On Fri, Jul 06, 2001 at 09:58:00AM -0400, Matthew Hagerty wrote:
Greetings,
I am going over the use of select() for a server I'm writing and I
*thought* I understood the man page's description for the use of the first
parameter, nfds.
From MAN:
The first nfds descriptors are checked in each set; i.e., the descriptors
from 0 through nfds-1 in the descriptor sets are examined.
I take this to mean that each descriptor set contains n descriptors and I
am interested in examining the first nfds descriptors referenced in my
sets. I also understood it to mean that nfds has absolutely nothing to do
with the actual *value* of a descriptor, i.e. the value returned by
fopen(), socket(), etc.. Is this correct thinking? What got me
second-guessing myself was a use of select() that seems to indicate that
you have to make sure nfds is larger than the value of the largest
descriptor you want checked. Here is the select() from the questionable
code (I can provide the whole function if necessary, it's not very big):
if (select(conn-sock + 1, input_mask, output_mask, except_mask,
(struct timeval *) NULL) 0)
Actually, this is the correct use. nfds should be larger than
the value of the largest fd in the set, no matter how many fd's
there are.
Is this improper use? conn-sock is set like this:
/* Open a socket */
if ((conn-sock = socket(family, SOCK_STREAM, 0)) 0)
Any clarification on how nfds should be set would be greatly appreciated.
Maybe it would help if you thought of an fd set as of a bit array,
with a '1' bit for each fd in the set, and a '0' for each fd
not in the set. The nfds argument tells select(2) how far into
the bit array to check for set bits. If you want to check fd's 0,
1 and 5, then nfds should be at least 6.
For more information, I would suggest reading W. Richard Stevens' book
Unix Network Programming, vol. 1.
G'luck,
Peter
--
You have, of course, just begun reading the sentence that you have just finished
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