gcc produces warning for qsort - c

Question

We have some code in spatialite that looks like:
static int cmp_pt_coords (const void *p1, const void *p2)
{
....
}
static gaiaGeomCollPtr auxPolygNodes (gaiaGeomCollPtr geom)
{
....
/* sorting points by coords */
qsort (sorted, count, sizeof (gaiaPointPtr), cmp_pt_coords);
....
}
This is obviously simplified - the real code can be seen at
https://www.gaia-gis.it/fossil/libspatialite/artifact/fe1d6e12c2f98dff23f9df9372afc23f745b50df
The error that I'm getting from gcc (gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-9ubuntu3)) is
/bin/bash ../../libtool --tag=CC --mode=compile gcc -DHAVE_CONFIG_H -I. -I../.. -g -Wall -Werror -fprofile-arcs -ftest-coverage -g -I../../src/headers -fvisibility=hidden -g -Wall -Werror -fprofile-arcs -ftest-coverage -g -MT libsplite_la-spatialite.lo -MD -MP -MF .deps/libsplite_la-spatialite.Tpo -c -o libsplite_la-spatialite.lo `test -f 'spatialite.c' || echo './'`spatialite.c
libtool: compile: gcc -DHAVE_CONFIG_H -I. -I../.. -g -Wall -Werror -fprofile-arcs -ftest-coverage -g -I../../src/headers -fvisibility=hidden -g -Wall -Werror -fprofile-arcs -ftest-coverage -g -MT libsplite_la-spatialite.lo -MD -MP -MF .deps/libsplite_la-spatialite.Tpo -c spatialite.c -fPIC -DPIC -o .libs/libsplite_la-spatialite.o
spatialite.c: In function 'auxPolygNodes':
spatialite.c:17843:5: error: passing argument 4 of 'qsort' from incompatible pointer type [-Werror]
/usr/include/stdlib.h:761:13: note: expected '__compar_fn_t' but argument is of type 'int (*)(void *, void *)'
cc1: all warnings being treated as errors
I've looked at some previous postings:
What are the parameters in this C qsort function call?
Warning when using qsort in C
However they don't really seem the same (or at least, the way I read the suggestions in those postings is what I think we're already doing here).
I can cast around this, using:
qsort (sorted, count, sizeof (gaiaPointPtr), (__compar_fn_t)cmp_pt_coords);
However I don't see why that should be necessary, and I'm worried about portability to other systems. It seems like the compiler is omitting the const-s from the arguments.

Answer 1

That cast is perfectly fine. GCC isn't smart enough to know that __compar_fn_t is
int (*)(const void *, const void *)
so it issues a warning.
However, __compar_fn_t is not portable -- so if you don't want to use it for casting, you should probably make GCC not warn about this using an appropriate compiler flag.
Or you can see whether __compar_fn_t is defined, and if not, define it yourself:
#ifndef __GNUC__
typedef int (*__compar_fn_t)(const void *, const void *);
#endif

Answer 2

The error probably comes from the visibility flag that you pass to the compiler. Your are saying that all functions in that compilation unit should be hidden. For gcc this changes the function API so your comparison function is then incompatible with the one expected by qsort.
You might want to deal with
#pragma visibility
or
__attribute__((__visibility(default)))
or similar for your comparison function.

Answer 3

The reason for the warning/error is that the GCC prototype of __compar_fn_t is:
typedef int (*__compar_fn_t)(__const void *, __const void );
and not:
typedef int (__compar_fn_t)(const void *, const void *);
Therefore, in order to solve the problem, simply define your function as:
static int cmp_pt_coords (__const void *p1, __const void *p2)