On Windows FormatMessageA can return a NULL message - for example when trying to load a 32bit dll from a 64 bit program (garbage.so in the test suite). Since the passed in buffer contains junk data the resulting error message full of garbage characters which makes it impossible to know what the problem is.

This commit updates the message so that it at least returns the error code (in this case 193) which makes it possible to debug the problem.

1 files changed, 10 insertions, 2 deletions

diff --git a/ext/ffi_c/DynamicLibrary.c b/ext/ffi_c/DynamicLibrary.c
index 9bfe86e..6c27ec6 100644
--- a/ext/ffi_c/DynamicLibrary.c
+++ b/ext/ffi_c/DynamicLibrary.c
@@ -225,8 +225,16 @@ dl_open(const char* name, int flags)
 static void
 dl_error(char* buf, int size)
 {
-    FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM, NULL, GetLastError(),
-            0, buf, size, NULL);
+    // Get the last error code
+    DWORD error = GetLastError();
+
+    // Get the associated message
+    LPSTR message = NULL;
+    FormatMessageA(FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_ALLOCATE_BUFFER, 
+                   NULL, error, 0, (LPSTR)&message, 0, NULL);
+
+    // Update the passed in buffer
+    snprintf(buf, size, "Failed with error %d: %s", error, message);
 }
 #endif