(i)

The select function uses a timeout that is a struct timeval (with seconds and microseconds), while pselect uses a struct timespec (with seconds and nanoseconds).

(ii)

The select function may update the timeout parameter to indicate how much time was left. The pselect function does not change this parameter.

(iii)

The select function has no sigmask parameter, and behaves as pselect called with NULL sigmask .

Three independent sets of descriptors are watched.  Those listed in readfds will be watched to see if characters become available for reading (more precisely, to see if a read will not block - in particular, a file descriptor is also ready on end-of-file), those in writefds will be watched to see if a write will not block, and those in exceptfds will be watched for exceptions.  On exit, the sets are modified in place to indicate which descriptors actually changed status.

Four macros are provided to manipulate the sets. FD_ZERO will clear a set. FD_SET and FD_CLR add or remove a given descriptor from a set. FD_ISSET tests to see if a descriptor is part of the set; this is useful after select returns.

n is the highest-numbered descriptor in any of the three sets, plus 1.

timeout is an upper bound on the amount of time elapsed before select returns. It may be zero, causing select to return immediately. (This is useful for polling.) If timeout is NULL (no timeout), select can block indefinitely.

sigmask is a pointer to a signal mask (see sigprocmask(2) ); if it is not NULL, then pselect first replaces the current signal mask by the one pointed to by sigmask , then does the `select' function, and then restores the original signal mask again.

The idea of pselect is that if one wants to wait for an event, either a signal or something on a file descriptor, an atomic test is needed to prevent race conditions. (Suppose the signal handler sets a global flag and returns. Then a test of this global flag followed by a call of select () could hang indefinitely if the signal arrived just after the test but just before the call. On the other hand, pselect allows one to first block signals, handle the signals that have come in, then call pselect () with the desired sigmask , avoiding the race.) Since Linux today does not have a pselect () system call, the current glibc2 routine still contains this race. "The timeout" The time structures involved are defined in <sys/time.h> and look like

struct timeval { 
    long    tv_sec;         /* seconds */
    long    tv_usec;        /* microseconds */
};

struct timespec {
    long    tv_sec;         /* seconds */
    long    tv_nsec;        /* nanoseconds */
};

EBADF

An invalid file descriptor was given in one of the sets.

EINTR

A non blocked signal was caught.

EINVAL

n is negative or the value contained within timeout is invalid.

ENOMEM

select was unable to allocate memory for internal tables.

#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int
main(void) {
    fd_set rfds;
    struct timeval tv;
    int retval;

    /* Watch stdin (fd 0) to see when it has input. */
    FD_ZERO(&rfds);
    FD_SET(0, &rfds);
    /* Wait up to five seconds. */
    tv.tv_sec = 5;
    tv.tv_usec = 0;

    retval = select(1, &rfds, NULL, NULL, &tv);
    /* Don't rely on the value of tv now! */

    if (retval == -1)
        perror("select()");
    else if (retval)
        printf("Data is available now.\n");
        /* FD_ISSET(0, &rfds) will be true. */
    else
        printf("No data within five seconds.\n");

    return 0;
}