read files in c

// C program to implement
// the above approach
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
// Driver code
int main()
{
    FILE* ptr;
    char ch;
 
    // Opening file in reading mode
    ptr = fopen("test.txt", "r");
 
    if (NULL == ptr) {
        printf("file can't be opened \n");
    }
 
    printf("content of this file are \n");
 
    // Printing what is written in file
    // character by character using loop.
    do {
        ch = fgetc(ptr);
        printf("%c", ch);
 
        // Checking if character is not EOF.
        // If it is EOF stop eading.
    } while (ch != EOF);
 
    // Closing the file
    fclose(ptr);
    return 0;
}

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

int main()
{
   int num;
   FILE *fptr;

   if ((fptr = fopen("C:\\program.txt","r")) == NULL){
       printf("Error! opening file");

       // Program exits if the file pointer returns NULL.
       exit(1);
   }

   fscanf(fptr,"%d", &num);

   printf("Value of n=%d", num);
   fclose(fptr); 
  
   return 0;
}

FILE *in_file  = fopen("name_of_file", "r"); // read only 
          FILE *out_file = fopen("name_of_file", "w"); // write only 
           
          // test for files not existing. 
          if (in_file == NULL || out_file == NULL) 
            {   
              printf("Error! Could not open file\n"); 
              exit(-1); // must include stdlib.h 
            } 
           
          // write to file vs write to screen 
          fprintf(file, "this is a test %d\n", integer); // write to file 
 
          fprintf(stdout, "this is a test %d\n", integer); // write to screen  
          printf(         "this is a test %d\n", integer); // write to screen  
 
          // read from file/keyboard. remember the ampersands!  
          fscanf(file, "%d %d", &int_var_1, &int_var_2);  
 
          fscanf(stdin, "%d %d", &int_var_1, &int_var_2);  
          scanf(        "%d %d", &int_var_1, &int_var_2);