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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 1.2 2003/06/17 04:26:47 dillon 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.
130 returns the number of ready descriptors that are contained in
132 or -1 if an error occurred.
133 If the time limit expires,
138 returns with an error,
139 including one due to an interrupted call,
140 the descriptor sets will be unmodified.
147 One of the descriptor sets specified an invalid descriptor.
149 A signal was delivered before the time limit expired and
150 before any of the selected events occurred.
152 The specified time limit is invalid. One of its components is
153 negative or too large.
161 .Xr getdtablesize 2 ,
172 In order to accommodate programs which might potentially
173 use a larger number of open files with
176 to increase this size by having the program define
178 before the inclusion of any header which includes
183 is greater than the number of open files,
185 is not guaranteed to examine the unused file descriptors. For historical
188 will always examine the first 256 descriptors.
191 allows systems to modify the original timeout in place.
192 Thus, it is unwise to assume that the timeout value will be unmodified
199 function call appeared in