select first parameter

RGummi · 09-06-2008, 05:27 AM

Hello,

what should be used as the first parameter in a select statement (sockets)?
Should it FD_SETSIZE or filedescriptor + 1 ?

Code:

 
 fd_set fs;
 FD_ZERO(&fs);
 FD_SET(fdSocket, &fs);

or

Code:

 
 select(fdSocket+1, &fs, NULL, NULL, &tout);

Thanks

dmail · 09-06-2008, 05:42 AM

The first parameter is the highest file descriptor plus one.
Beej example
FD_SETSIZE is an indication as to the possible maximum descriptors which can be contained in a set.

RGummi · 09-06-2008, 06:07 AM

Thanks,

here FD_SETSIZE is used

http://www.delorie.com/gnu/docs/glibc/libc_248.html

The select function checks only the first nfds file descriptors. The usual thing is to pass FD_SETSIZE as the value of this argument.

dmail · 09-06-2008, 08:01 AM

I personally find it strange that it says "the usual thing" as I have never seen it used like that and man select shows

Quote:

...The first nfds descriptors are checked in
each set; i.e., the descriptors from 0 through nfds-1 in the descriptor
sets are examined. (Example: If you have set two file descriptors "4"
and "17", nfds should not be "2", but rather "17 + 1" or "18".)

RGummi · 09-06-2008, 11:22 AM

Hell,

This is the reason why I'am confused and posted this question.

RGummi