From 62617a35de28df2a2ddc1a1daa6f55869c781426 Mon Sep 17 00:00:00 2001
From: Carter Li <zhangsongcui3371@sina.com>
Date: Sat, 10 Oct 2026 15:43:20 +0800
Subject: [PATCH] Common (Debug): fixes possible errno lost

---
 src/common/impl/debug.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/common/impl/debug.c b/src/common/impl/debug.c
index 86e76383f..b72c3dea3 100644
--- a/src/common/impl/debug.c
+++ b/src/common/impl/debug.c
@@ -28,16 +28,19 @@ void ffDebugPrint(const char* file, int line, const char* format, ...) {
     }
 
     static FFThreadMutex debugMutex = FF_THREAD_MUTEX_INITIALIZER;
-    ffThreadMutexLock(&debugMutex);
+    // Saved before the lock and restored after the unlock: FF_DEBUG() is called from error paths
+    // that go on to read `errno`, so nothing this function does -- neither the lock nor the
+    // formatting -- may be allowed to leave its own value behind.
     int errno_ = errno;
-    static char errmsg_[1024];
+    ffThreadMutexLock(&debugMutex);
+    static char errmsg_[1024]; // Don't increase stack size
     va_list args;
     va_start(args, format);
     vsnprintf(errmsg_, sizeof(errmsg_), format, args);
     va_end(args);
     fprintf(stderr, "[%s%4d, %s] %s\n", ffFindFileName(file), line, ffTimeToTimeStr(ffTimeGetNow()), errmsg_);
-    errno = errno_;
     ffThreadMutexUnlock(&debugMutex);
+    errno = errno_;
 }
 
     #if _WIN32