The Linux Programming Interface

Alternative I/O Models 1369

file descriptor that is monitored (along with other file descriptors) using

select(), poll(), or epoll_wait().

63.5.1 The pselect() System Call

The pselect() system call performs a similar task to select(). The main semantic differ-

ence is an additional argument, sigmask, that specifies a set of signals to be

unmasked while the call is blocked.

More precisely, suppose we have the following pselect() call:

ready = pselect(nfds, &readfds, &writefds, &exceptfds, timeout, &sigmask);

This call is equivalent to atomically performing the following steps:

sigset_t origmask;

sigprocmask(SIG_SETMASK, &sigmask, &origmask);

ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);

sigprocmask(SIG_SETMASK, &origmask, NULL); /* Restore signal mask */

Using pselect(), we can recode the first part of the body of our main program in

Listing 63-7 as shown in Listing 63-8.

Aside from the sigmask argument, select() and pselect() differ in the following

ways:

z The timeout argument to pselect() is a timespec structure (Section 23.4.2), which

allows the timeout to be specified with nanosecond (instead of microsecond)

precision.

z SUSv3 explicitly states that pselect() doesn’t modify the timeout argument on return.

If we specify the sigmask argument of pselect() as NULL, then pselect() is equivalent to

select() (i.e., it performs no manipulation of the process signal mask), except for the

differences just noted.

The pselect() interface is an invention of POSIX.1g, and is nowadays incorpo-

rated in SUSv3. It is not available on all UNIX implementations, and was added to

Linux only in kernel 2.6.16.

Previously, a pselect() library function was provided by glibc, but this implemen-

tation didn’t provide the atomicity guarantees that are required for the correct

operation of the call. Such guarantees can be provided only by a kernel

implementation of pselect().

#define _XOPEN_SOURCE 600

#include <sys/select.h>

int pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,

struct timespec *timeout, const sigset_t *sigmask);

Returns number of ready file descriptors, 0 on timeout, or –1 on error