value-result argument

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);

 

 

 The argument addrlen is a value-result  argument,which  the  caller 

 should initialize before the call to the size of the   buffer associated 

 with src_addr, and modified on return to indicate the

 actual  size  of the source address.