Tests: Test whether stdio streams are flushed correctly on exit

Tests/LibC/CMakeLists.txt

@@ -18,6 +18,7 @@ set(TEST_SOURCES
     TestSearch.cpp
     TestSnprintf.cpp
     TestStackSmash.cpp
+    TestStdio.cpp
     TestStrlcpy.cpp
     TestStrtodAccuracy.cpp
     TestWchar.cpp

Tests/LibC/TestStdio.cpp

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2022, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <stdio.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+// Test whether file writes are flushed to disk at program termination,
+// even if we do not close the files or call fflush().
+TEST_CASE(flush_on_exit)
+{
+    static constexpr auto test_str = "peekaboo";
+    auto pid = fork();
+    VERIFY(pid >= 0);
+
+    if (pid == 0) {
+        auto* fp = fopen("/tmp/flushtest", "w");
+        VERIFY(fp != nullptr);
+        auto nwritten = fwrite(test_str, 1, strlen(test_str), fp);
+        VERIFY(nwritten != 0);
+        // We intentionally leak `fp` here.
+        exit(0);
+    } else {
+        int wstatus;
+        auto rc = waitpid(pid, &wstatus, 0);
+        VERIFY(rc >= 0);
+
+        char buf[256];
+        auto* fp = fopen("/tmp/flushtest", "r");
+        VERIFY(fp != nullptr);
+        auto* sp = fgets(buf, sizeof(buf), fp);
+        VERIFY(sp != nullptr);
+        fclose(fp);
+        unlink("/tmp/flushtest");
+
+        EXPECT_EQ(strcmp(test_str, buf), 0);
+    }
+}