Bug#758911: libc6-dev: spurious sign-conversion warning for setrlimit, clang 3.5, _GNU_SOURCE

[Zack Weinberg <zackw@panix.com>]

Package: libc6-dev
Version: 2.19-9
Severity: minor
Tags: upstream

glibc changes the signature of setrlimit() when _GNU_SOURCE is defined, "to
provide better error checking":

extern int setrlimit (int, const struct rlimit *); // as specified by POSIX
extern int setrlimit (__rlimit_resource_t, const struct rlimit *); //
_GNU_SOURCE

where __rlimit_resource_t is an enum with no negative values.  This causes some
compilers (e.g. clang-3.5) to give spurious warnings for programs that pass
non-constant values to setrlimit(), e.g.

$ cat test.c
#define _GNU_SOURCE
#include <sys/resource.h>
int foo(int rsrc, struct rlimit *rl)
{
  return setrlimit(rsrc, rl);
}
$ clang-3.5 -Wsign-conversion -c test.c
test.c:5:20: warning: implicit conversion changes signedness: 'int' to
'__rlimit_resource_t' (aka 'enum __rlimit_resource')
      [-Wsign-conversion]
  return setrlimit(rsrc, rl);
         ~~~~~~~~~ ^~~~

I don't think it's reasonable for me to have to work around this in my code,
and indeed I don't see that there _is_ any workaround available to me in C.
(Maybe in C++ some template black magic to extract "the type of the first
argument to setrlimit"? Yech.)

I'm labeling this a libc bug rather than a clang bug because, if libc is going
to deviate from the standard "to provide better error checking", it needs to
ensure that standard-compliant code still works with no complaints, even at
quite aggressive warning levels.



-- System Information:
Debian Release: jessie/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.14-2-amd64 (SMP w/8 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages libc6-dev depends on:
ii  libc-dev-bin    2.19-9
ii  libc6           2.19-9
ii  linux-libc-dev  3.14.15-2

libc6-dev recommends no packages.

Versions of packages libc6-dev suggests:
pn  glibc-doc     <none>
ii  manpages-dev  3.71-1

-- no debconf information