Stream the output of a void function using printf()

February 5, 2023

I want to store the output of a function (matrix_output_printf()) printing the following output (a matrix):

0   1   2   
1   2   3   
2   3   4

I would like to save this output in a text file.

In a first attempt, I modified the original in matrix_output_fprintf() so that it stores the output continuously using fprintf(). I indeed stores the output but the code of matrix_output_printf() has been changed

However, I would like not to modify matrix_output_printf() as, let’s say, it is part of a package and want to test it without modifiying it.
Is there a way to store (using C and not bash) the output of matrix_output_printf() from outside the function (or without using fprintf()?

The content of file.txt is the following:

0   1   2   
1   2   3   
2   3   4

Here is the code:

/* main.c */
#include <stdio.h>
#include <stdlib.h>

void matrix_output();

void matrix_output_printf(){
  for (int i = 0; i < 3; i++){
    for (int j = 0; j < 3; j++){
      printf("%d\t", i+j);
    }
    printf("\n");
  }
}

void matrix_output_fprintf(){
  FILE * fp;
  fp = fopen("file.txt", "w");
  fclose(fp);
  fp = fopen("file.txt", "ab");
  for (int i = 0; i < 3; i++){
    for (int j = 0; j < 3; j++){
      fprintf(fp, "%d\t", i+j);
    }
    fprintf(fp, "\n");
  }
  fclose(fp);
}


int main ()
{
  matrix_output_printf();
  matrix_output_fprintf();
  return 0;
}

EDIT

The complete working code for me was:

/* main.c */
#include <stdio.h>
#include <stdlib.h>

void matrix_output();

void matrix_output_printf(){
  for (int i = 0; i < 3; i++){
    for (int j = 0; j < 3; j++){
      printf("%d\t", i+j);
    }
    printf("\n");
  }
}

int main ()
{ 
  freopen("file.txt", "wb", stdout);
  matrix_output_printf();
  return 0;
}