Sorting structs in C with qsort

Question

I have a struct in C like this:

typedef struct proces {
    char ime[60];
    char pid[60];
    char ppid[60];
    char stanje;
    int stdat;
    char niti[60];
    char poraba[60];
} proces ;

I create about 100 of them and put them into an array

proces** procesi = malloc(sizeof(proces));
int x;
for(x=0; x<st; x++){
    procesi[x] = (struct proces*)malloc(sizeof(proces));
}

Now I would like to sort them with qsort. But the qsort sorts it wrong. The function looks like this:

int compar_ppid(const void *v1, const void *v2){
   const proces *p1 = (proces*)v1;
   const proces *p2 = (proces*)v2;
   return(strcmp(p1->ppid, p2->ppid));
}

I checked the values that the compar_ppid is comparing and they seem to be something like �#d, when they should be numbers.

I guess I'm accessing the pointer and not the value, but I cant figure out what to change to get the right values.

Qsort call:

qsort(procesi, st, sizeof(proces*), compar_name);

proces** procesi = malloc(sizeof(proces));--->proces** procesi = malloc(st*sizeof(proces*)); — BLUEPIXY
– BLUEPIXY, Commented May 5, 2013 at 10:21

Bryan Olivier · Accepted Answer · 2013-05-05 10:56:36Z

1

The array you sort contains pointers to process, so your compare function should look like:

int compar_ppid(const void * v1, const void * v2)
{
    process *const*p1 = v1, *const*p2 = v2;
    return strcmp((*p1)->ppid, (*p2)->ppid);
}

and as BLUEPIXY points out the allocation of the array is not using the pointer size, but the struct size.

edited May 5, 2013 at 10:56

answered May 5, 2013 at 10:23

Bryan Olivier

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2 Comments

Pero44 Over a year ago

It works now, but I'm getting a warning: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]

Bryan Olivier Over a year ago

My bad. Fixed the example. I hardly ever use const myself, highly overrated.

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