Implementing RAII in pure C?

This is inherent implementation dependent, since the Standard doesn’t include such a possibility. For GCC, the cleanup attribute runs a function when a variable goes out of scope:

#include <stdio.h>

void scoped(int * pvariable) {
    printf("variable (%d) goes out of scope\n", *pvariable);
}

int main(void) {
    printf("before scope\n");
    {
        int watched __attribute__((cleanup (scoped)));
        watched = 42;
    }
    printf("after scope\n");
}

Prints:

before scope
variable (42) goes out of scope
after scope

See here