typedef provides no type safety

handletest.c

#include <stddef.h>

typedef size_t foo_h;
typedef size_t bar_h;

void foo(foo_h h) { (void)h; }
void bar(bar_h h) { (void)h; }

int main(void)
{
    foo_h f = 0;

    foo(f);
    bar(f);     /* Wrong handle type! */

    return 0;
}

Note that this compiles just fine, but will lead to serious trouble because the wrong "opaque pointer" was passed to bar().