timers/timed_read.c

/* timed_read.c

   Demonstrate the use of a timer to place a timeout on a blocking system call
   (read(2) in this case).
*/
#include <signal.h>
#include "tlpi_hdr.h"

#define BUF_SIZE 200

static void     /* SIGALRM handler: interrupts blocked system call */
handler(int sig)
{
    printf("Caught signal\n");          /* UNSAFE (see Section 21.1.2) */
}

int
main(int argc, char *argv[])
{
    struct sigaction sa;
    char buf[BUF_SIZE];
    ssize_t numRead;
    int savedErrno;

    if (argc > 1 && strcmp(argv[1], "--help") == 0)
        usageErr("%s [num-secs [restart-flag]]\n", argv[0]);

    /* Set up handler for SIGALRM. Allow system calls to be interrupted,
       unless second command-line argument was supplied. */

    sa.sa_flags = (argc > 2) ? SA_RESTART : 0;
    sigemptyset(&sa.sa_mask);
    sa.sa_handler = handler;
    if (sigaction(SIGALRM, &sa, NULL) == -1)
        errExit("sigaction");

    alarm((argc > 1) ? getInt(argv[1], GN_NONNEG, "num-secs") : 10);

    numRead = read(STDIN_FILENO, buf, BUF_SIZE);

    savedErrno = errno;                 /* In case alarm() changes it */
    alarm(0);                           /* Ensure timer is turned off */
    errno = savedErrno;

    /* Determine result of read() */

    if (numRead == -1) {
        if (errno == EINTR)
            printf("Read timed out\n");
        else
            errMsg("read");
    } else {
        printf("Successful read (%ld bytes): %.*s",
                (long) numRead, (int) numRead, buf);
    }

    exit(EXIT_SUCCESS);
}