[PATCH v3] libstdc++: Fix SIGSEGV in std::print with a setvbuf'd FILE

[Tomasz Kaminski]

On Wed, Aug 19, 2026 at 4:17 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Mon, 17 Aug 2026 at 09:58 +0200, Tomasz Kamiński wrote:
> >From: Anlai Lu <agicy@qq.com>
> >
> >_File_sink::_M_write_buf() returns a span with a null data pointer when
> >the FILE buffer was pre-allocated by setvbuf but never written (glibc
> >keeps _IO_write_ptr as nullptr until the first write). The empty() check
> >in the constructor then fails to force buffer initialization and the
> format
> >engine memcpys into nullptr.
>
> Please replace the paragraph above with the first two paragraphs
> from the original [PATCH 0/1] email, and a shorter form of the third
> paragraph:
>
>    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.
>
>
> >Calls __overflow when _IO_write_ptr is null in _File_sink constructor.
>
> Let's say:
>
>    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 separte 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_writebuf.
> >       * testsuite/27_io/print/1.cc: Add test_print_setvbuf.
> >
> >Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>
> >Signed-off-by: Anlai Lu <agicy@qq.com>
> >Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
> >---
> >v3 introduced _M_init_write_buf that I suggested on review.
> >
> >Tested on x86_64-linux. OK for trunk and 16?
> >
> > libstdc++-v3/include/bits/print.h       | 28 ++++++++++++++++---------
> > libstdc++-v3/testsuite/27_io/print/1.cc | 17 +++++++++++++++
> > 2 files changed, 35 insertions(+), 10 deletions(-)
> >
> >diff --git a/libstdc++-v3/include/bits/print.h
> b/libstdc++-v3/include/bits/print.h
> >index 20724844726..d63267c8ecf 100644
> >--- a/libstdc++-v3/include/bits/print.h
> >+++ b/libstdc++-v3/include/bits/print.h
> >@@ -80,20 +80,28 @@ 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 returns span
>
> s/returns span/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.
>
> The two comment lines above are indented inconsistently, the first one
> uses spaces and the second uses a tab.
>
> >+      if (!_M_file->_IO_write_ptr || _M_write_buf().empty())
> >+        if (::__overflow(_M_file, EOF) == EOF)
> >+          {
> >+            const int __err = errno;
> >+            __throw_system_error(__err);
>
> There's no need to store errno before throwing (because we're not
> calling funlockfile here) so it can be just:
>
>      __throw_system_error(errno);
>
> OK with those changes, thanks.
>
OK also to backport after some time to 16?

>
>
> >+          }
> >+      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 +166,8 @@ 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 4fd7f5dc925..641a97c1f9c 100644
> >--- a/libstdc++-v3/testsuite/27_io/print/1.cc
> >+++ b/libstdc++-v3/testsuite/27_io/print/1.cc
> >@@ -67,6 +67,22 @@ 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()
> > {
> >@@ -142,6 +158,7 @@ int main()
> >   test_print_file();
> >   test_println_file();
> >   test_print_raw();
> >+  test_print_setvbuf();
> >   test_vprint_nonunicode();
> > #ifdef __cpp_exceptions
> >   test_errors();
> >--
> >2.55.0
> >
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20260819/d560a4fb/attachment-0001.htm>