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32 .\" @(#)select.2 8.2 (Berkeley) 3/25/94
40 .Nd synchronous I/O multiplexing
46 .Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
49 .Fn FD_ISSET fd &fdset
55 examines the I/O descriptor sets whose addresses are passed in
60 to see if some of their descriptors
61 are ready for reading, are ready for writing, or have an exceptional
62 condition pending, respectively.
63 The only exceptional condition detectable is out-of-band
64 data received on a socket.
67 descriptors are checked in each set;
68 i.e., the descriptors from 0 through
70 in the descriptor sets are examined.
73 replaces the given descriptor sets
74 with subsets consisting of those descriptors that are ready
75 for the requested operation.
79 returns the total number of ready descriptors in all the sets.
81 The descriptor sets are stored as bit fields in arrays of integers.
82 The following macros are provided for manipulating such descriptor sets:
84 initializes a descriptor set
88 includes a particular descriptor
97 .Fn FD_ISSET fd &fdset
103 The behavior of these macros is undefined if
104 a descriptor value is less than zero or greater than or equal to
106 which is normally at least equal
107 to the maximum number of descriptors supported by the system.
111 is not a null pointer, it specifies the maximum interval to wait for the
112 selection to complete.
113 System activity can lengthen the interval by
114 an indeterminate amount.
118 is a null pointer, the select blocks indefinitely.
120 To effect a poll, the
122 argument should not be a null pointer,
123 but it should point to a zero-valued timeval structure.
130 may be given as null pointers if no descriptors are of interest.
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 system call,
145 the descriptor sets will be unmodified.
152 One of the descriptor sets specified an invalid descriptor.
155 .Fa readfds , writefds , exceptfds ,
158 points to an invalid address.
160 A signal was delivered before the time limit expired and
161 before any of the selected events occurred.
163 The specified time limit is invalid.
164 One of its components is
165 negative or too large.
175 .Xr getdtablesize 2 ,
188 In order to accommodate programs which might potentially
189 use a larger number of open files with
192 to increase this size by having the program define
194 before the inclusion of any header which includes
199 is greater than the number of open files,
201 is not guaranteed to examine the unused file descriptors.
205 will always examine the first 256 descriptors.
220 system call appeared in
224 allows systems to modify the original timeout in place.
225 Thus, it is unwise to assume that the timeout value will be unmodified