Bug report

Bug description:

The man(3) page for dlsym() states:

In unusual cases (see NOTES) the value of the symbol  could  actually  be
NULL. Therefore, a NULL return from dlsym() need not indicate an error.
The correct way to distinguish an error from a symbol whose value is NULL
is  to  call  dlerror(3) to clear any old error conditions, then call dl‐
sym(), and then call dlerror(3) again, saving its  return  value  into  a
variable, and check whether this saved value is not NULL.

As such, there can be cases where a call to dlsym returns NULL, while no error has been encountered and the string that dlerror() returns has not been (re)set.

Currently, (https://github.com/python/cpython/blob/main/Modules/_ctypes/_ctypes.c#L970) we do:

<...>
address = (void *)dlsym(handle, name);
if (!address) {
#ifdef __CYGWIN__
<snip Windows stuff>
#else
        PyErr_SetString(PyExc_ValueError, dlerror());
<...>

If dlsym() returns NULL, then by calling dlerror() we pass either NULL or a previous, unconsumed error string to PyErr_SetString.

In the context of ctypes, a NULL return by dlsym() might indeed always be considered an error.

To correctly express this, we should:

• Call dlerror() before dlsym(), to clear any previous error
• Call dlerror() after dlsym(), to see if dlsym() encountered any error
  • If dlerror() returns a non-NULL value, then pass its result to PyErr_SetString
  • If dlerror() returns NULL, then check if address is NULL, and if so, pass a custom error string to PyErr_SetString.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-126554: ctypes: Correctly handle NULL dlsym values #126555
• [3.13] gh-126554: ctypes: Correctly handle NULL dlsym values (GH-126555) #126861
• [3.12] gh-126554: ctypes: Correctly handle NULL dlsym values (GH-126555) #127764
• gh-126554: correct detection of gcc for TestNullDlsym.test_null_dlsym #129872
• [3.13] gh-126554: correct detection of gcc for TestNullDlsym.test_null_dlsym (GH-129872) #129944
• [3.12] gh-126554: correct detection of gcc for TestNullDlsym.test_null_dlsym (GH-129872) #129945