int filedes？ 系统调用读写

Question

Can any of you guys tell me what "int filedes" refers to? http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html

I've noticed I can put any int in there and it seems to work but I don't know what it's for...

Thanks.

Answer 1

The very first sentence of the description says, "the file associated with the open file descriptor, fildes ". In other words, it indicates the file you're reading from. If your read function call works no matter what file descriptor you pass it, your program isn't doing what you think it is.

Answer 2

It's a file descriptor. See http://en.wikipedia.org/wiki/File_descriptor . Since it represents an offset to a table lookup of files and pipes, there may be multiple descriptors that could return valid data. 0=stdin and 2=stderr will exist by default, or you can look at the open function to create your own.

Answer 3

Somewhere inside the kernel, there is a table comprises of file descriptor entries on a per-process base. A file descriptor is a structure which describes the state of the file. What kind of information has a file descriptor? First of all, position from which the next read/write operation can be performed. Then, the access mode of the file, specified by the open system call. And last but not least, a data structure which represent the on-disk information of a file. In *nix, this is an inode structure. Here, the main question to be answered is : Where resides the blocks of the file in the disk. If you have an inode of a file in the memory, you can find quickly, where is the Nth block of the file(which means you don't need to parse the path every time, and scan each directory in the path to resolve the inode).