select -- synchronous I/O multiplexing


cc . . . -lc

#include  <sys/types.h>
#include  <sys/times.h>
#include  <sys/select.h>

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

FD_CLR(fd, &fdset) int fd; fd_set fdset;

FD_ISSET(fd, &fdset) int fd; fd_set fdset;

FD_SET(fd, &fdset) int fd; fd_set fdset;

FD_ZERO(&fdset) fd_set fdset;


select 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 first nfds descriptors are checked in each set; that is, 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 return value of select is 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 nonzero 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 a non-NULL pointer, it specifies a maximum interval to wait for the selection to complete. If timeout is a NULL pointer, the select blocks indefinitely. To effect a poll, the timeout argument should be non-NULL, pointing to a zero-valued timeval structure.

Any of readfds, writefds, and exceptfds may be given as NULL pointers if no descriptors are of interest.

Return values

select returns the number of ready descriptors that are contained in the descriptor sets, or -1 if an error occurred. If the time limit expires then select returns 0. If select returns with an error, including one due to an interrupted call, the descriptor sets are unmodified.


An error return from select indicates:

One of the descriptor sets specified an invalid descriptor.

A signal was delivered before the time limit expired and before any of the selected events occurred.

The specified time limit is invalid. One of its components is negative or too large. Or, the device driver being polled has not implemented select support.


select should probably return the time remaining from the original timeout, if any, by modifying the time value in place. This may be implemented in future versions of the system. Thus, it is unwise to assume that the timeout value is unmodified by the select call.

See also

read(S), write(S)

Standards conformance

select is not part of any currently supported standard; it was developed at the University of California at Berkeley and is maintained by The SCO Group.
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003