[Bug c/45106] New: casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[Bug c/45106] New: casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[robert dot haschke at uni-bielefeld dot de]

Casting a variadic function pointer to normal function pointer (with adequate
number and type of arguments) causes a segfault, when the function is called.
Vice versa (casting normal function pointer to variadic function) works fine.
The problem only occurs for 64bit builds. Casting works nice in all directions
for 32bit builds.


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           Summary: casting of variadic function pointers to normal function
                    pointers causes segfaults on 64bit systems
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: robert dot haschke at uni-bielefeld dot de
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106

[Bug c/45106] casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[robert dot haschke at uni-bielefeld dot de]

------- Comment #1 from robert dot haschke at uni-bielefeld dot de  2010-07-28 11:52 -------
Created an attachment (id=21335)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21335&action=view)
preprocessed file


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106

[Bug c/45106] casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[robert dot haschke at uni-bielefeld dot de]

------- Comment #2 from robert dot haschke at uni-bielefeld dot de  2010-07-28 11:53 -------
Created an attachment (id=21336)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21336&action=view)
original .c file


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106

[Bug c/45106] casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2010-07-28 11:57 -------
Don't do it, it is not valid.
The x86-64 ABI requires that for varargs the %al register contains number of
SSE regs that need saving, while for non-varargs functions that register is
undefined.


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jakub at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106

[Bug c/45106] casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[rhaschke at techfak dot uni-bielefeld dot de]

------- Comment #4 from rhaschke at techfak dot uni-bielefeld dot de  2010-07-28 12:04 -------
Subject: Re:  casting of variadic function pointers to normal
 function pointers causes segfaults on 64bit systems

I was afraid of this answer.
However in my application, I have no clue which type of function pointer
is transferred. Thus, I should always cast to the variadic function
pointer in order to define the %al register?

Cheers, Robert


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106

[Bug c/45106] casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[schwab at linux-m68k dot org]

------- Comment #5 from schwab at linux-m68k dot org  2010-07-28 12:26 -------
Calling a function with a different signature than declared is undefined in any
case.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106

[Bug c/45106] casting of variadic function pointers to normal function pointers causes segfaults on 64bit systems

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2010-07-28 14:01 -------
(In reply to comment #4)
> Subject: Re:  casting of variadic function pointers to normal
>  function pointers causes segfaults on 64bit systems
> 
> I was afraid of this answer.
> However in my application, I have no clue which type of function pointer
> is transferred. Thus, I should always cast to the variadic function
> pointer in order to define the %al register?
> 
> Cheers, Robert

That doesn't work either.  You have to pass around some cookie that
tells you the function prototype and cast to the appropriate one.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45106