C-动态内存分配-复制字符串

Question

for the following program

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

int main() {
  int size = 16, len = 0, c;
  char* data = (char*)malloc(sizeof(char) * size);
  while((c = getchar()) != EOF) {
    if(len == size) {
      size *= 2;
      char* data_new = (char*)realloc(data, sizeof(char) * size);
      if(data_new == NULL) {
        free(data);
        return -1;
      }
      if(data_new != data) {
        memcpy(data_new, data, size);
        data = data_new;
      }
    }
    data[len] = c;
    len++;
  }
  data[len] = '0';
  printf("%s\n", data);
  free(data);
  return 0;
}

I think the error lies in the realloc part because if im setting the size int high everything works fine.

Does someone have a tip for me how i can do this realloc thing correctly?

thx for your support

Answer 1

You don't need to copy the data when realloc returns a different pointer - that is done for you. In fact it is a mistake because you are copying the data from a source that no longer exists, and twice the quantity that it had! So you only need

data = data_new;

Also this line is wrong

data[len] = '0';

it should be

data[len] = '\0';

and can also write beyond the memory bounds. You should be checking the size again before writing the terminator, or more simply, reallocate the memory when size-1 has been used, because you know you'll be needing that extra byte.

if(len == size - 1) {
    // . . . 
}

Note too that if you are entering characters each separated by a newline then getchar() will read every newline too.