linux-kernel - Re: recv function bug

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Date:	Tue, 11 Feb 2014 11:16:19 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	未卷起的浪 <ayjj_8109@...com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>
Subject: Re: recv function bug

Hello 未卷起的浪,

On Sat, Feb 8, 2014 at 5:32 PM, 未卷起的浪 <ayjj_8109@...com> wrote:
>     It is my pleasure to write here for you.
>     I have read 'Linux Programmer’s Manual RECV(2)'. The description of
> return value is like this: These calls return the number of bytes received,
> or -1 if  an  error occurred. The  return  value will be 0 when the peer has
> performed an orderly shutdown.
>     Notice the bold text. It means that if the "recv" function return 0, the
> connection has already been broken gracefully. But in the fallowing code, it
> is not the case:
>            char         buf[256];
>            ssize_t      rc;
>            size_t       buflen = 0;
>            rc = recv( sockfd, buf, buflen, 0 );   // the 'rc' will be 0, but
> the connection is still established
> If the value of 'buflen' is 0, the "recv" function also return 0. But the
> connection is still established. In another case, before the "recv" being
> called, the remote programe close the socket (it means that the connection
> has been broken), and the "recv" return 0 too.
>                 SERVER
> CLIENT
>            char         buf[256];
>            ssize_t      rc;
>            size_t       buflen = 0;
> ......
>
> close( sockfd );
>            // before call recv, the connection has been broken gracefully
> ......
>            rc = recv( sockfd, buf, buflen, 0 );   // the 'rc' will be 0, but
> the connection is broken
>
>     So, when I call the "recv" function with the value 0 in the 3rd
> argument, I'm not sure whether the connection is still OK by the return
> value.
>     I think there is a conflict between the definition of the 3rd argument
> and the return value.

Thanks for this report. Yes, there are several details missing there.
I've applied the patch below.

Cheers,

Michael

--- a/man2/recv.2
+++ b/man2/recv.2
@@ -403,8 +403,16 @@ if an error occurred.
 In the event of an error,
 .I errno
 is set to indicate the error.
-The return value will be 0 when the
-peer has performed an orderly shutdown.
+
+When a stream socket peer has performed an orderly shutdown,
+the return value will be 0 (the traditional "end-of-file" return).
+
+Datagram sockets in various domains (e.g., the UNIX and Internet domains)
+permit zero-length datagrams.
+When such a datagram is received, the return value is 0.
+
+The value 0 may also be returned if the requested number of bytes
+to receive from a stream socket was 0.
 .SH ERRORS
 These are some standard errors generated by the socket layer.
 Additional errors





-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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