3 files changed, 85 insertions, 1 deletions

diff --git a/src/regress/lib/libc/stdio_threading/Makefile b/src/regress/lib/libc/stdio_threading/Makefile
index e42481afc2..5823b6e284 100644
--- a/src/regress/lib/libc/stdio_threading/Makefile
+++ b/src/regress/lib/libc/stdio_threading/Makefile
@@ -1,3 +1,3 @@
-SUBDIR += fopen fread fwrite fgetln fgets fputs
+SUBDIR += fopen fread fwrite fgetln fgets fputs flockfile
 
 .include <bsd.subdir.mk>
diff --git a/src/regress/lib/libc/stdio_threading/flockfile/Makefile b/src/regress/lib/libc/stdio_threading/flockfile/Makefile
new file mode 100644
index 0000000000..171ccc7ac7
--- /dev/null
+++ b/src/regress/lib/libc/stdio_threading/flockfile/Makefile
@@ -0,0 +1,6 @@
+TOPDIR=${.CURDIR}
+PROG=flockfile_test
+CFLAGS+=-I${TOPDIR}/../include/
+LDFLAGS+=-lpthread
+
+.include <bsd.regress.mk>
diff --git a/src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c b/src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c
new file mode 100755
index 0000000000..034a753cd9
--- /dev/null
+++ b/src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c
@@ -0,0 +1,78 @@
+/*      $OpenBSD: flockfile_test.c,v 1.1 2026/01/19 23:01:00 guenther Exp $     */
+/*
+ * Copyright (c) 2026 Philip Guenther <guenther@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stdio.h>
+#include <pthread.h>
+#include "local.h"
+
+pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
+pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
+int did_test1 = 0;
+
+static void *
+test1(void *v)
+{
+        int r;
+
+        if (ftrylockfile(stdin) == 0)
+                errx(1, "Lock on stdin never held");
+
+        if ((r = pthread_mutex_lock(&lock)))
+                errc(1, r, "could not lock lock");
+        did_test1 = 1;
+        if ((r = pthread_cond_signal(&cond)))
+                errc(1, r, "could not signal cond");
+        if ((r = pthread_mutex_unlock(&lock)))
+                errc(1, r, "could not unlock lock");
+
+        flockfile(stdin);
+        funlockfile(stdin);
+        return NULL;
+}
+
+int
+main(void)
+{
+        int fd, r;
+        pthread_t thr;
+
+        /*
+         * Make sure flockfile() is handled correctly before the first
+         * thread is created.
+         */
+        flockfile(stdin);
+        flockfile(stdin);
+
+        if ((r = pthread_create(&thr, NULL, test1, NULL)))
+                warnc(r, "could not create thread");
+
+        if ((r = pthread_mutex_lock(&lock)))
+                errc(1, r, "could not lock lock");
+        while (did_test1 == 0)
+                if ((r = pthread_cond_wait(&cond, &lock)))
+                        errc(1, r, "could not wait on cond");
+        if ((r = pthread_mutex_unlock(&lock)))
+                errc(1, r, "could not unlock lock");
+
+        funlockfile(stdin);
+        funlockfile(stdin);
+
+        if ((r = pthread_join(thr, NULL)))
+                warnc(r, "could not join thread");
+
+        exit(0);
+}