Recvfrom() return value

Question

Im using UDP packets and I want to be cleared about some points :
1 - what exactly does "recvfrom" Returns ? I mean if i send a packet with size of 450 byte + 20 byte of IP header + 8 byte UDP header does recvfrom returns 478 bytes as a whole or there could be something like :
it received 10 bytes,300 bytes,100 bytes,68 bytes ?
2 - does the return value of "recvfrom" related to packet fragmentation ?

note :
* Im talking with the assumption that "recvfrom" was successful
* I chose 450 byte to be sure that Im less than the min MTU

Answer 1

1. For an UDP socket, recvfrom() reads the UDP data. So it returns 450 , provided you supply a buffer that is at least 450 bytes big. If you supply a buffer that is smaller than the received data, the data will be truncated, and recvfrom() will read as much data as can fit in the buffer you give it.

2. The IP layer will be the part that fragments an UDP packet, on the receiving host it will reassemble it. This is transparent to the sending/receiving application.