The #include statements were not exactly right here:
2021-01-04 Bruno Haible <br...@clisp.org>
ssfmalloc: Fix includes.
* lib/ssfmalloc.h: Include <strings.h>, for ffs().
* tests/test-ssfmalloc.c: Include <string.h>, for memset().
diff --git a/lib/ssfmalloc.h b/lib/ssfmalloc.h
index d5e0a1e..8fdb2b1 100644
--- a/lib/ssfmalloc.h
+++ b/lib/ssfmalloc.h
@@ -121,7 +121,8 @@ static void free_block (uintptr_t block);
*/
#include <stdlib.h>
-#include <string.h>
+#include <string.h> /* ffsll */
+#include <strings.h> /* ffs */
#include "flexmember.h"
#include "glthread/lock.h"
#include "thread-optim.h"
diff --git a/tests/test-ssfmalloc.c b/tests/test-ssfmalloc.c
index 640071d..6571ced 100644
--- a/tests/test-ssfmalloc.c
+++ b/tests/test-ssfmalloc.c
@@ -142,6 +142,7 @@ free_pages (uintptr_t pages, size_t size)
/* ================================= Tests ================================= */
#include <limits.h>
+#include <string.h>
#include "macros.h"
