[Swig-devel] [ swig-Bugs-2828711 ] compile warning in base.c/fio.c: ISO C forbids conversion...

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Bugs item #2828711, was opened at 2009-07-29 00:15
Message generated for change (Comment added) made by wsfulton
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Category: installation
Group: None
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Private: No
Submitted By: DCTodd (delicasso)
Assigned to: Nobody/Anonymous (nobody)
Summary: compile warning in base.c/fio.c: ISO C forbids conversion...

Initial Comment:
While compiling SWIG on Ubuntu Linux 9.04
gcc 4.3.3
Swig 1.3.39
Python 2.6.2

It still installed, and I will work with it, but the warnings made me a little nervous.


DOH -I../Source/CParse -I../Source/Preprocessor -I../Source/Swig -I../Source/Modules -g -O2 -Wall -W -ansi -pedantic -MT DOH/base.o -MD -MP -MF $depbase.Tpo -c -o DOH/base.o DOH/base.c &&\
	mv -f $depbase.Tpo $depbase.Po
DOH/base.c: In function ‘DohCall’:
DOH/base.c:938: warning: ISO C forbids conversion of object pointer to function pointer type

DOH -I../Source/CParse -I../Source/Preprocessor -I../Source/Swig -I../Source/Modules -g -O2 -Wall -W -ansi -pedantic -MT DOH/fio.o -MD -MP -MF $depbase.Tpo -c -o DOH/fio.o DOH/fio.c &&\
	mv -f $depbase.Tpo $depbase.Po
DOH/fio.c: In function ‘DohEncoding’:
DOH/fio.c:51: warning: ISO C forbids conversion of function pointer to object pointer type
DOH/fio.c: In function ‘encode’:
DOH/fio.c:75: warning: ISO C forbids conversion of object pointer to function pointer type

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>Comment By: William Fulton (wsfulton)
Date: 2009-08-16 01:07

Message:
As per Olly's comments, In practice this warning is harmless and it will
disappear when DOH is replaced, so for now we just put up with it.

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Comment By: DCTodd (delicasso)
Date: 2009-07-30 22:52

Message:
Thanks Olly.
I appreciate all the detail.
I agree that the second solution would be the best (i.e. arrange to not
pass the "-ansi" flag for these two files), but it sounds like the build
process might now allow that ... . easily.
I will forge ahead with confidence that nothing is wrong!
Thank again, David

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Comment By: Olly Betts (olly)
Date: 2009-07-29 14:48

Message:
ISO C says this is undefined behaviour, but the implementation is allowed
to define behaviour in such cases and POSIX requires this to work since
dlsym() returns a void * which is actually a function pointer.  So it's
ugly, but nothing to worry about (at least on platforms which conform to
POSIX, which is at least all modern Unix platforms).

The reason the warning is given is that we pass "-ansi" to gcc, which
tells it to warn in such cases, so we're getting what we asked for.

So I think either we drop "-ansi", or we arrange not to pass it for this
one file, or just put up with these warnings.  The only particularly useful
thing -ansi does is to make // comments an error in C code, but it's all
too easy to introduce them in a mixed C and C++ codebase.  C99 allows it,
but still not all compilers which matter support that.

The second option would be nicest, but SWIG uses automake in a somewhat
odd way, and my attempt to implement this using the technique suggested in
the automake manual failed.

The longer term plan seems to be to replace DOH with something else which
would also cure this.

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