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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 1.8 2007/08/02 00:21:54 swildner Exp $
41 .Nd synchronous I/O multiplexing
47 .Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
50 .Fn FD_ISSET fd &fdset
54 examines the I/O descriptor sets whose addresses are passed in
59 to see if some of their descriptors
60 are ready for reading, are ready for writing, or have an exceptional
61 condition pending, respectively.
62 The only exceptional condition detectable is out-of-band
63 data received on a socket.
66 descriptors are checked in each set;
67 i.e., the descriptors from 0 through
69 in the descriptor sets are examined.
72 replaces the given descriptor sets
73 with subsets consisting of those descriptors that are ready
74 for the requested operation.
76 returns the total number of ready descriptors in all the sets.
78 The descriptor sets are stored as bit fields in arrays of integers.
79 The following macros are provided for manipulating such descriptor sets:
81 initializes a descriptor set
85 includes a particular descriptor
94 .Fn FD_ISSET fd &fdset
100 The behavior of these macros is undefined if
101 a descriptor value is less than zero or greater than or equal to
103 which is normally at least equal
104 to the maximum number of descriptors supported by the system.
108 is a non-nil pointer, it specifies the maximum interval to wait for the
109 selection to complete. System activity can lengthen the interval by
110 an indeterminate amount.
114 is a nil pointer, the select blocks indefinitely.
116 To effect a poll, the
118 argument should be non-nil, pointing to a zero-valued timeval structure.
125 may be given as nil pointers if no descriptors are of interest.
127 Even if no descriptors are of interest, the timeout works as described,
128 effectively putting the process into an interruptible sleep for the
132 is nil, the process will block until a signal is received.
135 returns the number of ready descriptors that are contained in
137 or -1 if an error occurred.
138 If the time limit expires,
143 returns with an error,
144 including one due to an interrupted call,
145 the descriptor sets will be unmodified.
152 One of the descriptor sets specified an invalid descriptor.
154 A signal was delivered before the time limit expired and
155 before any of the selected events occurred.
157 The specified time limit is invalid. One of its components is
158 negative or too large.
166 .Xr getdtablesize 2 ,
177 In order to accommodate programs which might potentially
178 use a larger number of open files with
181 to increase this size by having the program define
183 before the inclusion of any header which includes
188 is greater than the number of open files,
190 is not guaranteed to examine the unused file descriptors. For historical
193 will always examine the first 256 descriptors.
197 function call appeared in
201 allows systems to modify the original timeout in place.
202 Thus, it is unwise to assume that the timeout value will be unmodified