[Bug c/61739] New: __builtin_printf optimization conflicts with POSIX semantics

[Bug c/61739] New: __builtin_printf optimization conflicts with POSIX semantics

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61739

            Bug ID: 61739
           Summary: __builtin_printf optimization conflicts with POSIX
                    semantics
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugdal at aerifal dot cx

Created attachment 33086
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33086&action=edit
test case

Per POSIX,

"All functions that reference (FILE *) objects, except those with names ending
in _unlocked, shall behave as if they use flockfile() and funlockfile()
internally to obtain ownership of these (FILE *) objects."

Source:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/flockfile.html

GCC's transformation that eliminates printf("") causes this requirement to be
ignored; the calling thread is allowed to proceed even if a lock on stdout held
by another thread should block printf.

The included test-case demonstrates the issue. It should deadlock, and indeed
it does with -fno-builtin. But with the default settings, the program
terminates normally.

Additional thoughts on the above POSIX requirement are on a blog post I wrote a
while back, http://ewontfix.com/8/

[Bug c/61739] __builtin_printf optimization conflicts with POSIX semantics

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61739

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
Since printf("") does not need to obtain ownership of stdout there is no
conflict here.  There is no requirement that it *must* obtain ownership even if
not needed.

[Bug c/61739] __builtin_printf optimization conflicts with POSIX semantics

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61739

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Agreed.

[Bug c/61739] __builtin_printf optimization conflicts with POSIX semantics

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61739

Rich Felker <bugdal at aerifal dot cx> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #3 from Rich Felker <bugdal at aerifal dot cx> ---
Yes it does need to obtain ownership of the file: to set the orientation to
byte mode. This is required even by ISO C (without POSIX). A simpler test case:

printf("");
printf("%d\n", fwide(stdout));

[Bug c/61739] __builtin_printf optimization conflicts with POSIX semantics

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61739

--- Comment #4 from Rich Felker <bugdal at aerifal dot cx> ---
Sorry, that should be:

printf("");
printf("%d\n", fwide(stdout, 0));

[Bug c/61739] __builtin_printf optimization conflicts with POSIX semantics

[bugdal at aerifal dot cx]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61739

--- Comment #5 from Rich Felker <bugdal at aerifal dot cx> ---
I reopened this, but now that I'm reporting a new symptom that's independent of
the original symptom, I'm wondering whether I should file a separate bug report
for it, or continue the issue under this original report. Which way is best?
Both have the same fix (removing the invalid optimization or replacing it with
a valid one), but the user-visible erroneous behaviors are distinct.