c - How can I prevent (not react to) a segmentation fault?

Question

I am not trying to handle a segmentation fault. I understand how exception handling works, more or less. I would rather not have the fault in the first place. What I want to do is call a function or perform an operation that returns a value telling me whether or not that particular memory location/block is accessible, without actually accessing it and getting the fault.

That is, I would like a C function to probe an address in Linux and/or Mac OS X before actually accessing it. Something like:

result = probe_memory(address,length)

where result is

 0 = writable
 1 = read-only
-1 = nonexistent

or something along those lines.

Is there anything like that in Linux and/or Mac OS X?

Answer 1

I believe something more or less like the following should work:

int probe_memory(void *address, size_t length) {
    int result, fd[2];

    pipe(fd);  /* Remember to check for errors! */

    errno = 0;
    result = write(fd[1], address, length);

    if (result < 0 || (size_t)result != length || errno == EFAULT) {
        result = 0;
    } else {
        result = 1;
    }

    close(fd[0]);
    close(fd[1]);

    return result;
}

This is a partial solution to your problem, as this code does not check for page protection.

The basic idea is to let the OS read length bytes from address through the call to write. If the memory is not accessible, it will return EFAULT without triggering a segfault.

Jonathan Leffler wrote a fully worked up implementation in his Stack Overflow Questions repository.