In C, Merge Sorted Data Files Into Single Output Data File

/** 
* your name, 
* the course number,
* the date the program was completed, 
* a brief description of theprogram.
*/

#include<stdio.h>

int merge(const char*, const char*, const char*);

int main(){
  int error = merge("Data1.txt","Data2.txt","Merged.txt");
  if(error){
    printf("Error: File merging failed\n");
  } else {
    printf("Success: File merged\n");
  }
  return error;
}

int merge(const char* n1, const char* n2, const char* n3){
  FILE *in1 = fopen(n1, "r");
  if(!in1){
    printf("Error: Could not read file: %s\n", n1);
    return 1;
  }
  FILE *in2 = fopen(n2, "r");
  if(!in2){
    fclose(in1);
    printf("Error: Could not read file: %s\n", n2);
    return 2;
  }
  FILE *out = fopen(n3, "w");
  if(!out){
    fclose(in1);
    fclose(in2);
    printf("Error: Could not write file: %s\n", n3);
    return 3;
  }
  int num1, num2;
  //until end of file
  while(!feof(in1) && !feof(in2)){
    //at beginning of file
    if(!ftell(in1)){
      fscanf(in1, "%d", &num1);
      fscanf(in2, "%d", &num2);
    }
    if(num1 < num2){
      fprintf(out, "%d\n", num1);
      fscanf(in1, "%d", &num1);
    } else if(num2 < num1){
      fprintf(out, "%d\n", num2);
      fscanf(in2, "%d", &num2);
    } else {
      fprintf(out, "%d\n", num1);
      fprintf(out, "%d\n", num2);
      fscanf(in1, "%d", &num1);
      fscanf(in2, "%d", &num2);
    }
  }
  while(!feof(in1)){
      fprintf(out, "%d\n", num1);
      fscanf(in1, "%d", &num1);
  }
  while(!feof(in2)){
      fprintf(out, "%d\n", num2);
      fscanf(in2, "%d", &num2);
  }
  fclose(out);
  fclose(in1);
  fclose(in2);
  return 0;
}

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