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RETURN VALUE
Upon successful completion, recvfrom() returns the length of the
message in bytes. If no messages are available to be received and
the peer has performed an orderly shutdown, recvfrom() returns 0.
Otherwise the function returns -1 and sets errno to indicate the error.
/* 文件:net/socket.c */
SYSCALL_DEFINE6(recvfrom, int, fd, void __user *, ubuf, size_t, size,
unsigned, flags, struct sockaddr __user *, addr,
int __user *, addr_len)
{
struct socket *sock;
struct iovec iov;
struct msghdr msg;
struct sockaddr_storage address;
int err, err2;
int fput_needed;
sock = sockfd_lookup_light(fd, &err, &fput_needed);
if (!sock)
goto out;
msg.msg_control = NULL;
msg.msg_controllen = 0;
msg.msg_iovlen = 1;
msg.msg_iov = &iov;
iov.iov_len = size;
iov.iov_base = ubuf;
msg.msg_name = (struct sockaddr *)&address;
msg.msg_namelen = sizeof(address);
if (sock->file->f_flags & O_NONBLOCK)
flags |= MSG_DONTWAIT;
err = sock_recvmsg(sock, &msg, size, flags);
if (err >= 0 && addr != NULL) {
err2 = move_addr_to_user((struct sockaddr *)&address,
msg.msg_namelen, addr, addr_len);
if (err2 < 0)
err = err2;
}
fput_light(sock->file, fput_needed);
out:
return err;
}