NAME

select − synchronous I/O multiplexing

SYNOPSIS

#include <sys/time.h>

nfds = select(width, readfds, writefds, execptfds, timeout)
int width, ∗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.  Width is the number of significant bits in each bit mask that represent a file descriptor.  Typically width has the value returned by getdtablesize(2) for the maximum number of file descriptors or is the constant 32 (number of bits in an int). 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 effect 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 select returns 0. 

ERRORS

An error return from select indicates:

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

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

SEE ALSO

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

BUGS

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

Sun Release 2.0  —  Last change: 1 March 1984