I have data from socket, which is header and message. Header if of 5 bytes, 3rd and 4th is message length.
I am reading from socket, can someone suggest me good function, it will return me msg. assuming that i am reading 1024 bytes and it may happen that i have recived only partial data.
Why would you read 1024 bytes? Read 5 bytes, repeating the read if necessary until you've actually got 5 bytes (or an error). Then you know how long the message itself is: so repeatedly read until you've got all the data for the message. (On each call you only ask for as much data as you actually want, of course - so if when you're reading the header you first read 2 bytes, then on the next call you request 3 bytes, i.e. the remainder of the header.)
Reading more than you know you need is just asking for complexity - it means you've got to remember that data somewhere for the next read.
You should put the call to read()/recv() in a loop that you exit once you have read enough data. If only partial data is received, you should keep on reading again until you received enough to parse the packet.