From 15a23abd7e65aaa11e639fcb681464f1ded5a453 Mon Sep 17 00:00:00 2001
From: yhirose <yuji.hirose.bug@gmail.com>
Date: Sat, 1 Aug 2026 22:21:26 -0400
Subject: [PATCH] Clarify the MAP_FAILED guard comment and assert the cleared
 size

Explain why `addr_` is reset before `close()`: `munmap()` must not be
called with the sentinel. The test also checks `size()`, since the hazard
is a stale size paired with a sentinel `data()`.
---
 httplib.h    | 5 ++---
 test/test.cc | 8 ++++----
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/httplib.h b/httplib.h
index e699b12..e4044c5 100644
--- a/httplib.h
+++ b/httplib.h
@@ -5577,10 +5577,9 @@ inline bool mmap::open(const char *path) {
     return false;
   }
 
-  // A failed mapping must not be left in `addr_`: `is_open()` only compares it
-  // against nullptr, so the MAP_FAILED sentinel would pass and `data()` would
-  // hand the caller (const char *)-1.
   if (addr_ == MAP_FAILED) {
+    // Clear the sentinel before `close()`, since `is_open()` only checks
+    // `addr_` against nullptr and `munmap()` must not be called with it.
     addr_ = nullptr;
     close();
     return false;
diff --git a/test/test.cc b/test/test.cc
index adc4ddb..588a1b6 100644
--- a/test/test.cc
+++ b/test/test.cc
@@ -9347,10 +9347,9 @@ TEST(MmapTest, OpenWhileFileHeldForWriting) {
 #endif
 
 #ifndef _WIN32
-// A failed ::mmap must not be reported as an open mapping. is_open() only
-// compares addr_ against nullptr, so the MAP_FAILED sentinel used to pass it
-// and data() handed the caller (const char *)-1. A directory opens and stats
-// fine but has no mapping, so ::mmap fails for it.
+// A failed ::mmap() must not be reported as an open mapping, otherwise data()
+// hands the caller the MAP_FAILED sentinel. A directory is the easiest way to
+// reach it, since ::open() and fstat() succeed for one but ::mmap() doesn't.
 TEST(MmapTest, FailedMappingIsNotOpen) {
   const char *path = "./mmap_failed_mapping_test_dir";
   ASSERT_EQ(0, ::mkdir(path, 0755));
@@ -9358,6 +9357,7 @@ TEST(MmapTest, FailedMappingIsNotOpen) {
 
   detail::mmap m(path);
   EXPECT_FALSE(m.is_open());
+  EXPECT_EQ(0U, m.size());
   EXPECT_NE(static_cast<const void *>(m.data()),
             static_cast<const void *>(MAP_FAILED));
 }