Question

I’ve got a small program that opens a file and does some operation on it. I subscribed the file closure to the program termination as follows:

static
void exit_handler (int ev, void *arg)
{
    fprintf(stderr, "bye %d\n", WEXITSTATUS(ev));
    fclose((FILE *)arg);
}

int main (int argc, char *argv[])
{
    FILE *out;

    ...

    out = fopen(argv[1], "wt");
    if (out == NULL) {
        perror("Opening output file");
        exit(EXIT_FAILURE);
    }
    on_exit(exit_handler, out);

    ...
}

Trying to execute this I notice that it works properly only if the program terminates normally. In case of CTRL+C (SIGINT) the exit_handler callback is not executed.

Isn’t that weird? Should I associate a exit(EXIT_FAILURE) call to the signal handler for SIGTERM? What is the best practice in this case?

Answer 1

First of all, on_exit isn’t specified by POSIX (atexit with the same semantics is). Second , the linux manual says:

The on_exit() function registers the given function to be called
at normal process termination, whether via exit(3) or via return from
the program’s main().

Getting killed by a signal is not a normal exit for a process so callbacks installed with on_exit and atexit aren’t implicitly called.