Question

While debugging a crash, I came across this issue in some code:

int func()
{
    char *p1 = malloc(...);
    if (p1 == NULL)
        goto err_exit;

    char *p2 = malloc(...);
    if (p2 == NULL)
        goto err_exit;

    ...

err_exit:
    free(p2);
    free(p1);

    return -1;
}

The problem occurs when the first malloc fails. Because we jump across the initialization of p2, it contains random data and the call to free(p2) can crash.

I would expect/hope that this would be treated the same way as in C++ where the compiler does not allow a goto to jump across an initialization.

My question: is jumping across an initialization allowed by the standard or is this a bug in gcc’s implementation of c99?

Answer 1

You can ask gcc to warn you when you jump over a variable definition by using -Wjump-misses-init and then you can use -Werror (or, more precisely, -Werror=jump-misses-init) to force the users to deal with it. This warning is included in -Wc++-compat so the gcc developers are aware that the code behaves differently in C versus C++.

You could also change the code slightly:

int func()
{
    char *p1 = malloc(...);
    if (p1 == NULL)
        goto err_exit_1;

    char *p2 = malloc(...);
    if (p2 == NULL)
        goto err_exit_2;

    ...

err_exit_2:
    free(p2);
err_exit_1:
    free(p1);

    return -1;
}

… and just keep pairing labels with initialized variables. You’ll have the same problem with calling many other functions with unitialized variables, free just happens to be a more obvious one.