[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #6 from Peter Eisentraut peter_e at gmx dot net ---
This particular case is in system headers, but there are other cases that are
not, so this isn't going to help in general.

I think this is a legitimate way to write a function-like macro that has a
side-effect and a return value, and the warning is bogus.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #4 from Peter Eisentraut peter_e at gmx dot net ---
Not really, because this code is in header files not under my control, and
those header files should presumably work with a variety of C compilers and
shouldn't need to rely on GCC extensions.

[Bug c/61081] New: excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

Bug ID: 61081
   Summary: excessive warnings: right-hand operand of comma
expression has no effect
   Product: gcc
   Version: 4.9.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: peter_e at gmx dot net

OS X has the following system header definitions:

#define sigaddset(set, signo)   (*(set) |= __sigbits(signo), 0)
#define sigdelset(set, signo)   (*(set) = ~__sigbits(signo), 0)
#define sigismember(set, signo) ((*(set)  __sigbits(signo)) != 0)
#define sigemptyset(set)(*(set) = 0, 0)
#define sigfillset(set) (*(set) = ~(sigset_t)0, 0)

gcc 4.9 with -Wall with complain about any use thereof with the newly
introduced

right-hand operand of comma expression has no effect

I think the above programming patterns is common and legitimate in macros
(search engines bring up similar cases) and without an elegant alternative. 
Warning about the right-hand expression having no effect is incorrect because
it supplies the return value of the expression (unlike the left-hand side).

I suggest that this warning be removed, or at least moved away from -Wall.

[Bug c/61081] excessive warnings: right-hand operand of comma expression has no effect

[peter_e at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61081

--- Comment #2 from Peter Eisentraut peter_e at gmx dot net ---
No, these functions need to have a usable return value, because someone could
write

if (!sigemptyset(...))
weirderror();

[Bug c/48778] New: gcc 4.6 -Waddress adds unhelpful new warning case

[peter_e at gmx dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48778

   Summary: gcc 4.6 -Waddress adds unhelpful new warning case
   Product: gcc
   Version: 4.6.0
Status: UNCONFIRMED
  Severity: minor
  Priority: P3
 Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: pete...@gmx.net


Using built-in specs.
COLLECT_GCC=/usr/bin/gcc-4.6
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.6.1/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.6.0-2'
--with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-plugin --enable-gold --enable-ld=default --with-plugin-ld
--enable-objc-gc --enable-targets=all --with-arch-32=i586 --with-tune=generic
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu
--target=i486-linux-gnu
Thread model: posix
gcc version 4.6.1 20110329 (prerelease) (Debian 4.6.0-2) 


Consider this test file:

#include stdlib.h

typedef struct buf {
size_t len;
char *stuff;
} buf;

#define buflen(b) ((b) == NULL ? 0 : (b)-len)

int
func()
{
buf b;

b.len = 5;
b.stuff = malloc(5);

if (buflen(b)  0)
return 1;
else
return 0;
}

This compiles without warnings with -Wall on gcc =4.5.  But

gcc-4.6 -c -Waddress test.c
test.c: In function ‘func’:
test.c:18:12: warning: the comparison will always evaluate as ‘false’ for the
address of ‘b’ will never be NULL [-Waddress]

I don't think this is really helpful.  Macros like in the above example are not
uncommon, and there is no obvious way to write the code better.