1 files changed, 40 insertions, 1 deletions

diff --git a/src/regress/lib/libc/stdio/test_fflush.c b/src/regress/lib/libc/stdio/test_fflush.c
index 76f2d88aba..a0586b7d14 100644
--- a/src/regress/lib/libc/stdio/test_fflush.c
+++ b/src/regress/lib/libc/stdio/test_fflush.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: test_fflush.c,v 1.2 2025/05/25 05:35:13 yasuoka Exp $ */
+/*      $OpenBSD: test_fflush.c,v 1.3 2025/06/08 08:53:53 yasuoka Exp $ */
 
 /*
  * Copyright (c) 2025 YASUOKA Masahiko <yasuoka@yasuoka.net>
@@ -38,6 +38,7 @@ void test_fflush_read3(void);
 void test_fflush_read4(void);
 void setupw(void);
 void test_fflush_read5(void);
+void test_fflush_read6(void);
 
 void
 setup(void)
@@ -290,6 +291,43 @@ test_fflush_read5(void)
         r = fclose(fp);
         assert(r == 0);
 }
+
+void
+test_fflush_read6(void)
+{
+        int      r, c;
+        FILE    *fp;
+
+        setup();
+        fp = fopen(TMPFILENAME, "r");
+        assert(fp != NULL);
+
+        /*
+         * https://pubs.opengroup.org/onlinepubs/9699919799/functions/fflush.html
+         * .. any characters pushed back onto the stream by ungetc() or ungetwc()
+         * that have not subsequently been read from the stream shall be discarded
+         * (without further changing the file offset).
+         */
+
+        assert(fgetc(fp) == 'H');
+        c = getc(fp);
+        ungetc(c, fp);  /* push back the character has been read */
+        r = fflush(fp);
+        assert(r == 0);
+        assert(getc(fp) == c);
+
+        fseek(fp, 0, SEEK_SET);
+        assert(fgetc(fp) == 'H');
+        c = getc(fp);
+        ungetc('X', fp); /* push back the character has not been read */
+        r = fflush(fp);
+        assert(r == 0);
+        assert(getc(fp) == 'l');
+
+        r = fclose(fp);
+        assert(r == 0);
+}
+
 int
 main(int argc, char *argv[])
 {
@@ -301,6 +339,7 @@ main(int argc, char *argv[])
         test_fflush_read3();
         test_fflush_read4();
         test_fflush_read5();
+        test_fflush_read6();
 
         exit(0);
 }