[LLVMbugs] [Bug 20742] New: Spurious -Wsign-conversion warning due to excessively clever glibc <sys/resource.h>

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Mon Aug 25 07:23:09 PDT 2014


            Bug ID: 20742
           Summary: Spurious -Wsign-conversion warning due to excessively
                    clever glibc <sys/resource.h>
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zackw at panix.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

This test program ...

    #define _GNU_SOURCE
    #include <sys/resource.h>

    int spurious_sign_conversion_warning(int r, struct rlimit *l)
        return setrlimit(r, l);

... produces a spurious -Wsign-conversion warning when compiled against
recent GNU libc's headers:

    $ clang-3.5 -Wsign-conversion -c setrlimit.c 
    setrlimit.c:6:22: warning: implicit conversion changes signedness: 'int' to
          '__rlimit_resource_t' (aka 'enum __rlimit_resource')
        return setrlimit(r, l);
               ~~~~~~~~~ ^
    1 warning generated.

The problem is this construct in <sys/resource.h>:

    /* The X/Open standard defines that all the functions below must use
       `int' as the type for the first argument.  When we are compiling with
       GNU extensions we change this slightly to provide better error
       checking.  */
    #if defined __USE_GNU && !defined __cplusplus
    typedef enum __rlimit_resource __rlimit_resource_t;
    typedef enum __rusage_who __rusage_who_t;
    typedef enum __priority_which __priority_which_t;
    typedef int __rlimit_resource_t;
    typedef int __rusage_who_t;
    typedef int __priority_which_t;

`enum __rlimit_resource` has no negative values and is therefore (I presume)
assigned an unsigned type, triggering the warning.

I don't think I should have to work around this in my code.  X/Open says the
type of `setrlimit`'s first argument is `int`, therefore `RLIM_*` constants
should be passed around in `int` variables.  Moreover, I don't see that there
*is* any workaround available to me in C that wouldn't harm portability to
other systems.  (A portable program can reasonably want to define `_GNU_SOURCE`
in order to take advantage of specific glibc features when available.)

I initially filed this as a bug with Debian's glibc (
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=758911 ) and was asked to
file an upstream clang bug inviting you to work with upstream glibc to resolve
this in a mutually satisfactory way.  I will also file an upstream glibc bug
report and add the URL here.

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