What is the difference between incrementing an array pointer and accessing a pointer's array vi-CodePudding

I have some code to read a file byte by byte, and store the data in blocks. I have the blocks separated via dynamically allocated memory. Here is my code:

  int ch = NULL;
  while (ch != EOF) {
    //Read 16-byte blocks of data from a text file or until the EOF flag is found
    for (size_t i = 0; i < UINT16_MAX && stream && ((ch = fgetc(stream)) != EOF);   i) {
      //(void)printf("%c", ch);
      *blocks  = (char)abs(ch);
    }

    //Increase allocated memory blocks if the previous has been filled 
    if (ch != EOF) {
      uint32_t* tempMem = realloc(blocks,   size * sizeof(uint32_t));
      if (tempMem == NULL) {
        free(blocks);
        (void)printf("Error: could not allocate memory");
        return EXIT_FAILURE;
      }
      blocks = tempMem;
      *(  blocks) = 0; //zero-initialize new memory location
    }
  }

With this code, I get a corrupted heap address breakpoint, but if I change the memory access method, like so:

  int ch = NULL;
  while (ch != EOF) {
    //Read 16-byte blocks of data from a text file or until the EOF flag is found
    for (size_t i = 0; i < UINT16_MAX && stream && ((ch = fgetc(stream)) != EOF);   i) {
      //(void)printf("%c", ch);
      blocks[size - 1]  = (char)abs(ch);
    }

    //Increase allocated memory blocks if the previous has been filled 
    if (ch != EOF) {
      uint32_t* tempMem = realloc(blocks,   size * sizeof(uint32_t));
      if (tempMem == NULL) {
        free(blocks);
        (void)printf("Error: could not allocate memory");
        return EXIT_FAILURE;
      }
      blocks = tempMem;
      blocks[size - 1] = 0;
    }
  }

I get no errors and the code runs fine (albeit with logic errors, but this is an educational project anyways). How come doing array[ index] works fine, but *( pointer_to_an_array) breaks everything?

CodePudding user response：

blocks changes blocks, after which it is no longer the address of the start of the allocated memory, so it is not the correct address to pass to free or realloc.