Using Linux C select system call to monitor files

Question

I creates two files in the execution folder of the following code, one named test and the other is test2. I run the following code in one terminal which "monitors" those two files for changes, but the select call returns all the time even when the file is not touched.

#include <fcntl.h>      /* fcntl */
#include <sys/select.h> /* select */
#include <stdio.h> /* NULL */

int main() {

    fd_set readfds, writefds;
    int max_fd;

    int fd_1 = open("test", O_RDWR  | O_NONBLOCK);
    int fd_2 = open("test2", O_RDWR | O_NONBLOCK);

    if(fd_1 == -1)
        return -1;

    if(fd_2 == -1)
        return -1;


    while(1) {

        FD_ZERO(&readfds);
        FD_ZERO(&writefds);

        FD_SET(fd_1, &readfds);
        FD_SET(fd_2, &readfds);
        FD_SET(fd_1, &writefds);
        FD_SET(fd_2, &writefds);

        max_fd = (fd_2 > fd_1 ? fd_2 : fd_1) + 1;

        int t_rdy = select(max_fd, &readfds, &writefds, NULL, NULL);

        if(t_rdy == -1)
            return -1;

        int t_fd;
        for(t_fd = 0; t_fd < max_fd; t_fd++) {

            if(t_fd <= 2) continue;

            printf("READ  LIST %d: %s \n", t_fd, (FD_ISSET(t_fd, &readfds) ? "set" : "nope"));
            printf("WRITE LIST %d: %s \n", t_fd, (FD_ISSET(t_fd, &writefds) ? "set" : "nope"));
        }

    }

    return 0;
}

shouldn't you do `sleep` in this cycle too? also, perhaps just `continue` instead of `return` if you don't get a value that makes sense. — Ashalynd, Sep 10 '14 at 23:48
if return of select is -1, that is an error why continue. Regardless that might not be relevant in this case. — ArmenB, Sep 10 '14 at 23:51

score 2 · Accepted Answer · answered Sep 10 '14 at 23:52

2

This is exactly the behavior I would expect. As far as select() is concerned, disk files are always ready to be read from or written to.

answered Sep 10 '14 at 23:52

Jim Lewis

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Then I cannot use select to monitor file changes in a folder for example? – ArmenB Sep 11 '14 at 00:11
@ArmenB. You can, just not like that. – David Schwartz Sep 11 '14 at 00:43
1

(Lookup inotify) – i336_ Oct 10 '20 at 03:22

Using Linux C select system call to monitor files

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