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34 .\" $Id: recv.2,v 1.3 1999/05/13 11:33:38 freitag Exp $
36 .\" Modified Sat Jul 24 00:22:20 1993 by Rik Faith <faith@cs.unc.edu>
37 .\" Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esr@thyrsus.com>
38 .\" Modified 1998,1999 by Andi Kleen
39 .\" 2001-06-19 corrected SO_EE_OFFENDER, bug report by James Hawtin
41 .TH RECV 2 2017-09-15 "Linux" "Linux Programmer's Manual"
43 recv, recvfrom, recvmsg \- receive a message from a socket
45 .\" .B #include <sys/uio.h>
48 .B #include <sys/types.h>
50 .B #include <sys/socket.h>
52 .BI "ssize_t recv(int " sockfd ", void *" buf ", size_t " len ", int " flags );
54 .BI "ssize_t recvfrom(int " sockfd ", void *" buf ", size_t " len ", int " flags ,
55 .BI " struct sockaddr *" src_addr ", socklen_t *" addrlen );
57 .BI "ssize_t recvmsg(int " sockfd ", struct msghdr *" msg ", int " flags );
65 calls are used to receive messages from a socket.
67 to receive data on both connectionless and connection-oriented sockets.
68 This page first describes common features of all three system calls,
69 and then describes the differences between the calls.
71 The only difference between
81 is generally equivalent to
84 Also, the following call
86 recv(sockfd, buf, len, flags);
90 recvfrom(sockfd, buf, len, flags, NULL, NULL);
92 All three calls return the length of the message on successful
94 If a message is too long to fit in the supplied buffer, excess
95 bytes may be discarded depending on the type of socket the message is
98 If no messages are available at the socket, the receive calls wait for a
99 message to arrive, unless the socket is nonblocking (see
101 in which case the value \-1 is returned and the external variable
104 .BR EAGAIN " or " EWOULDBLOCK .
105 The receive calls normally return any data available, up to the requested
106 amount, rather than waiting for receipt of the full amount requested.
108 An application can use
113 to determine when more data arrives on a socket.
114 .SS The flags argument
117 argument is formed by ORing one or more of the following values:
119 .BR MSG_CMSG_CLOEXEC " (" recvmsg "() only; since Linux 2.6.23)"
120 Set the close-on-exec flag for the file descriptor received
121 via a UNIX domain file descriptor using the
123 operation (described in
125 This flag is useful for the same reasons as the
130 .BR MSG_DONTWAIT " (since Linux 2.2)"
131 Enables nonblocking operation; if the operation would block,
132 the call fails with the error
133 .BR EAGAIN " or " EWOULDBLOCK .
134 This provides similar behavior to setting the
139 operation), but differs in that
141 is a per-call option, whereas
143 is a setting on the open file description (see
145 which will affect all threads in the calling process
146 and as well as other processes that hold file descriptors
147 referring to the same open file description.
149 .BR MSG_ERRQUEUE " (since Linux 2.2)"
151 specifies that queued errors should be received from the socket error queue.
152 The error is passed in
153 an ancillary message with a type dependent on the protocol (for IPv4
155 The user should supply a buffer of sufficient size.
160 for more information.
161 The payload of the original packet that caused the error
162 is passed as normal data via
164 The original destination address of the datagram that caused the error
168 The error is supplied in a
174 #define SO_EE_ORIGIN_NONE 0
175 #define SO_EE_ORIGIN_LOCAL 1
176 #define SO_EE_ORIGIN_ICMP 2
177 #define SO_EE_ORIGIN_ICMP6 3
179 struct sock_extended_err
181 uint32_t ee_errno; /* Error number */
182 uint8_t ee_origin; /* Where the error originated */
183 uint8_t ee_type; /* Type */
184 uint8_t ee_code; /* Code */
185 uint8_t ee_pad; /* Padding */
186 uint32_t ee_info; /* Additional information */
187 uint32_t ee_data; /* Other data */
188 /* More data may follow */
191 struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
198 number of the queued error.
200 is the origin code of where the error originated.
201 The other fields are protocol-specific.
204 returns a pointer to the address of the network object
205 where the error originated from given a pointer to the ancillary message.
206 If this address is not known, the
212 and the other fields of the
215 The payload of the packet that caused the error is passed as normal data.
217 For local errors, no address is passed (this
218 can be checked with the
227 After an error has been passed, the pending socket error
228 is regenerated based on the next queued error and will be passed
229 on the next socket operation.
232 This flag requests receipt of out-of-band data that would not be received
233 in the normal data stream.
234 Some protocols place expedited data
235 at the head of the normal data queue, and thus this flag cannot
236 be used with such protocols.
239 This flag causes the receive operation to
240 return data from the beginning of the
241 receive queue without removing that data from the queue.
243 subsequent receive call will return the same data.
245 .BR MSG_TRUNC " (since Linux 2.2)"
248 Internet datagram (since Linux 2.4.27/2.6.8),
249 netlink (since Linux 2.6.22), and UNIX datagram
250 .\" commit 9f6f9af7694ede6314bed281eec74d588ba9474f
251 (since Linux 3.4) sockets:
252 return the real length of the packet or datagram,
253 even when it was longer than the passed buffer.
255 For use with Internet stream sockets, see
258 .BR MSG_WAITALL " (since Linux 2.2)"
259 This flag requests that the operation block until the full request is
261 However, the call may still return less data than requested if
262 a signal is caught, an error or disconnect occurs, or the next data to be
263 received is of a different type than that returned.
264 This flag has no effect for datagram sockets.
268 places the received message into the buffer
270 The caller must specify the size of the buffer in
276 and the underlying protocol provides the source address of the message,
277 that source address is placed in the buffer pointed to by
279 .\" (Note: for datagram sockets in both the UNIX and Internet domains,
283 .\" is also filled in for stream sockets in the UNIX domain, but is not
284 .\" filled in for stream sockets in the Internet domain.)
285 .\" [The above notes on AF_UNIX and AF_INET sockets apply as at
286 .\" Kernel 2.4.18. (MTK, 22 Jul 02)]
289 is a value-result argument.
291 it should be initialized to the size of the buffer associated with
295 is updated to contain the actual size of the source address.
296 The returned address is truncated if the buffer provided is too small;
299 will return a value greater than was supplied to the call.
301 If the caller is not interested in the source address,
305 should be specified as NULL.
310 call is normally used only on a
314 It is equivalent to the call:
316 recvfrom(fd, buf, len, flags, NULL, 0);
323 structure to minimize the number of directly supplied arguments.
324 This structure is defined as follows in
329 struct iovec { /* Scatter/gather array items */
330 void *iov_base; /* Starting address */
331 size_t iov_len; /* Number of bytes to transfer */
335 void *msg_name; /* Optional address */
336 socklen_t msg_namelen; /* Size of address */
337 struct iovec *msg_iov; /* Scatter/gather array */
338 size_t msg_iovlen; /* # elements in msg_iov */
339 void *msg_control; /* Ancillary data, see below */
340 size_t msg_controllen; /* Ancillary data buffer len */
341 int msg_flags; /* Flags on received message */
348 field points to a caller-allocated buffer that is used to
349 return the source address if the socket is unconnected.
350 The caller should set
352 to the size of this buffer before this call;
353 upon return from a successful call,
355 will contain the length of the returned address.
356 If the application does not need to know the source address,
358 can be specified as NULL.
364 describe scatter-gather locations, as discussed in
371 points to a buffer for other protocol control-related messages or
372 miscellaneous ancillary data.
377 should contain the length of the available buffer in
379 upon return from a successful call it will contain the length
380 of the control message sequence.
382 The messages are of the form:
387 size_t cmsg_len; /* Data byte count, including header
388 (type is socklen_t in POSIX) */
389 int cmsg_level; /* Originating protocol */
390 int cmsg_type; /* Protocol-specific type */
392 unsigned char cmsg_data[]; */
397 Ancillary data should be accessed only by the macros defined in
400 As an example, Linux uses this ancillary data mechanism to pass extended
401 errors, IP options, or file descriptors over UNIX domain sockets.
409 It can contain several flags:
412 indicates end-of-record; the data returned completed a record (generally
413 used with sockets of type
414 .BR SOCK_SEQPACKET ).
417 indicates that the trailing portion of a datagram was discarded because the
418 datagram was larger than the buffer supplied.
421 indicates that some control data was discarded due to lack of space in the
422 buffer for ancillary data.
425 is returned to indicate that expedited or out-of-band data was received.
428 indicates that no data was received but an extended error from the socket
431 These calls return the number of bytes received, or \-1
432 if an error occurred.
433 In the event of an error,
435 is set to indicate the error.
437 When a stream socket peer has performed an orderly shutdown,
438 the return value will be 0 (the traditional "end-of-file" return).
440 Datagram sockets in various domains (e.g., the UNIX and Internet domains)
441 permit zero-length datagrams.
442 When such a datagram is received, the return value is 0.
444 The value 0 may also be returned if the requested number of bytes
445 to receive from a stream socket was 0.
447 These are some standard errors generated by the socket layer.
449 may be generated and returned from the underlying protocol modules;
450 see their manual pages.
452 .BR EAGAIN " or " EWOULDBLOCK
453 .\" Actually EAGAIN on Linux
454 The socket is marked nonblocking and the receive operation
455 would block, or a receive timeout had been set and the timeout expired
456 before data was received.
457 POSIX.1 allows either error to be returned for this case,
458 and does not require these constants to have the same value,
459 so a portable application should check for both possibilities.
464 is an invalid file descriptor.
467 A remote host refused to allow the network connection (typically
468 because it is not running the requested service).
471 The receive buffer pointer(s) point outside the process's
475 The receive was interrupted by delivery of a signal before
476 any data was available; see
480 Invalid argument passed.
481 .\" e.g., msg_namelen < 0 for recvmsg() or addrlen < 0 for recvfrom()
484 Could not allocate memory for
488 The socket is associated with a connection-oriented protocol
489 and has not been connected (see
497 does not refer to a socket.
499 POSIX.1-2001, POSIX.1-2008,
500 4.4BSD (these interfaces first appeared in 4.2BSD).
502 POSIX.1 describes only the
509 If a zero-length datagram is pending,
515 argument of zero provide different behavior.
516 In this circumstance,
518 has no effect (the datagram remains pending), while
520 consumes the pending datagram.
524 type was invented by POSIX.
528 According to POSIX.1,
529 .\" POSIX.1-2001, POSIX.1-2008
534 structure should be typed as
536 but glibc currently types it as
538 .\" glibc bug raised 12 Mar 2006
539 .\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
540 .\" The problem is an underlying kernel issue: the size of the
541 .\" __kernel_size_t type used to type this field varies
542 .\" across architectures, but socklen_t is always 32 bits.
546 for information about a Linux-specific system call
547 that can be used to receive multiple datagrams in a single call.
549 An example of the use of