The Linux Programming Interface

1340 Chapter 63

The following shell session shows an example of what we see when running this

program. The command-line arguments to the program specify that ten pipes

should be created, and writes should be made to three randomly selected pipes.

$ ./poll_pipes 10 3

Writing to fd: 4 (read fd: 3)

Writing to fd: 14 (read fd: 13)

Writing to fd: 14 (read fd: 13)

poll() returned: 2

Readable: 3

Readable: 13

From the above output, we can see that poll() found two pipes had data available

for reading.

Listing 63-2: Using poll() to monitor multiple file descriptors

––––––––––––––––––––––––––––––––––––––––––––––––––––––– altio/poll_pipes.c

#include <time.h>

#include <poll.h>

#include "tlpi_hdr.h"

int

main(int argc, char *argv[])

{

int numPipes, j, ready, randPipe, numWrites;

int (*pfds)[2]; /* File descriptors for all pipes */

struct pollfd *pollFd;

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");

pfds = calloc(numPipes, sizeof(int [2]));

if (pfds == NULL)

errExit("malloc");

pollFd = calloc(numPipes, sizeof(struct pollfd));

if (pollFd == NULL)

errExit("malloc");

/* 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 */

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