char *new_name(char *old_name, char *operation)
{
    char file_num[2];

    char *edited_name = ( char* ) malloc( strlen( old_name ) + strlen( operation ) );
    strcat( edited_name, old_name );
    strcat( edited_name, operation );

    // Iterate through names of new file to see which doesn't exist
    int fnum = 1;
    char *tempname = ( char* ) malloc( strlen( old_name ) + strlen( operation ) + sizeof( file_num ) + sizeof( ".txt" ) );
    do
    {
        strcpy( tempname, edited_name );
        sprintf( file_num, "%d", fnum++ );
        strcat( tempname, file_num );
        strcat( tempname, ".txt" );
    } while ( file_exists( tempname ) );
    free(edited_name);

    return tempname;
}

int main()
{

    char *old_name = "textfile";
    char *operation = "_join";
    char *out_name = new_name(old_name, operation);

    printf( "%s", out_name );

    return 0;
}