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28 .\" @(#)select.2 8.2 (Berkeley) 3/25/94
35 .Nd synchronous I/O multiplexing
41 .Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
44 .Fn FD_ISSET fd &fdset
50 examines the I/O descriptor sets whose addresses are passed in
55 to see if some of their descriptors
56 are ready for reading, are ready for writing, or have an exceptional
57 condition pending, respectively.
58 The only exceptional condition detectable is out-of-band
59 data received on a socket.
62 descriptors are checked in each set;
63 i.e., the descriptors from 0 through
65 in the descriptor sets are examined.
68 replaces the given descriptor sets
69 with subsets consisting of those descriptors that are ready
70 for the requested operation.
74 returns the total number of ready descriptors in all the sets.
76 The descriptor sets are stored as bit fields in arrays of integers.
77 The following macros are provided for manipulating such descriptor sets:
79 initializes a descriptor set
83 includes a particular descriptor
92 .Fn FD_ISSET fd &fdset
98 The behavior of these macros is undefined if
99 a descriptor value is less than zero or greater than or equal to
101 which is normally at least equal
102 to the maximum number of descriptors supported by the system.
106 is not a null pointer, it specifies the maximum interval to wait for the
107 selection to complete.
108 System activity can lengthen the interval by
109 an indeterminate amount.
113 is a null pointer, the select blocks indefinitely.
115 To effect a poll, the
117 argument should not be a null pointer,
118 but it should point to a zero-valued timeval structure.
125 may be given as null 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 system call,
140 the descriptor sets will be unmodified.
147 One of the descriptor sets specified an invalid descriptor.
150 .Fa readfds , writefds , exceptfds ,
153 points to an invalid address.
155 A signal was delivered before the time limit expired and
156 before any of the selected events occurred.
158 The specified time limit is invalid.
159 One of its components is
160 negative or too large.
170 .Xr getdtablesize 2 ,
184 In order to accommodate programs which might potentially
185 use a larger number of open files with
188 to increase this size by having the program define
190 before the inclusion of any header which includes
195 is greater than the number of open files,
197 is not guaranteed to examine the unused file descriptors.
201 will always examine the first 256 descriptors.
216 system call appeared in
220 allows systems to modify the original timeout in place.
221 Thus, it is unwise to assume that the timeout value will be unmodified
226 does not modify the return value, which can cause problems for applications
227 ported from other systems.