Writing a 2D character array into a text file

Question

I am trying to write my 2D character array "A" into a .txt file "some_file.txt". I tried to use the Sergey Kalinichenko's answer from this question for my code (see below). However, it does not work. Could you please tell me what I am doing wrong.

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


char A[10][10] =
{
  {'~', 'S', 'S', 'S', 'S', 'S', '~', '~', '~', 'S'},
  {'S', '~', '~', '~', '~', '~', '~', '~', '~', 'S'},
  {'S', '~', '~', 'S', 'S', 'S', '~', 'S', '~', '~'},
  {'S', '~', '~', '~', '~', '~', '~', 'S', '~', '~'},
  {'S', '~', 'S', 'S', 'S', '~', '~', '~', '~', '~'},
  {'~', 'S', '~', '~', '~', '~', '~', '~', '~', '~'},
  {'~', 'S', '~', '~', '~', '~', 'S', '~', '~', '~'},
  {'~', 'S', '~', '~', '~', '~', 'S', '~', '~', 'S'},
  {'~', 'S', '~', 'S', 'S', '~', '~', '~', '~', 'S'},
  {'~', '~', '~', '~', '~', '~', '~', '~', '~', 'S'}
};

void saveImage(int height, int width, char image[height][width], const char* file_name)
{
  FILE *file = fopen(file_name, "wb");
  fwrite(image, sizeof(char), sizeof(image), file);
  fclose(file);
}

int main()
{
  saveImage(10, 10, A, "some_file.txt");

  return 0;
}

image is a pointer, so it's only 8 bytes (on 64bit systems). use `width*height` instead of `sizeof(image)`. — robthebloke, May 17 '23 at 23:16
The trick is to not let arrays decay into pointers - or do, but take that into consideration — Ted Lyngmo, May 17 '23 at 23:21
...and to pay attention to compiler warnings (after turning them on). — atru, May 17 '23 at 23:22
@atru I think an answer in your words would work. The question has undoubtedly a duplicate answer, but your finess does not. — Ted Lyngmo, May 17 '23 at 23:35
@TedLyngmo Thank you for the encouragement :) Added - if you think something is missing in it, let me know and I'll add that too :) — atru, May 18 '23 at 01:01

score 4 · Accepted Answer · edited May 18 '23 at 01:25

When sending an array to a function as a parameter, the array decays into a pointer. This post may be a good reading start on the decay.

Because of this decay, your sizeof(image) is actually the size of the pointer and not the number of all array elements. As you already saw, you need to replace it with the actual element number:

fwrite(image, sizeof(char), height*width, file);

The important part is that when compiling, you should get a warning indicating the above. For instance, on GCC 9.4.0 (Ubuntu) I get:

cscratch.c: In function ‘saveImage’:
cscratch.c:23:35: warning: ‘sizeof’ on array function parameter ‘image’ will return size of ‘char (*)[(sizetype)(width)]’ [-Wsizeof-array-argument]
   23 | fwrite(image, sizeof(char), sizeof(image), file);
      |                                   ^
cscratch.c:19:44: note: declared here
   19 | void saveImage(int height, int width, char image[height][width], const char* file_name)


  |                                       ~~~~~^~~~~~~~~~~~~~~~~~~~

As part of a basic compilation that does not have any special warning flags specified (it is highly recommended to use stricter and more comprehensive warning flags than the default ones).

In this message, you can clearly see the issue and its source - point being that one should enable strict warnings and pay close attention to them.

Sorry, I rolled back your edit. My comments have led you astray. My fault. — Ted Lyngmo, May 18 '23 at 01:26
Sorry on my end, it was getting late after a long day! Thank you :) — atru, May 18 '23 at 02:41

0___________ · Answer 2 · 2023-05-18T00:03:16.667

sizeof in your question is giving the size of the pointer. Use a pointer to the array and sizeof will work fine :). Also, use the correct types.

void saveImage(size_t height, size_t width, char (*image)[height][width], const char* file_name)
{
  FILE *file = fopen(file_name, "wb");
  if(file) 
  {
     fwrite(image, sizeof(char), sizeof(*image), file);
     fclose(file);
  }
}

//usage saveImage(10, 10, &A, "some_file.txt");

or

void saveImage(size_t height, size_t width, char (*image)[width], const char* file_name)
{
  FILE *file = fopen(file_name, "wb");
  if(file) 
  {
     fwrite(image, sizeof(char), height * sizeof(*image), file);
     fclose(file);
  }
}

//usage   saveImage(10, 10, A, "some_file.txt");

Writing a 2D character array into a text file

2 Answers2