https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81751
Bug ID: 81751
Summary: __basic_file::sync() may flush _all_ files
Product: gcc
Version: 7.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: bugzilla.volker at kabelmail dot de
Target Milestone: ---
When creating a stdio_filebuf from a FILE* the constructor calls
__basic_file::sys_open() which in turn calls __basic_file::sync(). Sync()
unconditionally makes a call to fflush() even if the _M_cfile member is NULL
which then causes _all_ open streams to be sync'd.
This behaviour makes it impossible to use stdio_filebuf in a multithreaded
environment as there may be a buffer that is flushed multiple times.
My proposed solution to file libstdc++-v3/config/io/basic_file_stdio.cc is to
call fflush() in sync() only if _M_cfile is not NULL:
int
__basic_file<char>::sync()
{ return _M_cfile ? fflush(_M_cfile) : 0; }
This change makes sure only the controlled sequence is synchronized.
Alternatively the call to sync in function sys_open (in the same file) may be
made conditional on _M_cfile.
This issue is present in versions back to at least 4.7.2.
