Functions return file pointer main and function split over files.

compile

#!/bin/sh
gcc file_example_main.c file_example.c -o file_example

file_example.c

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


static  int     Sfile_open=0;

FILE *Open_Sfile(char *modname){
  char file_name[256];
  FILE *Sfile;
  if (Sfile_open==0) {
    strcpy(file_name,modname);
    Sfile=fopen(file_name,"w");
    printf("// Writing file %s\n",file_name);
    Sfile_open=1;
  };
  return Sfile;
}

file_example_main.c

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

FILE *Open_Sfile(char *);

int main(int argc, char **argv) {
  FILE *source_file;
  source_file = Open_Sfile("example.txt");
  fprintf(source_file, "Helloworld\n");
  fclose(source_file);
  exit(0);
}