altio/self_pipe.c

This is altio/self_pipe.c (Listing 63-9, page 1371), an example from the book, The Linux Programming Interface.

The source code file is copyright 2010, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/*  self_pipe.c

   Employ the self-pipe trick so that we can avoid race conditions while both
   selecting on a set of file descriptors and also waiting for a signal.

   Usage as shown in synopsis below; for example:

        self_pipe - 0
*/
#include <sys/time.h>
#if ! defined(__hpux)   /* HP-UX 11 doesn't have this header file */
#include <sys/select.h>
#endif
#include <fcntl.h>
#include <signal.h>
#include "tlpi_hdr.h"

static int pfd[2];                      /* File descriptors for pipe */
static void
handler(int sig)
{
    int savedErrno;                     /* In case we change 'errno' */

    savedErrno = errno;
    if (write(pfd[1], "x", 1) == -1 && errno != EAGAIN)
        errExit("write");
    errno = savedErrno;
}
int
main(int argc, char *argv[])
{
    fd_set readfds;
    int ready, nfds, flags;
    struct timeval timeout;
    struct timeval *pto;
    struct sigaction sa;
    char ch;
    int fd, j;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s {timeout|-} fd...\n"
                "\t\t('-' means infinite timeout)\n", argv[0]);

    /* Initialize 'timeout', 'readfds', and 'nfds' for select() */

    if (strcmp(argv[1], "-") == 0) {
        pto = NULL;                     /* Infinite timeout */
    } else {
        pto = &timeout;
        timeout.tv_sec = getLong(argv[1], 0, "timeout");
        timeout.tv_usec = 0;            /* No microseconds */
    }

    nfds = 0;

    /* Build the 'readfds' from the fd numbers given in command line */

    FD_ZERO(&readfds);
    for (j = 2; j < argc; j++) {
        fd = getInt(argv[j], 0, "fd");
        if (fd >= FD_SETSIZE)
            cmdLineErr("file descriptor exceeds limit (%d)\n", FD_SETSIZE);

        if (fd >= nfds)
            nfds = fd + 1;              /* Record maximum fd + 1 */
        FD_SET(fd, &readfds);
    }

    /* Create pipe before establishing signal handler to prevent race */

    if (pipe(pfd) == -1)
        errExit("pipe");

    FD_SET(pfd[0], &readfds);           /* Add read end of pipe to 'readfds' */
    nfds = max(nfds, pfd[0] + 1);       /* And adjust 'nfds' if required */

    /* Make read and write ends of pipe nonblocking */

    flags = fcntl(pfd[0], F_GETFL);
    if (flags == -1)
        errExit("fcntl-F_GETFL");
    flags |= O_NONBLOCK;                /* Make read end nonblocking */
    if (fcntl(pfd[0], F_SETFL, flags) == -1)
        errExit("fcntl-F_SETFL");

    flags = fcntl(pfd[1], F_GETFL);
    if (flags == -1)
        errExit("fcntl-F_GETFL");
    flags |= O_NONBLOCK;                /* Make write end nonblocking */
    if (fcntl(pfd[1], F_SETFL, flags) == -1)
        errExit("fcntl-F_SETFL");

    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART;           /* Restart interrupted reads()s */
    sa.sa_handler = handler;
    if (sigaction(SIGINT, &sa, NULL) == -1)
        errExit("sigaction");

    while ((ready = select(nfds, &readfds, NULL, NULL, pto)) == -1 &&
            errno == EINTR)
        continue;                       /* Restart if interrupted by signal */
    if (ready == -1)                    /* Unexpected error */
        errExit("select");

    if (FD_ISSET(pfd[0], &readfds)) {   /* Handler was called */
        printf("A signal was caught\n");

        for (;;) {                      /* Consume bytes from pipe */
            if (read(pfd[0], &ch, 1) == -1) {
                if (errno == EAGAIN)
                    break;              /* No more bytes */
                else
                    errExit("read");    /* Some other error */
            }

            /* Perform any actions that should be taken in response to signal */
        }
    }

    /* Examine file descriptor sets returned by select() to see
       which other file descriptors are ready */

    printf("ready = %d\n", ready);
    for (j = 2; j < argc; j++) {
        fd = getInt(argv[j], 0, "fd");
        printf("%d: %s\n", fd, FD_ISSET(fd, &readfds) ? "r" : "");
    }

    /* And check if read end of pipe is ready */

    printf("%d: %s   (read end of pipe)\n", pfd[0],
            FD_ISSET(pfd[0], &readfds) ? "r" : "");

    if (pto != NULL)
        printf("timeout after select(): %ld.%03ld\n",
               (long) timeout.tv_sec, (long) timeout.tv_usec / 1000);

    exit(EXIT_SUCCESS);
}

 

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