在c中怎样用qsort对结构体数组进行多级排序？

#include <stdlib.h>
#include <string.h>

struct st_test
{
        int cat;
        char caTrain[ 100 ];
        double plane;
        float  tree;
};

int cmp( const void *arg1, const void *arg2 );

int
main( void )
{
        int i = 0;
        struct st_test stArr[] = {
                { 2, "defghi", 5.2, 6.7 },
                { 2, "aefghi", 5.0, 6.9 },
                { 1, "aefghi", 5.1, 6.0 },
                { 1, "aefghi", 5.9, 5.0 },
                { 3, "elflfg", 9.0, 2.0 },
                { 3, "elflfg", 9.0, 1.0 }
        };

        qsort( stArr, 6, sizeof( struct st_test ), cmp );

        for( i = 0; i < 6; i++ )
        {
                printf( "%2d %8s %3.1lf %3.1f\n", stArr[ i ].cat, stArr[ i ].caTrain,
                        stArr[ i ].plane, stArr[ i ].tree );       
        }
        return 0;
}

int 
cmp( const void *arg1, const void *arg2 )
{
        static const double DSTDZERO = 0.0000001;
        static const float  FSTDZERO = 0.0000001;
        struct st_test *stArg1 = ( struct st_test * )arg1;
        struct st_test *stArg2 = ( struct st_test * )arg2;

        if( stArg1->cat != stArg2->cat )
                return  stArg1->cat > stArg2->cat ? 1 : -1;
        else if( strcmp( stArg1->caTrain, stArg2->caTrain ) != 0 )
                return -strcmp( stArg1->caTrain, stArg2->caTrain );
        else if( !( ( stArg1->plane - stArg2->plane >= -DSTDZERO ) && ( stArg1->plane - stArg2->plane <= DSTDZERO ) ) )
        {
                if( stArg1->plane - stArg2->plane < -DSTDZERO )
                        return 1;

                if( stArg1->plane - stArg2->plane > DSTDZERO )
                        return -1;
        } 
        else if( !( ( stArg1->tree - stArg2->tree >= -FSTDZERO ) && ( stArg1->tree - stArg2->tree <= FSTDZERO ) ) )
        {
                if( stArg1->tree - stArg2->tree < -FSTDZERO )
                        return -1;

                if( stArg1->tree - stArg2->tree > FSTDZERO )
                        return 1;
        } 

        return 0;
}