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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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


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

#include <sys/time.h>
#include <sys/select.h>
#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);
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)

    /* 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;

    for (j = 2; j < argc; j++) {
        numRead = sscanf(argv[j], "%d%2[rw]", &fd, buf);
        if (numRead != 2)
        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)

    /* 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);


Download altio/t_select.c

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