qsort() in C for an array of char pointers [duplicate]

Question

I've been trying to write a function to sort an array of char pointers and print them out. Using the qsort function, but for some reason, I can't seem to be able to sort them in the output. The inputs are basically an array of MAC addresses in the format:

84:1b:5e:a8:bf:7c

74:e2:f5:17:96:89

00:8e:f2:c0:13:cc

74:e2:f5:17:96:89

I've looked around and tried different functions of for the string compare section and other examples of qsort, but I still can't get it to work. My code is:

int compare_function(const void *a,const void *b) {
return (strcmp((char *)a,(char *)b));
}


void countDistinct(char* MAClist[], int n){
char* bufList[n];
for(int i = 0; i < n; i++){
    printf("success1 ");
    bufList[i] = str_dup(MAClist[i]);
    printf("success2 ");
    printf("%s \n", bufList[i]);
    printf("success \n");
}

qsort(bufList, n, sizeof(*bufList), compare_function);
for(int j = 0; j < n; j++){
    printf("%s \n", bufList[j]);
}

}

The reason why I'm trying to sort it is so that I can count the number of distinct objects efficiently. But the sorting has to be done again at a later stage anyway, so I'll need to reuse the function. Any help is appreciated!

Try return (strcmp((char *)a,(char *)b)); --> return (strcmp(*(char **)a,*(char **)b)); — BLUEPIXY
– BLUEPIXY, Commented Sep 10, 2017 at 16:16

P0W · Accepted Answer · 2017-09-10 16:18:49Z

2

Need to fix your compare function like following :

int compare_function(const void * a, const void * b ) {
    // a, b are pointer to const char*, when passed from qsort 
    const char *pa = *(const char**)a;
    const char *pb = *(const char**)b;

    return strcmp(pa,pb);
}

answered Sep 10, 2017 at 16:18

P0W

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