altio/poll_pipes.c

/* 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);
}