排序动态分配的字符串 [英] Sorting dynamically allocated strings

问题描述

我有奇怪的问题：

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]);

给出预期：

before
c
b
a
after
a
b
c

但

names[0] =  (char *) calloc(1024,sizeof(char));
names[1] =  (char *) 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]);

给

before
c
b
a
after
b
a
c

为什么字符串是不正确排序？

Why strings are not sorted correctly?

推荐答案

您比较函数接收数组中的项的地址。您需要提领，要获得数组中的指针：

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);
}

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