diff --git a/src/stdio/fmemopen.c b/src/stdio/fmemopen.c
index 91d52bc1..d7849609 100644
--- a/src/stdio/fmemopen.c
+++ b/src/stdio/fmemopen.c
@@ -108,12 +108,13 @@ FILE *fmemopen(void *restrict buf, size_t size, const char *restrict mode)
 	f->seek = mseek;
 	f->close = mclose;
 
-	if (!libc.threaded) {
-		f->lock = -1;
-		f->next = libc.ofl_head;
-		if (libc.ofl_head) libc.ofl_head->prev = f;
-		libc.ofl_head = f;
-	}
+	if (!libc.threaded) f->lock = -1;
+
+	OFLLOCK();
+	f->next = libc.ofl_head;
+	if (libc.ofl_head) libc.ofl_head->prev = f;
+	libc.ofl_head = f;
+	OFLUNLOCK();
 
 	return f;
 }
diff --git a/src/stdio/open_memstream.c b/src/stdio/open_memstream.c
index c7330abe..9eafdfba 100644
--- a/src/stdio/open_memstream.c
+++ b/src/stdio/open_memstream.c
@@ -77,12 +77,13 @@ FILE *open_memstream(char **bufp, size_t *sizep)
 	f->seek = ms_seek;
 	f->close = ms_close;
 
-	if (!libc.threaded) {
-		f->lock = -1;
-		f->next = libc.ofl_head;
-		if (libc.ofl_head) libc.ofl_head->prev = f;
-		libc.ofl_head = f;
-	}
+	if (!libc.threaded) f->lock = -1;
+
+	OFLLOCK();
+	f->next = libc.ofl_head;
+	if (libc.ofl_head) libc.ofl_head->prev = f;
+	libc.ofl_head = f;
+	OFLUNLOCK();
 
 	return f;
 }
diff --git a/src/stdio/open_wmemstream.c b/src/stdio/open_wmemstream.c
index 2fe504c7..35370309 100644
--- a/src/stdio/open_wmemstream.c
+++ b/src/stdio/open_wmemstream.c
@@ -79,12 +79,13 @@ FILE *open_wmemstream(wchar_t **bufp, size_t *sizep)
 	f->seek = wms_seek;
 	f->close = wms_close;
 
-	if (!libc.threaded) {
-		f->lock = -1;
-		f->next = libc.ofl_head;
-		if (libc.ofl_head) libc.ofl_head->prev = f;
-		libc.ofl_head = f;
-	}
+	if (!libc.threaded) f->lock = -1;
+
+	OFLLOCK();
+	f->next = libc.ofl_head;
+	if (libc.ofl_head) libc.ofl_head->prev = f;
+	libc.ofl_head = f;
+	OFLUNLOCK();
 
 	return f;
 }