From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 26 Nov 2025 14:44:03 +0000 (+0000)
Subject: libstdc++: Fix std::counting_semaphore<> default max value
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=89552346e3cef8ef32e2ebe643bb96d085447e68;p=thirdparty%2Fgcc.git

libstdc++: Fix std::counting_semaphore<> default max value

My recent (uncommitted) changes to support a 64-bit __platform_wait_t
for FreeBSD and Darwin revealed a problem in std::counting_semaphore.
When the default template argument is used and __platform_wait_t is a
64-bit type, the numeric_limits<__platform_wait_t>::max() value doesn't
fit in ptrdiff_t and so we get ptrdiff_t(-1), which fails a
static_assert in the class body.

The solution is to cap the value to PTRDIFF_MAX instead of allowing it
to go negative.

libstdc++-v3/ChangeLog:

	* include/bits/semaphore_base.h (__platform_semaphore::_S_max):
	Limit to PTRDIFF_MAX to avoid negative values.
	* testsuite/30_threads/semaphore/least_max_value.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
---

diff --git a/libstdc++-v3/include/bits/semaphore_base.h b/libstdc++-v3/include/bits/semaphore_base.h
index 82871ce3518..cb815d340ac 100644
--- a/libstdc++-v3/include/bits/semaphore_base.h
+++ b/libstdc++-v3/include/bits/semaphore_base.h
@@ -173,8 +173,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   {
     using __count_type = __detail::__platform_wait_t;
 
-    static constexpr ptrdiff_t _S_max
-      = _Binary ? 1 : __gnu_cxx::__int_traits<__count_type>::__max;
+    static consteval ptrdiff_t
+    _S_calc_max()
+    {
+      if (_Binary)
+	return 1;
+      else if ((ptrdiff_t)__gnu_cxx::__int_traits<__count_type>::__max < 0)
+	return __gnu_cxx::__int_traits<ptrdiff_t>::__max;
+      else
+	return __gnu_cxx::__int_traits<__count_type>::__max;
+    }
+
+    static constexpr ptrdiff_t _S_max = _S_calc_max();
 
     constexpr explicit
     __platform_semaphore_impl(__count_type __count) noexcept
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value.cc b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value.cc
new file mode 100644
index 00000000000..67fa1258b7f
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/least_max_value.cc
@@ -0,0 +1,9 @@
+// { dg-do compile { target c++20 } }
+// { dg-require-effective-target gthreads { target { ! *-*-linux* } } }
+// { dg-require-effective-target hosted }
+
+#include <semaphore>
+
+std::counting_semaphore<> sem(0);
+std::counting_semaphore<> sem2(2);
+std::counting_semaphore sem3(3);