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

[Jonathan Wakely]

On Thu, 20 Aug 2026 at 15:21, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Wed, Aug 19, 2026 at 4:40 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> On Wed, 19 Aug 2026 at 15:24, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>> >
>> >
>> >
>> > 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?
>>
>> 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.

I'd still just push to gcc-16 now. If it causes a new FAIL, we can fix
it on both branches.

setvbuf is in C89 and the testcase has dg-require-fileio, which
requires POSIX open, lseek and read. I would expect any system
providing those POSIX APIs to also provide C89 stdio.

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