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);
 }