lock_test.c

gistfile1.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/file.h>
#include <errno.h>

void lockFile(char *filename) {
    int fd;

    /* To lock it, we need to open the file in a way it is created if
     * it does not exist, otherwise there is a race condition with other
     * processes. */
    fd = open(filename,O_WRONLY|O_CREAT,0644);
    if (fd == -1) {
        perror("Can't open the file in order to acquire a lock");
        exit(1);
    }

    if (flock(fd,LOCK_EX|LOCK_NB) == -1) {
        if (errno == EWOULDBLOCK) {
            printf("Sorry, this file is already locked by another process.\n");
        } else {
            perror("flock error");
        }
        exit(1);
    }
    /* Note: leak the 'fd' by not closing it, so that we'll retain the
     * lock to the file as long as the process exists. */
    printf("Lock acquired!\n");
}

int main(void) {
    char *filename = "/tmp/foo.txt";

    lockFile(filename);

    while(1) {
        int fd = open(filename,O_WRONLY|O_CREAT,0644);
        int len;
        char buf[32];

        if (fd == -1) {
            perror("Can't open the file for writing");
            exit(1);
        }
        len = snprintf(buf,sizeof(buf),"%d", rand());
        write(fd,buf,len);
        ftruncate(fd,len);
        close(fd);

        sleep(1);
    }
}