Passing null to dlclose()

Question

Is it safe to pass null (0, NULL, nullptr, ..) to dlclose()?

I could not find anything regarding this in the manual.

Answer 1

If it is not documented, it is most likely not (or at "best" undefined). Actually a simple test could verify that for your platform:

Compiling this

#include <dlfcn.h>
int main()
{
   dlclose(0);
}

Already yields the following compiler warning (gcc):

test.c: In function ‘main’:
test.c:5:2: warning: null argument where non-null required (argument 1) [-Wnonnull]
    5 |  dlclose(0);
      |  ^~~~~~~
/usr/bin/ld: /tmp/ccYF9oqz.o: in function `main':

An running it, crashes as expected with a segmentation fault.

Why you want to do or need to know this in the first place is the question. It should be trivial to do a null-check before calling dlclose or provide a wrapper that does it, if otherwise inconvenient.

Answer 2

It is not safe.

Looking at the glibc implementation, the handle passed to this function is a pointer to a struct link_map instance. This pointer is then dereferenced without a NULL check.

So you need to check for NULL before calling dlclose.