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Synchronous I/O multiplexing.

#include <sys/select.h>

select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);


FD_SET(fd, &fdset);

FD_CLR(fd, &fdset);

FD_ISSET(fd, &fdset);


The select() system call examines the I/O descriptor sets whose addresses a re 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:

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 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 descriptors.

select() should not be regarded as a high precision timer. Despite the timeval structure having a microseconds value (tv_usec), the minimum time select() will wait is 10 milliseconds. If timeout is all zeros, select() returns immediately.


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.

Return Values

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.


Versions Link to
INtime 4.0 netlib.lib

See Also

accept, connect, poll, read, recv, send, write, fcntl