inv_file = fopen( filename, "r+b" );
    if( inv_file == NULL ){
        if( errno == ENOENT ){
            inv_file = fopen( filename, "w+b" );
            if( inv_file != 0 ){
                part_number[ LAST]=0;
                part_number[ NEXT]=1;
                write_part_numbers();
            }
        }
第5行代码为何判断fopen的返回值是否!=0,,,fopen的返回值不是NULL或一个指向成功打开此流的指针么?这里咋就判断是否!=0呢??难道还有别的返回值?求解,,,