timers/timed_read.c

This is timers/timed_read.c (Listing 23-2, page 486), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* 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);
        /*FIXME: above: should use %zd here, and remove (long) cast */
    }

    exit(EXIT_SUCCESS);
}

 

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