使用动态内存分配为C附加函数

Question

Following this question , I'm trying to modify the code provided in this blog post to create an append function using dynamic memory allocation. This is what I have so far:

#include <stdio.h>
#include <stdlib.h>

typedef struct intlist_ {
  int size;
  int* list;
} intlist;

void append(intlist* arr, int value){
  realloc((*arr).list, sizeof((*arr).list) + sizeof(int));
  (*arr).size = (*arr).size + 1;
  (*arr).list[(*arr).size -1] = value;
}

int main() {

  intlist arr;
  arr.size = 4;
  arr.list = malloc(arr.size * sizeof(int));

  arr.list[0] = 0;
  arr.list[1] = 5;
  arr.list[2] = 3;
  arr.list[3] = 64;

  append(&arr, 12);

  for (int ii = 0; ii < arr.size; ii++)
    printf("%d, ", arr.list[ii]);

  free(arr.list);
  return 0;
}

However the result I get is wrong:

clang version 7.0.0-3~ubuntu0.18.04.1 (tags/RELEASE_700/final) main.c:10:3: warning: ignoring return value of function declared with 'warn_unused_result' attribute [-Wunused-result] realloc((*arr).list, sizeof((*arr).list) + sizeof(int)); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1 warning generated. 0, 5, 3, 64, 0, 5, 3, 64, 12,

I'm using this online compiler for testing where you can also see the latest versions of the above code. I would appreciate if you could help me know where is my mistake and how I can solve it. Thanks for your support in advance.

PS You may a final version of the code here in this Gist .

Answer 1

Close, but:

1. sizeof((*arr).list) wont give you the size of the array. Instead it will give you the size of a int* .
2. realloc invalidates the original pointer, and continuing to use it is undefined behaviour. Use the returned value instead.

So change your realloc line to use the stored list size instead, and update the pointer with the return value:

(*arr).list = realloc((*arr).list, ((*arr).size + 1) * sizeof(int));

Couple of other tips:

• ptr-> is the same as (*ptr). but easier to read. I suggest changing all your (*arr).size and (*arr).list to arr->size and arr->list .
• realloc , like its brethren, is not guaranteed to succeed. You should check the return value for null to catch errors.
• The clang warning is (as often is the case) helpful - checking the return value would have solved a couple of issues.

Answer 2

• Wrong size allocated. sizeof((*arr).list) is the size of a pointer, not int .

• Return value not used. realloc() returns the new pointer.

• No NULL check

Rather than use the error prone ptr = some_alloc(sizeof(type) * n) , use ptr = some_alloc(sizeof *ptr * n)

---

void append(intlist *arr, int value){
  int *new_ptr = realloc(arr->list, sizeof *(arr->list) * (arr->size + 1u));
  if (new_ptr == NULL) {
    fprintf(stderr, "Out of memory\n");
    exit (EXIT_FAILURE);
  }
  arr->list = new_ptr;
  arr->list[arr->size] = value;
  arr->size++;
}