Poll0

William Baxter

2024-12-07

1 min read til c

In a pipeline you may have one segment that must wait for input before executing. Think xargs but passing data through rather than using it as command-line arguments. Today I addressed this with a simple C program, called poll0. It polls file descriptor 0 and exits 0 if data is ready, and 1 otherwise. It takes an optional argument giving a millisecond timeout for the call to the poll() system call.

It’s does one thing only. It’s simple. And it solves a useful problem. And the bonus: Claude wrote it.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <poll.h>

int main(int argc, char *argv[]) {
    int timeout = -1;

    if (argc > 2) {
        fprintf(stderr, "Usage: %s [timeout_ms]\n", argv[0]);
        return 1;
    }

    if (argc == 2) {
        timeout = atoi(argv[1]);
    }

    struct pollfd fds[1];
    fds[0].fd = 0;  // stdin
    fds[0].events = POLLIN;

    int result = poll(fds, 1, timeout);
    if (result < 0) {
        perror("poll");
        return 1;
    }

    return (result == 0) ? 1 : 0;  // 1 on timeout, 0 on input available
}