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