Read from socket: Is it guaranteed to at least get x bytes?

Question

I have a rare bug that seems to occur reading a socket.

It seems, that during reading of data sometimes I get only 1-3 bytes of a data package that is bigger than th

Answer 1

This is just the way TCP works. You aren't going to get all of your data at once. There are just too many timing issues between sender and receiver including the senders operating system, NIC, routers, switches, the wires themselves, the receivers NIC, OS, etc. There are buffers in the hardware, and in the OS.

You can't assume that the TCP network is the same as a OS pipe. With the pipe, it's all software so there's no cost in delivering the whole message at once for most messages. With the network, you have to assume there will be timing issues, even in a simple network.

That's why recv() can't give you all the data at once, it may just not be available, even if everything is working right. Normally, you will call recv() and catch the output. That should tell you how many bytes you've received. If it's less than you expect, you need to keep calling recv() (as has been suggested) until you get the correct number of bytes. Be aware that in most cases, recv() returns -1 on error, so check for that and check your documentation for ERRNO values. EAGAIN in particular seems to cause people problems. You can read about it on the internet for details, but if I recall, it means that no data is available at the moment and you should try again.

Also, it sounds like from your post that you're sure the sender is sending the data you need sent, but just to be complete, check this: http://beej.us/guide/bgnet/output/html/multipage/advanced.html#sendall

You should be doing something similar on the recv() end to handle partial receives. If you have a fixed packet size, you should read until you get the amount of data you expect. If you have a variable packet size, you should read until you have the header that tells you how much data you send(), then read that much more data.