Question

I’m writting an app and its in the specification that I need to lock
a file everytime I write on it (this file will be read for other apps
that other team is working on):

I made the following function:

int lock_file (int fd)
{
    if (fd == -1)
        return -1;
    struct flock file_locker;
    file_locker.l_type = F_WRLCK;
    file_locker.l_whence = SEEK_SET;
    file_locker.l_start = 0;
    file_locker.l_len = 0; //lock the entire file

    int locked = fcntl(fd, F_SETLK, &file_locker);
    if (locked == -1){
        /*handle errors*/
        return 0;
    }
    return 1;
}

I can get the 1 return (means everything is ok) but when I made a test case
I could write in the locked file Oo

the test code was:

char *file = "lock_test_ok";
int fd = open(file, O_RDWR);
int locked = lock_file(fd);
/* call popen and try write 'ERROR' in the file */

/* if the file contains ERROR, than fail */

Answer 1

Locking in Unix is advisory: only programs testing the lock will not write in it. (Some offers mandatory locking, but not that way. It usually involves setting up special properties on the locked file.)