Bug report

Bug description:

The man(3) page fordlsym() states:

In unusual cases (see NOTES) the value of the symbol  could  actually  beNULL. Therefore, a NULL return from dlsym() need not indicate an error.The correct way to distinguish an error from a symbol whose value is NULLis  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  avariable, and check whether this saved value is not NULL.

As such, there can be cases where a call todlsym returnsNULL, while no error has been encountered and the string thatdlerror() 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__<snipWindowsstuff>#elsePyErr_SetString(PyExc_ValueError,dlerror());<...>

Ifdlsym() returnsNULL, then by callingdlerror() we pass eitherNULL or a previous, unconsumed error string toPyErr_SetString.

In the context ofctypes, aNULL return bydlsym() might indeed always be considered an error.

To correctly express this, we should:

• Calldlerror() beforedlsym(), to clear any previous error
• Calldlerror() afterdlsym(), to see ifdlsym() encountered any error
  • Ifdlerror() returns a non-NULL value, then pass its result toPyErr_SetString
  • Ifdlerror() returns NULL, then check ifaddress is NULL, and if so, pass a custom error string toPyErr_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 ofgcc forTestNullDlsym.test_null_dlsym #129872
• [3.13] gh-126554: correct detection ofgcc forTestNullDlsym.test_null_dlsym (GH-129872) #129944
• [3.12] gh-126554: correct detection ofgcc forTestNullDlsym.test_null_dlsym (GH-129872) #129945