NAME

select − synchronous I/O multiplexing

SYNTAX

#include <sys/time.h>

nfound = select(nfds, readfds, writefds, execptfds, timeout)
int nfound, nfds, *readfds, *writefds, *execptfds;
struct timeval *timeout;

DESCRIPTION

The select system call examines the I/O descriptors specified by the bit masks readfds, writefds, and execptfds to see if they are ready for reading, writing, or have an exceptional condition pending, respectively.  The I/O descriptors can be pointers to arrays of integers if multiple fd’s are required to be selected.  File descriptor f is represented by the bit “1<<f” in the mask.  The nfds descriptors are checked, that is the bits from 0 through nfds-1 in the masks are examined. The select system call returns, in place, a mask of those descriptors which are ready. The total number of ready descriptors is returned in nfound.

If timeout is a nonzero pointer, it specifies a maximum interval to wait for the selection to complete.  If timeout is a zero pointer, the select blocks indefinitely.  To affect a poll, the timeout argument should be nonzero, pointing to a zero valued timeval structure. 

Any of readfds, writefds, and execptfds may be given as 0 if no descriptors are of interest. 

RETURN VALUE

The select system call returns the number of descriptors which are contained in the bit masks, or −1 if an error occurred. If the time limit expires then select returns 0.

RESTRICTIONS

If a process is blocked on a select waiting for input from a socket and the sending process closes the socket, the select notes this as an exception rather than as data.  Hence, if the select is not currently looking for exceptions, it will wait forever.

The descriptor masks are always modified on return, even if the call returns as the result of the timeout. 

DIAGNOSTICS

An error return from select indicates:

[EBADF] One of the bit masks specified an invalid descriptor. 

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

[EINVAL] The specified time limit is unacceptable.  One of its components is negative or too large. 

SEE ALSO

accept(2), connect(2), read(2), recv(2), send(2), write(2)

System Calls