[gcc r16-9570] libstdc++: Fix SIGSEGV in std::print with a setvbuf'd FILE

https://gcc.gnu.org/g:a8ce20256d4ffac873c81bae1a339e4eac9472f5

commit r16-9570-ga8ce20256d4ffac873c81bae1a339e4eac9472f5
Author: Anlai Lu <[email protected]>
Date:   Sun Aug 16 04:01:03 2026 +0000

    libstdc++: Fix SIGSEGV in std::print with a setvbuf'd FILE
    
    This is a regression introduced by the P3107R5 implementation in
    r16-4350-g8bd872f1ea7414. Before that formatting to a FILE* wrote to
    _Str_sink and used fwrite, which handles this legal stream state.
    
    This fixes it by calling __overflow from the _File_sink constructor
    when _IO_write_ptr is null.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/print.h (_File_sink::_File::_File)
            [_GLIBCXX_USE_STDIO_LOCKING && _GLIBCXX_USE_GLIBC_STDIO_EXT]:
            Move __overflow call to separate member function.
            (_File_sink::_File::_M_init_write_buf)
            [_GLIBCXX_USE_STDIO_LOCKING && _GLIBCXX_USE_GLIBC_STDIO_EXT]:
            Extracted from constructor, call __overflow for null
            _M_file->_IO_write_ptr.
            (_File_sink::_File_sink)
            [_GLIBCXX_USE_STDIO_LOCKING && _GLIBCXX_USE_GLIBC_STDIO_EXT]:
            Call _M_init_write_buf instead of _M_write_buf.
            * testsuite/27_io/print/1.cc: Add test_print_setvbuf.
    
    Reviewed-by: Jonathan Wakely <[email protected]>
    Co-authored-by: Tomasz Kamiński <[email protected]>
    Signed-off-by: Anlai Lu <[email protected]>
    Signed-off-by: Tomasz Kamiński <[email protected]>
            (cherry picked from commits 4cb7258da9bd637ccce023693ae6a167d179784d)
            (74b3180bb616fbc35d1b655da75d8f6d8b39d368)

Diff:
---
 libstdc++-v3/include/bits/print.h       | 25 +++++++++++++++----------
 libstdc++-v3/testsuite/27_io/print/1.cc | 17 +++++++++++++++++
 2 files changed, 32 insertions(+), 10 deletions(-)

diff --git a/libstdc++-v3/include/bits/print.h b/libstdc++-v3/include/bits/print.h
index 207248447264..722d82146ac8 100644
--- a/libstdc++-v3/include/bits/print.h
+++ b/libstdc++-v3/include/bits/print.h
@@ -80,20 +80,25 @@ namespace __format
 	    ::funlockfile(__f);
 	    __throw_system_error(EACCES);
 	  }
-	// Allocate buffer if needed:
-	if (_M_write_buf().empty())
-	  if (::__overflow(__f, EOF) == EOF)
-	    {
-	      const int __err = errno;
-	      ::funlockfile(__f);
-	      __throw_system_error(__err);
-	    }
       }
 
       ~_File() { ::funlockfile(_M_file); }
 
       _File(_File&&) = delete;
 
+      // Allocate FILE's output buffer if needed, and return a span
+      // viewing unused portion of it.
+      std::span<char>
+      _M_init_write_buf()
+      {
+	// After setvbuf glibc pre-allocates the buffer but _IO_write_ptr
+	// remains null until the first write.
+	if (!_M_file->_IO_write_ptr || _M_write_buf().empty())
+	  if (::__overflow(_M_file, EOF) == EOF)
+	    __throw_system_error(errno);
+	return _M_write_buf();
+      }
+
       // A span viewing the unused portion of the stream's output buffer.
       std::span<char>
       _M_write_buf() noexcept
@@ -158,8 +163,8 @@ namespace __format
     : _M_file(__f), _M_add_newline(__add_newline)
     {
       if (!_M_file._M_unbuffered())
-	// Write directly to the FILE's output buffer.
-	this->_M_reset(_M_file._M_write_buf());
+	// Allocate FILE's output buffer if needed, and write directly to it.
+	this->_M_reset(_M_file._M_init_write_buf());
     }
 
     // This calls I/O functions which are cancellation points, so they
diff --git a/libstdc++-v3/testsuite/27_io/print/1.cc b/libstdc++-v3/testsuite/27_io/print/1.cc
index 58f1eb163dfa..41314adb8203 100644
--- a/libstdc++-v3/testsuite/27_io/print/1.cc
+++ b/libstdc++-v3/testsuite/27_io/print/1.cc
@@ -65,6 +65,22 @@ test_print_raw()
   VERIFY( txt == "\xa3" );
 }
 
+void
+test_print_setvbuf()
+{
+  __gnu_test::scoped_file f;
+  FILE* strm = std::fopen(f.path.string().c_str(), "w");
+  VERIFY( strm );
+  VERIFY( std::setvbuf(strm, nullptr, _IOFBF, 4096) == 0 );
+  std::string str{"Hello, World!"};
+  std::print(strm, "{}", str);
+  std::fclose(strm);
+
+  std::ifstream in(f.path);
+  std::string txt(std::istreambuf_iterator<char>(in), {});
+  VERIFY( txt == "Hello, World!" );
+}
+
 void
 test_vprint_nonunicode()
 {
@@ -140,6 +156,7 @@ int main()
   test_print_file();
   test_println_file();
   test_print_raw();
+  test_print_setvbuf();
   test_vprint_nonunicode();
 #ifdef __cpp_exceptions
   test_errors();