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


const int MAX_FILE_NAME = 254;


const char *sys_file_name_from_usr_file_name(char *usr_file_name)
{
	for(char * ptr = usr_file_name; *ptr != '\0'; ++ptr) {
		if ( *ptr == ' ' ) {
			*ptr = '_';
		}
	}
	
	return usr_file_name;
}

const char *usr_file_name_from_sys_file_name(char *sys_file_name)
{
	for(char * ptr = sys_file_name; *ptr != '\0'; ++ptr) {
		if ( *ptr == '_' ) {
			*ptr = ' ';
		}
	}
	
	return sys_file_name;
}

int main(void)
{
	char file_name[MAX_FILE_NAME];
	
	printf( "Give me a file name: " );
	fgets( file_name, MAX_FILE_NAME, stdin );
	
	printf( "\nSys file name: %s", sys_file_name_from_usr_file_name( file_name ) );
	printf( "\nUser file name: %s",
				usr_file_name_from_sys_file_name(
					sys_file_name_from_usr_file_name( file_name ) ) );
	return EXIT_SUCCESS;
}