[Bug stdio/27602] New: fflush(NULL) hangs when getc() ongoing

[Bug stdio/27602] New: fflush(NULL) hangs when getc() ongoing

[samuel.thibault@ens-lyon.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

            Bug ID: 27602
           Summary: fflush(NULL) hangs when getc() ongoing
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: samuel.thibault@ens-lyon.org
  Target Milestone: ---

Created attachment 13320
  --> https://sourceware.org/bugzilla/attachment.cgi?id=13320&action=edit
testcase

Hello,

The attached testcase hangs: a thread is blocked in getc(stdin), and the main
thread tries to call fflush(NULL), but that tries to lock stdin, and that's
already locked by getc, and thus cannot proceed.

Samuel

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[Bug stdio/27602] fflush(NULL) hangs when getc() ongoing

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
And that's a bug why?

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[Bug stdio/27602] fflush(NULL) hangs when getc() ongoing

[samuel.thibault@ens-lyon.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

--- Comment #2 from Samuel Thibault <samuel.thibault@ens-lyon.org> ---
Well, that is very surprising: fflush is supposed to flush the output, not
interfere with input.

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[Bug stdio/27602] fflush(NULL) hangs when getc() ongoing

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

--- Comment #3 from Andreas Schwab <schwab@linux-m68k.org> ---
But fflush(NULL) needs to inspect each and every stream, in a thread safe way.

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[Bug stdio/27602] fflush(NULL) hangs when getc() ongoing

[samuel.thibault@ens-lyon.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

--- Comment #4 from Samuel Thibault <samuel.thibault@ens-lyon.org> ---
Sure, I understand why technically it does hang, but it is still surprising to
see that it would block just because a getc is ongoing. If it was a write on a
pipe which is full, that wouldn't be surprising.

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[Bug stdio/27602] fflush(NULL) hangs when getc() ongoing

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

--- Comment #5 from Andreas Schwab <schwab@linux-m68k.org> ---
The stream is locked, so no other thread can use it.

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[Bug stdio/27602] fflush(NULL) hangs when getc() ongoing

[samuel.thibault@ens-lyon.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=27602

Samuel Thibault <samuel.thibault@ens-lyon.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |NOTABUG
             Status|NEW                         |RESOLVED

--- Comment #6 from Samuel Thibault <samuel.thibault@ens-lyon.org> ---
> The stream is locked

“
All functions that reference ( FILE *) objects shall behave as if they use
flockfile() and funlockfile() internally to obtain ownership of these (FILE *)
objects.
”

Ah, alright then.

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