select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - 同步I / O复用

内容简介

/* According to POSIX.1-2001 */
#include <sys/select.h> 
/* According to earlier standards */
#include <sys/time.h> 
#include <sys/types.h> 
#include <unistd.h> 

描述

select() and pselect() allow a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without blocking.

The operation of select() and pselect() is identical, with three differences:

Three independent sets of file 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 inwritefds 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 file descriptors actually changed status. Each of the three file descriptor sets may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.

Four macros are provided to manipulate the sets.

• FD_ZERO() clears a set.

• FD_SET() and

• FD_CLR() respectively add and remove a given file descriptor from a set.

• FD_ISSET() tests to see if a file descriptor is part of the set;

this is useful after select() returns.

nfds is the highest-numbered file 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.) Iftimeout is NULL (no timeout), select() can block indefinitely.

sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL, thenpselect() 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.

Other than the difference in the precision of the timeout argument, the followingpselect() call:

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

is equivalent to atomically executing the following calls:

sigset_t origmask;

sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);

The reason that pselect() is needed is that if one wants to wait for either a signal or for a file descriptor to become ready, then 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. By contrast, pselect() allows one to first block signals, handle the signals that have come in, then call pselect() with the desiredsigmask, avoiding the 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 */
};

and

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

(However, see below on the POSIX.1-2001 versions.). Some code calls select() with all three sets empty, n zero, and a non-NULL timeout as a fairly portable way to sleep with subsecond precision.

On Linux, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this. (POSIX.1-2001 permits either behaviour.) This causes problems both when Linux code which reads timeout is ported to other operating systems, and when code is ported to Linux that reuses a struct timeval for multipleselect()s in a loop without reinitializing it. Consider timeout to be undefined afterselect() returns.

返回值

On success, select() and pselect() return the number of file descriptors contained in the three returned descriptor sets (that is, the total number of bits that are set in readfds,writefds, exceptfds) which may be zero if the timeout expires before anything interesting happens. On error, -1 is returned, and errno is set appropriately; the sets and timeoutbecome undefined, so do not rely on their contents after an error.

错误

EXAMPLE

#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;
}

遵循于

select() conforms to POSIX.1-2001 and 4.4BSD

pselect() is defined in POSIX.1g, and in POSIX.1-2001.