Sorting dynamically allocated strings
I've got strange problem:
int cmp(const void *a, const void *b) {
const char *ia = (const char *) a;
const char *ib = (const char *) b;
return strcmp(ia, ib);
}
char ** names = NULL;
if((names = (char **) calloc(3,sizeof(char*))) == NULL)
{
fprintf(stderr,"Unable to allocate the memory");
return 1;
}
...
names[0] = "c";
names[1] = "b";
names[2] = "a";
printf("before\n");
printf("%s\n",names[0]);
printf("%s\n",names[1]);
printf("%s\n",names[2]);
qsort(names,3,sizeof(char *),cmp);
printf("after\n");
printf("%s\n",names[0]);
printf("%s\n",names[1]);
printf("%s\n",names[2]);
Gives as expected:
before
c
b
a
after
a
b
c
But
names[0] = (char *) calloc(1024,sizeof(char));
names[1] = (ch开发者_JAVA百科ar *) calloc(1024,sizeof(char));
names[2] = (char *) calloc(1024,sizeof(char));
scanf("%s",names[0]);
scanf("%s",names[1]);
scanf("%s",names[2]);
printf("before\n");
printf("%s\n",names[0]);
printf("%s\n",names[1]);
printf("%s\n",names[2]);
qsort(names,3,sizeof(char *),cmp);
printf("after\n");
printf("%s\n",names[0]);
printf("%s\n",names[1]);
printf("%s\n",names[2]);
gives
before
c
b
a
after
b
a
c
Why strings are not sorted correctly?
Your comparison function receives the address of the item in the array. You need to dereference that to get the pointer in the array:
int cmp(const void *a, const void *b) {
const char *ia = *(const char **) a;
const char *ib = *(const char **) b;
return strcmp(ia, ib);
}
The qsort comparator function's arguments are pointers to the arguments being compared. I think you need an additional dereference. In other words, a and b are not char*, they are char**.
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