Test whether or not a DLL's atexit handler is called on FreeLibrary

_README.md

This is a demonstration of how a DLL's atexit handler is called on FreeLibrary if Microsoft's CRT is used.

---

Mixing Cygwin and Microsoft's CRT does not seem to work well. However according to Cygwin it is possible:

https://cygwin.com/faq/faq.html#faq.programming.msvs-mingw

https://github.com/openunix/cygwin/blob/master/winsup/testsuite/winsup.api/cygload.cc

I tried a program linked to Microsoft's CRT and a DLL linked to cygwin's CRT and the atexit handlers weren't called.

---

If you use a program and DLL that both use Microsoft's CRT (doesn't have to be the same version) or both use Cygwin's CRT then the test should PASS:

>>> main
libfoo was loaded.
>>> libfoo_init
<<< libfoo_init
libfoo was initialized.
>>> libfoo atexit handler
<<< libfoo atexit handler
libfoo was unloaded.
PASS: libfoo's atexit handler ran before main end.
<<< main
>>> main atexit handler
<<< main atexit handler

---

Other references:

curl/curl#1055

openssl/openssl#1693

libfoo.c

/* libfoo
Test that a DLL atexit handler is called during FreeLibrary.

gcc -Wall -Wextra -o main main.c
gcc -Wall -Wextra -shared -o libfoo.dll libfoo.c

cl.exe /W4 /MD main.c
cl.exe /W4 /LD /MD libfoo.c
If you withhold MD from either the results should be the same.

Ref https://github.com/openssl/openssl/pull/1693

https://gist.github.com/jay/423e12ea7f5b86c72f59719c8f5a006c
*/

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>

int *atexit_handler_was_called = NULL;

void goodbye(void) {
  fprintf(stderr, ">>> libfoo atexit handler\n");
  fprintf(stderr, "<<< libfoo atexit handler\n");
  *atexit_handler_was_called = TRUE;
}

__declspec(dllexport)
int libfoo_init(int *notify) {
  fprintf(stderr, ">>> libfoo_init\n");

  if(!notify)
    return FALSE;

  atexit_handler_was_called = notify;
  *atexit_handler_was_called = FALSE;

  if(atexit(goodbye))
    return FALSE;

  fprintf(stderr, "<<< libfoo_init\n");
  return TRUE;
}

main.c

/* main
Test that a DLL atexit handler is called during FreeLibrary.

gcc -Wall -Wextra -o main main.c
gcc -Wall -Wextra -shared -o libfoo.dll libfoo.c

cl.exe /W4 /MD main.c
cl.exe /W4 /LD /MD libfoo.c
If you withhold MD from either the results should be the same.

Ref https://github.com/openssl/openssl/pull/1693

https://gist.github.com/jay/423e12ea7f5b86c72f59719c8f5a006c
*/

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>

void goodbye(void) {
  fprintf(stderr, ">>> main atexit handler\n");
  fprintf(stderr, "<<< main atexit handler\n");
}

int main() {
  HMODULE dll;
  int (*libfoo_init)(int *);
  int libfoo_atexit_handler_was_called = FALSE;

  fprintf(stderr, ">>> main\n");

  if(atexit(goodbye)) {
    fprintf(stderr, "main atexit handler failed to register\n");
    goto cleanup;
  }

  dll = LoadLibraryA("libfoo");
  if(!dll) {
    fprintf(stderr, "libfoo not found\n");
    goto cleanup;
  }

  fprintf(stderr, "libfoo was loaded.\n");

  libfoo_init = (int (*)(int *))GetProcAddress(dll, "libfoo_init");
  if(!libfoo_init) {
    fprintf(stderr, "libfoo does not have an libfoo_init function\n");
    goto cleanup;
  }

  if(!libfoo_init(&libfoo_atexit_handler_was_called)) {
    fprintf(stderr, "libfoo_init() failed.\n");
    goto cleanup;
  }

  fprintf(stderr, "libfoo was initialized.\n");

  FreeLibrary(dll);
  fprintf(stderr, "libfoo was unloaded.\n");

cleanup:
  if(libfoo_atexit_handler_was_called)
    fprintf(stderr, "PASS: libfoo's atexit handler ran before main end.\n");
  else
    fprintf(stderr, "\a\aFAIL: libfoo's atexit handler hasn't run yet.\n");

  fprintf(stderr, "<<< main\n");
  return !libfoo_atexit_handler_was_called;
}

Test-DLL-atexit-handler