[PATCH 1/1] libstdc++: Fix SIGSEGV in std::print with a setvbuf'd FILE

_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.

Return an empty span when the write pointer is null.

libstdc++-v3/ChangeLog:

	* include/bits/print.h (_File_sink::_File::_M_write_buf): Return
	an empty span when _IO_write_ptr is null.
	* testsuite/27_io/print/1.cc: Add test_print_setvbuf.

Signed-off-by: Anlai Lu <[email protected]>
---
 libstdc++-v3/include/bits/print.h       |  5 +++++
 libstdc++-v3/testsuite/27_io/print/1.cc | 17 +++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/libstdc++-v3/include/bits/print.h b/libstdc++-v3/include/bits/print.h
index 20724844726..c208d9a979a 100644
--- a/libstdc++-v3/include/bits/print.h
+++ b/libstdc++-v3/include/bits/print.h
@@ -95,9 +95,14 @@ namespace __format
       _File(_File&&) = delete;
 
       // A span viewing the unused portion of the stream's output buffer.
+      // Empty when the buffer is not yet initialized: after setvbuf
+      // glibc pre-allocates the buffer but keeps _IO_write_ptr as
+      // nullptr until the first write.
       std::span<char>
       _M_write_buf() noexcept
       {
+	if (_M_file->_IO_write_ptr == nullptr) [[unlikely]]
+	  return {};
 	return {_M_file->_IO_write_ptr,
 		size_t(_M_file->_IO_buf_end - _M_file->_IO_write_ptr)};
       }
diff --git a/libstdc++-v3/testsuite/27_io/print/1.cc b/libstdc++-v3/testsuite/27_io/print/1.cc
index 4fd7f5dc925..bf79d3575e7 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 );
+  std::setvbuf(strm, nullptr, _IOFBF, 4096);
+  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();
-- 
2.34.1