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up vote 4 down vote favorite

I want to write a specific number of bytes to a socket, 

n=write(sock_fd, buf, len);

if n<len, I would rather no bytes are written into the socket, is it possible or not? thanks!

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1  
What protocol are you using? – Tim Cooper 2 days ago

2 Answers

up vote 3 down vote accepted
  1. If you're using blocking mode, the condition can never arise: send() will block until all the data has been sent. This is not the way the documentation reads, but it's how all known implementations behave.
  2. If you're talking about TCP, it doesn't make any difference anyway: it's a byte-steam protocol, not a datagram protocol.
  3. If you're talking about UDP, it already behaves that way in non-blocking ,ode, and for blocking mode see (1).
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A partial write is still possible even in blocking mode. An interruption by a signal (after some bytes have already been written) is one example. – jxh 2 days ago
how about TCP and non-blocking socket? or file descriptor and non-blocking mode? – misteryes 2 days ago
1  
@jxh No. See the man page. EINTR means 'a signal occurred before any data was transmitted' [my emphasis]. Otherwise you would have no way of knowing how much data had been transferred. – EJP 2 days ago
1  
@misteryes TCP in non-blocking mode transfers as much as it can to the socket send buffer and returns that count. I'm not aware you can use files in non-blocking mode, but if so, at worst they would behave the same way as TCP. – EJP 2 days ago
2  
@EJP: If the signal occurred after (partial) data was transmitted, the system call is still interrupted, but it would be woken up with a partial write result instead of -1. – jxh 2 days ago
up vote 1 down vote

If the socket is in blocking mode then, as EJP (+1) says, write() will not return until len bytes have been written to the socket's outgoing buffer.

However, it's important to note that that doesn't tell you anything about how many bytes have been received at the receiving end of the connection. In fact for small values of len when write() returns no data has been sent at all (yet). And when the other end calls read() it will block until some data has arrived, but not necessarily len bytes. If you want to read len bytes then you may have to keep calling read() until you have got them all.

The only real way to know what's actually happened and when it finished is to have some sort of message being returned from the other end of the socket to say that everything has now arrived. A confirmatory return from write() is not enough by itself.

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