SELECT(2) — System Interface Manual — System Calls

NAME

select − synchronous i/o multiplexing

SYNOPSIS

#include <time.h>

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

DESCRIPTION

Select 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. File descriptor f is represented by the bit “1<<f” in the mask. Select returns, in place, a mask of those descriptors which are ready. The total number of ready descriptors is returned in nfds.

If timeout is a non-zero 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 non-zero, 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

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

ERRORS

An error return from select indicates:

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

[EINTR] An signal was delivered before any of the selected for events occured or the time limit expired.

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