Xueting Zhu wrote:
> I am studying socket programming. I have a problem about the function
> send(). Generally it is thought that the calls return the number of
> characters sent, or -1 if an error occurred. But when I practice
> programming about udp socket, the server return 0 when it actually
> sent data. I am really puzzled. Can anyone help me ? The program I
> practice is as follows.
> *in line 38:count=sendto(sd,"test from server",sizeof("test from server"),0,(struct sockaddr *) &s_addr,sizeof(s_addr))<0)
> *when captured with etheral, it is found that the data has been sent,
> *while the printed message shows that "0 byte send success!"
> */
> if(count=sendto(sd,bufftosend,sizeof(bufftosend),0,(struct sockaddr *) &s_addr,sizeof(s_addr))<0)
sendto() isn't returning zero.
The "<" operator has higher precedence than "=" (the only operator
with a lower precedence than assignment operators is the sequencing
operator ","), so the above call is equivalent to:
if (count = (sendto(...) < 0))
I.e. "count" is being assigned the value of the expression (sendto(...) < 0),
which will be 1 if sendto() returns a negative value and zero otherwise.
To get the behaviour which you desire, you need additional parentheses, i.e.:
if ((count = sendto(...)) < 0)
--
Glynn Clements <glynn@xxxxxxxxxxxxxxxxxx>
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