read/write file in C

functs.c

#include <stdio.h>

#define USED_FILE "file.txt"

void write(void)
{
	FILE *f=fopen(USED_FILE,"rb");
	//I always use 'b' for binary because it does not make any difference except the thing with EOF.…
	//in text mode -1==EOF, that means if a file contains the byte 0xff this is interpreted as EOF even if the file is longer
	
	if(f)
	{
		//file already exists
		//(maybe convert this to a while loop and try to ask the user for another file here and set f accordingly)
		return;
	}
	
	f=fopen(USED_FILE,"wb");//there is also 'a' which means append
	
	//now write with either fprintf, fwrite, fputs, fputc
	//link: http://en.cppreference.com/w/c/io/<function name>

	int i;
	for(i=0;i<=10;i++)
	{
		fprintf(f,"i=%2d;%s;i^2=%3d\n",i,(i%2)?"odd":"even",i*i);
	}
}

void read(void)
{
	FILE *f=fopen(USED_FILE,"rb");
	
	if(!f)
	{
		//does not exist
		return;
	}
	
	while(!feof(f))
	{
		putchar(fgetc(f));
	}
	
	//now write with either fscanf (DO NOT USE), fread, fgets, fgetc
	//link: http://en.cppreference.com/w/c/io/<function name>
}