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32 .\" @(#)select.2 8.2 (Berkeley) 3/25/94
33 .\" $FreeBSD: src/lib/libc/sys/select.2,v 1.14.2.5 2001/12/14 18:34:01 ru Exp $
34 .\" $DragonFly: src/lib/libc/sys/select.2,v 1.7 2006/05/26 21:39:37 swildner Exp $
41 .Nd synchronous I/O multiplexing
49 .Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
52 .Fn FD_ISSET fd &fdset
56 examines the I/O descriptor sets whose addresses are passed in
61 to see if some of their descriptors
62 are ready for reading, are ready for writing, or have an exceptional
63 condition pending, respectively.
64 The only exceptional condition detectable is out-of-band
65 data received on a socket.
68 descriptors are checked in each set;
69 i.e., the descriptors from 0 through
71 in the descriptor sets are examined.
74 replaces the given descriptor sets
75 with subsets consisting of those descriptors that are ready
76 for the requested operation.
78 returns the total number of ready descriptors in all the sets.
80 The descriptor sets are stored as bit fields in arrays of integers.
81 The following macros are provided for manipulating such descriptor sets:
83 initializes a descriptor set
87 includes a particular descriptor
96 .Fn FD_ISSET fd &fdset
102 The behavior of these macros is undefined if
103 a descriptor value is less than zero or greater than or equal to
105 which is normally at least equal
106 to the maximum number of descriptors supported by the system.
110 is a non-nil pointer, it specifies the maximum interval to wait for the
111 selection to complete. System activity can lengthen the interval by
112 an indeterminate amount.
116 is a nil pointer, the select blocks indefinitely.
118 To effect a poll, the
120 argument should be non-nil, pointing to a zero-valued timeval structure.
127 may be given as nil pointers if no descriptors are of interest.
129 Even if no descriptors are of interest, the timeout works as described,
130 effectively putting the process into an interruptible sleep for the
134 is nil, the process will block until a signal is received.
137 returns the number of ready descriptors that are contained in
139 or -1 if an error occurred.
140 If the time limit expires,
145 returns with an error,
146 including one due to an interrupted call,
147 the descriptor sets will be unmodified.
154 One of the descriptor sets specified an invalid descriptor.
156 A signal was delivered before the time limit expired and
157 before any of the selected events occurred.
159 The specified time limit is invalid. One of its components is
160 negative or too large.
168 .Xr getdtablesize 2 ,
179 In order to accommodate programs which might potentially
180 use a larger number of open files with
183 to increase this size by having the program define
185 before the inclusion of any header which includes
190 is greater than the number of open files,
192 is not guaranteed to examine the unused file descriptors. For historical
195 will always examine the first 256 descriptors.
199 function call appeared in
203 allows systems to modify the original timeout in place.
204 Thus, it is unwise to assume that the timeout value will be unmodified