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select — synchronous I/O multiplexing
Standard C Library (libc, -lc)
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
struct timeval *timeout);
The select() system call examines the I/O descriptor sets whose addresses
are passed in readfds, writefds, and exceptfds to see if some of their
descriptors are ready for reading, are ready for writing, or have an
exceptional condition pending, respectively. The only exceptional
condition detectable is out-of-band data received on a socket. The first
nfds descriptors are checked in each set; i.e., the descriptors from 0
through nfds-1 in the descriptor sets are examined. On return, select()
replaces the given descriptor sets with subsets consisting of those
descriptors that are ready for the requested operation. The select()
system call returns the total number of ready descriptors in all the
The descriptor sets are stored as bit fields in arrays of integers. The
following macros are provided for manipulating such descriptor sets:
FD_ZERO(&fdset) initializes a descriptor set fdset to the null set.
FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.
FD_CLR(fd, &fdset) removes fd from fdset. FD_ISSET(fd, &fdset) is non-
zero if fd is a member of fdset, zero otherwise. The behavior of these
macros is undefined if a descriptor value is less than zero or greater
than or equal to FD_SETSIZE, which is normally at least equal to the
maximum number of descriptors supported by the system.
If timeout is not a null pointer, it specifies the maximum interval to
wait for the selection to complete. System activity can lengthen the
interval by an indeterminate amount.
If timeout is a null pointer, the select blocks indefinitely.
To effect a poll, the timeout argument should not be a null pointer, but
it should point to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds may be given as null pointers if
no descriptors are of interest.
The select() system call returns the number of ready descriptors that are
contained in the descriptor sets, or -1 if an error occurred. If the
time limit expires, select() returns 0. If select() returns with an
error, including one due to an interrupted system call, the descriptor
sets will be unmodified.
An error return from select() indicates:
[EBADF] One of the descriptor sets specified an invalid
[EFAULT] One of the arguments readfds, writefds, exceptfds, or
timeout points to an invalid address.
[EINTR] A signal was delivered before the time limit expired
and before any of the selected events occurred.
[EINVAL] The specified time limit is invalid. One of its
components is negative or too large.
[EINVAL] The nfds argument was invalid.
accept(2), connect(2), getdtablesize(2), gettimeofday(2), kqueue(2),
poll(2), read(2), recv(2), send(2), write(2), clocks(7)
The default size of FD_SETSIZE is currently 1024. In order to
accommodate programs which might potentially use a larger number of open
files with select(), it is possible to increase this size by having the
program define FD_SETSIZE before the inclusion of any header which
If nfds is greater than the number of open files, select() is not
guaranteed to examine the unused file descriptors. For historical
reasons, select() will always examine the first 256 descriptors.
The select() system call and FD_CLR(), FD_ISSET(), FD_SET(), and
FD_ZERO() macros conform with IEEE Std 1003.1-2001 (“POSIX.1”).
The select() system call appeared in 4.2BSD.
Version 2 of the Single UNIX Specification (“SUSv2”) allows systems to
modify the original timeout in place. Thus, it is unwise to assume that
the timeout value will be unmodified by the select() system call.
FreeBSD does not modify the return value, which can cause problems for
applications ported from other systems.