their way, it sends a
2451 @code{SIGURG} signal to the owner process or process group of the
2452 socket. You can specify the owner using the @code{F_SETOWN} command
2453 to the @code{fcntl} function; see @ref{Interrupt Input}. You must
2454 also establish a handler for this signal, as described in @ref{Signal
2455 Handling}, in order to take appropriate action such as reading the
2458 Alternatively, you can test for pending out-of-band data, or wait
2459 until there is out-of-band data, using the @code{select} function; it
2460 can wait for an exceptional condition on the socket. @xref{Waiting
2461 for I/O}, for more information about @code{select}.
2463 Notification of out-of-band data (whether with @code{SIGURG} or with
2464 @code{select}) indicates that out-of-band data are on the way; the data
2465 may not actually arrive until later. If you try to read the
2466 out-of-band data before it arrives, @code{recv} fails with an
2467 @code{EWOULDBLOCK} error.
2469 Sending out-of-band data automatically places a ``mark'' in the stream
2470 of ordinary data, showing where in the sequence the out-of-band data
2471 ``would have been''. This is useful when the meaning of out-of-band
2472 data is ``cancel everything sent so far''. Here is how you can test,
2473 in the receiving process, whether any ordinary data was sent before
2477 success = ioctl (socket, SIOCATMARK, &atmark);
2480 The @code{integer} variable @var{atmark} is set to a nonzero value if
2481 the socket's read pointer has reached the ``mark''.
2483 @c Posix 1.g specifies sockatmark for this ioctl. sockatmark is not
2486 Here's a function to discard any ordinary data preceding the
2491 discard_until_mark (int socket)
2495 /* @r{This is not an arbitrary limit; any size will do.} */
2497 int atmark, success;
2499 /* @r{If we have reached the mark, return.} */
2500 success = ioctl (socket, SIOCATMARK, &atmark);
2506 /* @r{Otherwise, read a bunch of ordinary data and discard it.}
2507 @r{This is guaranteed not to read past the mark}
2508 @r{if it starts before the mark.} */
2509 success = read (socket, buffer, sizeof buffer);
2516 If you don't want to discard the ordinary data preceding the mark, you
2517 may need to read some of it anyway, to make room in internal system
2518 buffers for the out-of-band data. If you try to read out-of-band data
2519 and get an @code{EWOULDBLOCK} error, try reading some ordinary data
2520 (saving it so that you can use it when you want it) and see if that
2521 makes room. Here is an example:
2528 struct buffer *next;
2531 /* @r{Read the out-of-band data from SOCKET and return it}
2532 @r{as a `struct buffer', which records the address of the data}
2535 @r{It may be necessary to read some ordinary data}
2536 @r{in order to make room for the out-of-band data.}
2537 @r{If so, the ordinary data are saved as a chain of buffers}
2538 @r{found in the `next' field of the value.} */
2541 read_oob (int socket)
2543 struct buffer *tail = 0;
2544 struct buffer *list = 0;
2548 /* @r{This is an arbitrary limit.}
2549 @r{Does anyone know how to do this without a limit?} */
2551 char *buf = (char *) xmalloc (BUF_SZ);
2555 /* @r{Try again to read the out-of-band data.} */
2556 success = recv (socket, buf, BUF_SZ, MSG_OOB);
2559 /* @r{We got it, so return it.} */
2561 = (struct buffer *) xmalloc (sizeof (struct buffer));
2563 link->size = success;
2568 /* @r{If we fail, see if we are at the mark.} */
2569 success = ioctl (socket, SIOCATMARK, &atmark);
2574 /* @r{At the mark; skipping past more ordinary data cannot help.}
2575 @r{So just wait a while.} */
2580 /* @r{Otherwise, read a bunch of ordinary data and save it.}
2581 @r{This is guaranteed not to read past the mark}
2582 @r{if it starts before the mark.} */
2583 success = read (socket, buf, BUF_SZ);
2587 /* @r{Save this data in the buffer list.} */
2590 = (struct buffer *) xmalloc (sizeof (struct buffer));
2592 link->size = success;
2594 /* @r{Add the new link to the end of the list.} */
2606 @section Datagram Socket Operations
2608 @cindex datagram socket
2609 This section describes how to use communication styles that don't use
2610 connections (styles @code{SOCK_DGRAM} and @code{SOCK_RDM}). Using
2611 these styles, you group data into packets and each packet is an
2612 independent communication. You specify the destination for each
2613 packet individually.
2615 Datagram packets are like letters: you send each one independently
2616 with its own destination address, and they may arrive in the wrong
2617 order or not at all.
2619 The @code{listen} and @code{accept} functions are not allowed for
2620 sockets using connectionless communication styles.
2623 * Sending Datagrams:: Sending packets on a datagram socket.
2624 * Receiving Datagrams:: Receiving packets on a datagram socket.
2625 * Datagram Example:: An example program: packets sent over a
2626 datagram socket in the local namespace.
2627 * Example Receiver:: Another program, that receives those packets.
2630 @node Sending Datagrams
2631 @subsection Sending Datagrams
2632 @cindex sending a datagram
2633 @cindex transmitting datagrams
2634 @cindex datagrams, transmitting
2636 @pindex sys/socket.h
2637 The normal way of sending data on a datagram socket is by using the
2638 @code{sendto} function, declared in @file{sys/socket.h}.
2640 You can call @code{connect} on a datagram socket, but this only
2641 specifies a default destination for further data transmission on the
2642 socket. When a socket has a default destination you can use
2643 @code{send} (@pxref{Sending Data}) or even @code{write} (@pxref{I/O
2644 Primitives}) to send a packet there. You can cancel the default
2645 destination by calling @code{connect} using an address format of
2646 @code{AF_UNSPEC} in the @var{addr} argument. @xref{Connecting}, for
2647 more information about the @code{connect} function.
2649 @comment sys/socket.h
2651 @deftypefun ssize_t sendto (int @var{socket}, const void *@var{buffer}, size_t @var{size}, int @var{flags}, struct sockaddr *@var{addr}, socklen_t @var{length})
2652 The @code{sendto} function transmits the data in the @var{buffer}
2653 through the socket @var{socket} to the destination address specified
2654 by the @var{addr} and @var{length} arguments. The @var{size} argument
2655 specifies the number of bytes to be transmitted.
2657 The @var{flags} are interpreted the same way as for @code{send}; see
2658 @ref{Socket Data Options}.
2660 The return value and error conditions are also the same as for
2661 @code{send}, but you cannot rely on the system to detect errors and
2662 report them; the most common error is that the packet is lost or there
2663 is no-one at the specified address to receive it, and the operating
2664 system on your machine usually does not know this.
2666 It is also possible for one call to @code{sendto} to report an error
2667 owing to a problem related to a previous call.
2669 This function is defined as a cancellation point in multi-threaded
2670 programs, so one has to be prepared for this and make sure that
2671 allocated resources (like memory, files descriptors, semaphores or
2672 whatever) are freed even if the thread is canceled.
2673 @c @xref{pthread_cleanup_push}, for a method how to do this.
2676 @node Receiving Datagrams
2677 @subsection Receiving Datagrams
2678 @cindex receiving datagrams
2680 The @code{recvfrom} function reads a packet from a datagram socket and
2681 also tells you where it was sent from. This function is declared in
2682 @file{sys/socket.h}.
2684 @comment sys/socket.h
2686 @deftypefun ssize_t recvfrom (int @var{socket}, void *@var{buffer}, size_t @var{size}, int @var{flags}, struct sockaddr *@var{addr}, socklen_t *@var{length-ptr})
2687 The @code{recvfrom} function reads one packet from the socket
2688 @var{socket} into the buffer @var{buffer}. The @var{size} argument
2689 specifies the maximum number of bytes to be read.
2691 If the packet is longer than @var{size} bytes, then you get the first
2692 @var{size} bytes of the packet and the rest of the packet is lost.
2693 There's no way to read the rest of the packet. Thus, when you use a
2694 packet protocol, you must always know how long a packet to expect.
2696 The @var{addr} and @var{length-ptr} arguments are used to return the
2697 address where the packet came from. @xref{Socket Addresses}. For a
2698 socket in the local domain the address information won't be meaningful,
2699 since you can't read the address of such a socket (@pxref{Local
2700 Namespace}). You can specify a null pointer as the @var{addr} argument
2701 if you are not interested in this information.
2703 The @var{flags} are interpreted the same way as for @code{recv}
2704 (@pxref{Socket Data Options}). The return value and error conditions
2705 are also the same as for @code{recv}.
2707 This function is defined as a cancellation point in multi-threaded
2708 programs, so one has to be prepared for this and make sure that
2709 allocated resources (like memory, files descriptors, semaphores or
2710 whatever) are freed even if the thread is canceled.
2711 @c @xref{pthread_cleanup_push}, for a method how to do this.
2714 You can use plain @code{recv} (@pxref{Receiving Data}) instead of
2715 @code{recvfrom} if you don't need to find out who sent the packet
2716 (either because you know where it should come from or because you
2717 treat all possible senders alike). Even @code{read} can be used if
2718 you don't want to specify @var{flags} (@pxref{I/O Primitives}).
2721 @c sendmsg and recvmsg are like readv and writev in that they
2722 @c use a series of buffers. It's not clear this is worth
2723 @c supporting or that we support them.
2724 @c !!! they can do more; it is hairy
2726 @comment sys/socket.h
2728 @deftp {Data Type} {struct msghdr}
2731 @comment sys/socket.h
2733 @deftypefun ssize_t sendmsg (int @var{socket}, const struct msghdr *@var{message}, int @var{flags})
2735 This function is defined as a cancellation point in multi-threaded
2736 programs, so one has to be prepared for this and make sure that
2737 allocated resources (like memory, files descriptors, semaphores or
2738 whatever) are freed even if the thread is cancel.
2739 @c @xref{pthread_cleanup_push}, for a method how to do this.
2742 @comment sys/socket.h
2744 @deftypefun ssize_t recvmsg (int @var{socket}, struct msghdr *@var{message}, int @var{flags})
2746 This function is defined as a cancellation point in multi-threaded
2747 programs, so one has to be prepared for this and make sure that
2748 allocated resources (like memory, files descriptors, semaphores or
2749 whatever) are freed even if the thread is canceled.
2750 @c @xref{pthread_cleanup_push}, for a method how to do this.
2754 @node Datagram Example
2755 @subsection Datagram Socket Example
2757 Here is a set of example programs that send messages over a datagram
2758 stream in the local namespace. Both the client and server programs use
2759 the @code{make_named_socket} function that was presented in @ref{Local
2760 Socket Example}, to create and name their sockets.
2762 First, here is the server program. It sits in a loop waiting for
2763 messages to arrive, bouncing each message back to the sender.
2764 Obviously this isn't a particularly useful program, but it does show
2765 the general ideas involved.
2768 @include filesrv.c.texi
2771 @node Example Receiver
2772 @subsection Example of Reading Datagrams
2774 Here is the client program corresponding to the server above.
2776 It sends a datagram to the server and then waits for a reply. Notice
2777 that the socket for the client (as well as for the server) in this
2778 example has to be given a name. This is so that the server can direct
2779 a message back to the client. Since the socket has no associated
2780 connection state, the only way the server can do this is by
2781 referencing the name of the client.
2784 @include filecli.c.texi
2787 Keep in mind that datagram socket communications are unreliable. In
2788 this example, the client program waits indefinitely if the message
2789 never reaches the server or if the server's response never comes
2790 back. It's up to the user running the program to kill and restart
2791 it if desired. A more automatic solution could be to use
2792 @code{select} (@pxref{Waiting for I/O}) to establish a timeout period
2793 for the reply, and in case of timeout either re-send the message or
2794 shut down the socket and exit.
2797 @section The @code{inetd} Daemon
2799 We've explained above how to write a server program that does its own
2800 listening. Such a server must already be running in order for anyone
2803 Another way to provide a service on an Internet port is to let the daemon
2804 program @code{inetd} do the listening. @code{inetd} is a program that
2805 runs all the time and waits (using @code{select}) for messages on a
2806 specified set of ports. When it receives a message, it accepts the
2807 connection (if the socket style calls for connections) and then forks a
2808 child process to run the corresponding server program. You specify the
2809 ports and their programs in the file @file{/etc/inetd.conf}.
2813 * Configuring Inetd::
2817 @subsection @code{inetd} Servers
2819 Writing a server program to be run by @code{inetd} is very simple. Each time
2820 someone requests a connection to the appropriate port, a new server
2821 process starts. The connection already exists at this time; the
2822 socket is available as the standard input descriptor and as the
2823 standard output descriptor (descriptors 0 and 1) in the server
2824 process. Thus the server program can begin reading and writing data
2825 right away. Often the program needs only the ordinary I/O facilities;
2826 in fact, a general-purpose filter program that knows nothing about
2827 sockets can work as a byte stream server run by @code{inetd}.
2829 You can also use @code{inetd} for servers that use connectionless
2830 communication styles. For these servers, @code{inetd} does not try to accept
2831 a connection since no connection is possible. It just starts the
2832 server program, which can read the incoming datagram packet from
2833 descriptor 0. The server program can handle one request and then
2834 exit, or you can choose to write it to keep reading more requests
2835 until no more arrive, and then exit. You must specify which of these
2836 two techniques the server uses when you configure @code{inetd}.
2838 @node Configuring Inetd
2839 @subsection Configuring @code{inetd}
2841 The file @file{/etc/inetd.conf} tells @code{inetd} which ports to listen to
2842 and what server programs to run for them. Normally each entry in the
2843 file is one line, but you can split it onto multiple lines provided
2844 all but the first line of the entry start with whitespace. Lines that
2845 start with @samp{#} are comments.
2847 Here are two standard entries in @file{/etc/inetd.conf}:
2850 ftp stream tcp nowait root /libexec/ftpd ftpd
2851 talk dgram udp wait root /libexec/talkd talkd
2854 An entry has this format:
2857 @var{service} @var{style} @var{protocol} @var{wait} @var{username} @var{program} @var{arguments}
2860 The @var{service} field says which service this program provides. It
2861 should be the name of a service defined in @file{/etc/services}.
2862 @code{inetd} uses @var{service} to decide which port to listen on for
2865 The fields @var{style} and @var{protocol} specify the communication
2866 style and the protocol to use for the listening socket. The style
2867 should be the name of a communication style, converted to lower case
2868 and with @samp{SOCK_} deleted---for example, @samp{stream} or
2869 @samp{dgram}. @var{protocol} should be one of the protocols listed in
2870 @file{/etc/protocols}. The typical protocol names are @samp{tcp} for
2871 byte stream connections and @samp{udp} for unreliable datagrams.
2873 The @var{wait} field should be either @samp{wait} or @samp{nowait}.
2874 Use @samp{wait} if @var{style} is a connectionless style and the
2875 server, once started, handles multiple requests as they come in.
2876 Use @samp{nowait} if @code{inetd} should start a new process for each message
2877 or request that comes in. If @var{style} uses connections, then
2878 @var{wait} @strong{must} be @samp{nowait}.
2880 @var{user} is the user name that the server should run as. @code{inetd} runs
2881 as root, so it can set the user ID of its children arbitrarily. It's
2882 best to avoid using @samp{root} for @var{user} if you can; but some
2883 servers, such as Telnet and FTP, read a username and password
2884 themselves. These servers need to be root initially so they can log
2885 in as commanded by the data coming over the network.
2887 @var{program} together with @var{arguments} specifies the command to
2888 run to start the server. @var{program} should be an absolute file
2889 name specifying the executable file to run. @var{arguments} consists
2890 of any number of whitespace-separated words, which become the
2891 command-line arguments of @var{program}. The first word in
2892 @var{arguments} is argument zero, which should by convention be the
2893 program name itself (sans directories).
2895 If you edit @file{/etc/inetd.conf}, you can tell @code{inetd} to reread the
2896 file and obey its new contents by sending the @code{inetd} process the
2897 @code{SIGHUP} signal. You'll have to use @code{ps} to determine the
2898 process ID of the @code{inetd} process as it is not fixed.
2900 @c !!! could document /etc/inetd.sec
2902 @node Socket Options
2903 @section Socket Options
2904 @cindex socket options
2906 This section describes how to read or set various options that modify
2907 the behavior of sockets and their underlying communications protocols.
2909 @cindex level, for socket options
2910 @cindex socket option level
2911 When you are manipulating a socket option, you must specify which
2912 @dfn{level} the option pertains to. This describes whether the option
2913 applies to the socket interface, or to a lower-level communications
2917 * Socket Option Functions:: The basic functions for setting and getting
2919 * Socket-Level Options:: Details of the options at the socket level.
2922 @node Socket Option Functions
2923 @subsection Socket Option Functions
2925 @pindex sys/socket.h
2926 Here are the functions for examining and modifying socket options.
2927 They are declared in @file{sys/socket.h}.
2929 @comment sys/socket.h
2931 @deftypefun int getsockopt (int @var{socket}, int @var{level}, int @var{optname}, void *@var{optval}, socklen_t *@var{optlen-ptr})
2932 The @code{getsockopt} function gets information about the value of
2933 option @var{optname} at level @var{level} for socket @var{socket}.
2935 The option value is stored in a buffer that @var{optval} points to.
2936 Before the call, you should supply in @code{*@var{optlen-ptr}} the
2937 size of this buffer; on return, it contains the number of bytes of
2938 information actually stored in the buffer.
2940 Most options interpret the @var{optval} buffer as a single @code{int}
2943 The actual return value of @code{getsockopt} is @code{0} on success
2944 and @code{-1} on failure. The following @code{errno} error conditions
2949 The @var{socket} argument is not a valid file descriptor.
2952 The descriptor @var{socket} is not a socket.
2955 The @var{optname} doesn't make sense for the given @var{level}.
2959 @comment sys/socket.h
2961 @deftypefun int setsockopt (int @var{socket}, int @var{level}, int @var{optname}, const void *@var{optval}, socklen_t @var{optlen})
2962 This function is used to set the socket option @var{optname} at level
2963 @var{level} for socket @var{socket}. The value of the option is passed
2964 in the buffer @var{optval} of size @var{optlen}.
2969 The return value and error codes for @code{setsockopt} are the same as
2970 for @code{getsockopt}.
2973 The return value and error codes for @code{setsockopt} are the same as
2974 for @code{getsockopt}.
2979 @node Socket-Level Options
2980 @subsection Socket-Level Options
2982 @comment sys/socket.h
2984 @deftypevr Constant int SOL_SOCKET
2985 Use this constant as the @var{level} argument to @code{getsockopt} or
2986 @code{setsockopt} to manipulate the socket-level options described in
2990 @pindex sys/socket.h
2992 Here is a table of socket-level option names; all are defined in the
2993 header file @file{sys/socket.h}.
2996 @comment sys/socket.h
2999 @c Extra blank line here makes the table look better.
3001 This option toggles recording of debugging information in the underlying
3002 protocol modules. The value has type @code{int}; a nonzero value means
3004 @c !!! should say how this is used
3005 @c OK, anyone who knows, please explain.
3007 @comment sys/socket.h
3010 This option controls whether @code{bind} (@pxref{Setting Address})
3011 should permit reuse of local addresses for this socket. If you enable
3012 this option, you can actually have two sockets with the same Internet
3013 port number; but the system won't allow you to use the two
3014 identically-named sockets in a way that would confuse the Internet. The
3015 reason for this option is that some higher-level Internet protocols,
3016 including FTP, require you to keep reusing the same port number.
3018 The value has type @code{int}; a nonzero value means ``yes''.
3020 @comment sys/socket.h
3023 This option controls whether the underlying protocol should
3024 periodically transmit messages on a connected socket. If the peer
3025 fails to respond to these messages, the connection is considered
3026 broken. The value has type @code{int}; a nonzero value means
3029 @comment sys/socket.h
3032 This option controls whether outgoing messages bypass the normal
3033 message routing facilities. If set, messages are sent directly to the
3034 network interface instead. The value has type @code{int}; a nonzero
3035 value means ``yes''.
3037 @comment sys/socket.h
3040 This option specifies what should happen when the socket of a type
3041 that promises reliable delivery still has untransmitted messages when
3042 it is closed; see @ref{Closing a Socket}. The value has type
3043 @code{struct linger}.
3045 @comment sys/socket.h
3047 @deftp {Data Type} {struct linger}
3048 This structure type has the following members:
3052 This field is interpreted as a boolean. If nonzero, @code{close}
3053 blocks until the data are transmitted or the timeout period has expired.
3056 This specifies the timeout period, in seconds.
3060 @comment sys/socket.h
3063 This option controls whether datagrams may be broadcast from the socket.
3064 The value has type @code{int}; a nonzero value means ``yes''.
3066 @comment sys/socket.h
3069 If this option is set, out-of-band data received on the socket is
3070 placed in the normal input queue. This permits it to be read using
3071 @code{read} or @code{recv} without specifying the @code{MSG_OOB}
3072 flag. @xref{Out-of-Band Data}. The value has type @code{int}; a
3073 nonzero value means ``yes''.
3075 @comment sys/socket.h
3078 This option gets or sets the size of the output buffer. The value is a
3079 @code{size_t}, which is the size in bytes.
3081 @comment sys/socket.h
3084 This option gets or sets the size of the input buffer. The value is a
3085 @code{size_t}, which is the size in bytes.
3087 @comment sys/socket.h
3090 @comment sys/socket.h
3093 This option can be used with @code{getsockopt} only. It is used to
3094 get the socket's communication style. @code{SO_TYPE} is the
3095 historical name, and @code{SO_STYLE} is the preferred name in GNU.
3096 The value has type @code{int} and its value designates a communication
3097 style; see @ref{Communication Styles}.
3099 @comment sys/socket.h
3102 @c Extra blank line here makes the table look better.
3104 This option can be used with @code{getsockopt} only. It is used to reset
3105 the error status of the socket. The value is an @code{int}, which represents
3106 the previous error status.
3107 @c !!! what is "socket error status"? this is never defined.
3110 @node Networks Database
3111 @section Networks Database
3112 @cindex networks database
3113 @cindex converting network number to network name
3114 @cindex converting network name to network number
3116 @pindex /etc/networks
3118 Many systems come with a database that records a list of networks known
3119 to the system developer. This is usually kept either in the file
3120 @file{/etc/networks} or in an equivalent from a name server. This data
3121 base is useful for routing programs such as @code{route}, but it is not
3122 useful for programs that simply communicate over the network. We
3123 provide functions to access this database, which are declared in
3128 @deftp {Data Type} {struct netent}
3129 This data type is used to represent information about entries in the
3130 networks database. It has the following members:
3134 This is the ``official'' name of the network.
3136 @item char **n_aliases
3137 These are alternative names for the network, represented as a vector
3138 of strings. A null pointer terminates the array.
3140 @item int n_addrtype
3141 This is the type of the network number; this is always equal to
3142 @code{AF_INET} for Internet networks.
3144 @item unsigned long int n_net
3145 This is the network number. Network numbers are returned in host
3146 byte order; see @ref{Byte Order}.
3150 Use the @code{getnetbyname} or @code{getnetbyaddr} functions to search
3151 the networks database for information about a specific network. The
3152 information is returned in a statically-allocated structure; you must
3153 copy the information if you need to save it.
3157 @deftypefun {struct netent *} getnetbyname (const char *@var{name})
3158 The @code{getnetbyname} function returns information about the network
3159 named @var{name}. It returns a null pointer if there is no such
3165 @deftypefun {struct netent *} getnetbyaddr (uint32_t @var{net}, int @var{type})
3166 The @code{getnetbyaddr} function returns information about the network
3167 of type @var{type} with number @var{net}. You should specify a value of
3168 @code{AF_INET} for the @var{type} argument for Internet networks.
3170 @code{getnetbyaddr} returns a null pointer if there is no such
3174 You can also scan the networks database using @code{setnetent},
3175 @code{getnetent} and @code{endnetent}. Be careful when using these
3176 functions because they are not reentrant.
3180 @deftypefun void setnetent (int @var{stayopen})
3181 This function opens and rewinds the networks database.
3183 If the @var{stayopen} argument is nonzero, this sets a flag so that
3184 subsequent calls to @code{getnetbyname} or @code{getnetbyaddr} will
3185 not close the database (as they usually would). This makes for more
3186 efficiency if you call those functions several times, by avoiding
3187 reopening the database for each call.
3192 @deftypefun {struct netent *} getnetent (void)
3193 This function returns the next entry in the networks database. It
3194 returns a null pointer if there are no more entries.
3199 @deftypefun void endnetent (void)
3200 This function closes the networks database.