altio/poll_pipes.c

This is altio/poll_pipes.c (Listing 63-2, page 1340), 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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/* poll_pipes.c

   Example of the use of poll() to monitor multiple file descriptors.

   Usage: poll_pipes num-pipes [num-writes]
                                  def = 1

   Create 'num-pipes' pipes, and perform 'num-writes' writes to
   randomly selected pipes. Then use poll() to inspect the read ends
   of the pipes to see which pipes are readable.
*/
#include <time.h>
#include <poll.h>
#include "tlpi_hdr.h"
int
main(int argc, char *argv[])
{
    int numPipes, ready, randPipe, numWrites, j;
    struct pollfd *pollFd;
    int (*pfds)[2];                     /* File descriptors for all pipes */

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s num-pipes [num-writes]\n", argv[0]);

    /* Allocate the arrays that we use. The arrays are sized according
       to the number of pipes specified on command line */

    numPipes = getInt(argv[1], GN_GT_0, "num-pipes");
    numWrites = (argc > 2) ? getInt(argv[2], GN_GT_0, "num-writes") : 1;

    pfds = calloc(numPipes, sizeof(int [2]));
    if (pfds == NULL)
        errExit("calloc");
    pollFd = calloc(numPipes, sizeof(struct pollfd));
    if (pollFd == NULL)
        errExit("calloc");

    /* Create the number of pipes specified on command line */

    for (j = 0; j < numPipes; j++)
        if (pipe(pfds[j]) == -1)
            errExit("pipe %d", j);

    /* Perform specified number of writes to random pipes */

    srandom((int) time(NULL));
    for (j = 0; j < numWrites; j++) {
        randPipe = random() % numPipes;
        printf("Writing to fd: %3d (read fd: %3d)\n",
                pfds[randPipe][1], pfds[randPipe][0]);
        if (write(pfds[randPipe][1], "a", 1) == -1)
            errExit("write %d", pfds[randPipe][1]);
    }

    /* Build the file descriptor list to be supplied to poll(). This list
       is set to contain the file descriptors for the read ends of all of
       the pipes. */

    for (j = 0; j < numPipes; j++) {
        pollFd[j].fd = pfds[j][0];
        pollFd[j].events = POLLIN;
    }

    ready = poll(pollFd, numPipes, 0);
    if (ready == -1)
        errExit("poll");

    printf("poll() returned: %d\n", ready);

    /* Check which pipes have data available for reading */

    for (j = 0; j < numPipes; j++)
        if (pollFd[j].revents & POLLIN)
            printf("Readable: %3d\n", pollFd[j].fd);

    exit(EXIT_SUCCESS);
}

 

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