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

On Wed, Aug 19, 2026 at 4:40 PM Jonathan Wakely <[email protected]> wrote:

> On Wed, 19 Aug 2026 at 15:24, Tomasz Kaminski <[email protected]> wrote:
> >
> >
> >
> > On Wed, Aug 19, 2026 at 4:17 PM Jonathan Wakely <[email protected]>
> wrote:
> >>
> >> On Mon, 17 Aug 2026 at 09:58 +0200, Tomasz Kamiński wrote:
> >> >From: Anlai Lu <[email protected]>
> >> >
> >> >_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 <[email protected]>
> >> >Signed-off-by: Anlai Lu <[email protected]>
> >> >Signed-off-by: Tomasz Kamiński <[email protected]>
> >> >---
> >> >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?
>
> Yes, OK for gcc-16 too.
> No need to wait, I think this is obviously more correct and safer than
> the current code.
>
I was thinking about test, i.e. that there is some architecture on which
setvbuf does not work, and I will need to add some dg-requires to it.

>
> >>
> >>
> >>
> >> >+          }
> >> >+      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
> >> >
> >> >
> >>
>
>