CODEWITHSUNDEEP
clinuxsockets
Sandip
Sandip

Reputation: 31

accept() returning "0" zero (stdin fd) in c socket prog

I am trying to create server which accepts multiple clients using c socket prog. but manytimes when client connects, sockfd=accept() returns 0 (zero) which is stdin fd.

As far as I know file descriptor returns minimum value which not in use, so it may returning 0 . But when I try to listen on sockfd (which is 0) or send() to sockfd it do nothing and both returns 0.

How can I overcome this?

I am working on RaspberryPi Raspbian (debian).

Upvotes: 3

Views: 4307

Answers (1)

Antti Haapala -- Слава Україні
Antti Haapala -- Слава Україні

Reputation: 133899

It is behaviour on POSIX that the file descriptor returned from a function is the smallest available file descriptor. If you've closed STDIN_FILENO or stdin, then 0 could be available, and would be the one returned by connect.

Note that there is nothing special about file descriptor 0; that it is used for standard input is just a convention only; and accept(2) manual on Linux explicitly uses the word nonnegative which includes the possibility of it returning 0:

RETURN VALUE

On success, these system calls return a nonnegative integer that is a descriptor for the accepted socket. On error, -1 is returned, and errno is set appropriately.

You can avoid this by not closing STDIN_FILENO, but opening say /dev/null for reading and dup2:n this file descriptor to STDIN_FILENO (error checking omitted for brevity):

int fd = open("/dev/null", O_RDONLY);
dup2(fd, STDIN_FILENO);
close(fd);

(or closing STDIN_FILENO and then immediately afterwards opening /dev/null, but this is a bit obscure):

close(STDIN_FILENO);
open("/dev/null", O_RDONLY);

I am not sure what you mean by listen; a connected TCP socket couldn't be listened. Also, 0 from send would mean that you're explicitly sending 0 bytes; because on error -1 would be returned instead.

Upvotes: 4

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