[gcc r16-6554] libstdc++: Override detection of flockfile support in newlib [PR123406]

[Jonathan Wakely]

https://gcc.gnu.org/g:412eb96b1ca384f4bebe996dba341decbc42362c

commit r16-6554-g412eb96b1ca384f4bebe996dba341decbc42362c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Jan 6 14:00:09 2026 +0000

    libstdc++: Override detection of flockfile support in newlib [PR123406]
    
    As explained in the PR, flockfile and funlockfile are always declared by
    newlib and there's no easy way to detect whether they're actually
    defined. Ensure that ac_stdio_locking=no gets set for non-cygwin newlib
    targets.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/123406
            * acinclude.m4 (GLIBCXX_CHECK_STDIO_LOCKING): Override detection
            of flockfile for non-cygwin newlib targets.
            * configure: Regenerate.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/acinclude.m4 | 9 ++++++++-
 libstdc++-v3/configure    | 7 +++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 103515c000f4..e6723edf880e 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -5822,13 +5822,20 @@ AC_LANG_SAVE
   AC_MSG_CHECKING([whether flockfile and putc_unlocked are defined in <stdio.h>])
   AC_TRY_COMPILE([
   #include <stdio.h>
+  #if __has_include(<newlib.h>)
+  # ifdef __CYGWIN__
+    // Cygwin has working flockfile
+  # else
+  #  error No usable flockfile on most newlib targets
+  # endif
+  #endif
   ],[
     FILE* f = ::fopen("", "");
     ::flockfile(f);
     ::putc_unlocked(' ', f);
     ::funlockfile(f);
     ::fclose(f);
-  ], [ac_stdio_locking=yes], [ac_stdio_locking=no])
+  ],[ac_stdio_locking=yes],[ac_stdio_locking=no])
   AC_MSG_RESULT($ac_stdio_locking)
 
   if test "$ac_stdio_locking" = yes; then
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index 0469cbe0f59c..3d825a69fa2e 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -55051,6 +55051,13 @@ $as_echo_n "checking whether flockfile and putc_unlocked are defined in <stdio.h
 /* end confdefs.h.  */
 
   #include <stdio.h>
+  #if __has_include(<newlib.h>)
+  # ifdef __CYGWIN__
+    // Cygwin has working flockfile
+  # else
+  #  error No usable flockfile on most newlib targets
+  # endif
+  #endif
 
 int
 main ()