cp_u2b() copies size (the fourth parameter) bytes from an address in a user process (src, the second parameter) to a linked list of buffers within the network service. The address of the first buffer in the linked list is returned through the third parameter. The process number of the user process is the first parameter and is a field in the message requesting the copy.
cp_u2b() is called only by sr_get_userdata().
"u2b" stands for "user to buffer".