SELECT(2) BSD System Calls Manual SELECT(2)
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 sets.
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 mem-
ber 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 selec-
tion 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
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 descriptor.
[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 includes <sys/types.h>.
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
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 prob-
lems for applications ported from other systems.
BSD November 17, 2002 BSD