altio/t_select.c

This is altio/t_select.c (Listing 63-1, page 1334), 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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/* t_select.c

   Example of the use of the select() system call to monitor multiple
   file descriptors.

   Usage as shown in usageError().
*/
#include <sys/time.h>
#if ! defined(__hpux)
/* HP-UX 11 doesn't have this header file */
#include <sys/select.h>
#endif
#include "tlpi_hdr.h"
static void
usageError(const char *progName)
{
    fprintf(stderr, "Usage: %s {timeout|-} fd-num[rw]...\n", progName);
    fprintf(stderr, "    - means infinite timeout; \n");
    fprintf(stderr, "    r = monitor for read\n");
    fprintf(stderr, "    w = monitor for write\n\n");
    fprintf(stderr, "    e.g.: %s - 0rw 1w\n", progName);
    exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
    fd_set readfds, writefds;
    int ready, nfds, fd, numRead, j;
    struct timeval timeout;
    struct timeval *pto;
    char buf[10];                       /* Large enough to hold "rw\0" */

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageError(argv[0]);

    /* Timeout for select() is specified in argv[1] */

    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 */
    }

    /* Process remaining arguments to build file descriptor sets */

    nfds = 0;
    FD_ZERO(&readfds);
    FD_ZERO(&writefds);

    for (j = 2; j < argc; j++) {
        numRead = sscanf(argv[j], "%d%2[rw]", &fd, buf);
        if (numRead != 2)
            usageError(argv[0]);
        if (fd >= FD_SETSIZE)
            cmdLineErr("file descriptor exceeds limit (%d)\n", FD_SETSIZE);

        if (fd >= nfds)
            nfds = fd + 1;              /* Record maximum fd + 1 */
        if (strchr(buf, 'r') != NULL)
            FD_SET(fd, &readfds);
        if (strchr(buf, 'w') != NULL)
            FD_SET(fd, &writefds);
    }

    /* We've built all of the arguments; now call select() */

    ready = select(nfds, &readfds, &writefds, NULL, pto);
                                        /* Ignore exceptional events */
    if (ready == -1)
        errExit("select");

    /* Display results of select() */

    printf("ready = %d\n", ready);
    for (fd = 0; fd < nfds; fd++)
        printf("%d: %s%s\n", fd, FD_ISSET(fd, &readfds) ? "r" : "",
                FD_ISSET(fd, &writefds) ? "w" : "");

    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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