NAME

select − synchronous i/o multiplexing

SYNOPSIS

#include <sys/time.h>
nfound = select(nfds, readfds, writefds, execptfds, timeout)
int nfound, nfds;
fd_set ∗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.  Nfds desciptors are checked, i.e. the bits from 0 through nfds-1 in the masks are examined. Select 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 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 select 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 occurred or the time limit expired. 

[EFAULT] The memory pointed to by readfds, writefds, exceptfds, or timeout lies outside the valid address space for the process. 

SEE ALSO

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

BUGS

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

4BSD