File descriptor assignment in C

Question

When sockets are created or files are opened / created in C, is the file descriptor that is assigned to the socket / file guaranteed to be available for the descriptor with the lowest value? What does the C spec say about the purpose of the file descriptor in this regard, if anything?

Answer 1

It is not guaranteed to be the lowest and depends on implementation (1). In general, however, a procedure that assigns open file descriptors uses a method that gives you the first open. Perhaps right after a few lower ones, free you, leaving you with a higher descriptor than you might expect.

, , - select, , , .

(1) , , IEEE, , . IEEE open(), , , .

Answer 2

, C, , . Linux , .

Answer 3

- ? ( stdin/stdout/stderr) . int ( ), , .

Answer 4

; C , / [ ] FILE, .

Answer 5

@aib open(), close(), lseek(), read(), write() . -.

@Kyle - select(). .

Answer 6

C spec , . Unix, man- open (2) : " , , , ".

This helps if you are trying to attach a specific file to a specific descriptor. Suppose you want to redirect stderr to / dev / null. Sort of

close (2); open ("/ dev / null", O_WRONLY);

must do it. You must, of course, grab the fd returned open and make sure it is 2.