KERNEL_RECVMSG(9) Linux Networking KERNEL_RECVMSG(9)NAME
kernel_recvmsg - Receive a message from a socket (kernel space)
SYNOPSIS
int kernel_recvmsg(struct socket * sock, struct msghdr * msg, struct kvec * vec, size_t num, size_t size, int flags);
ARGUMENTS
sock
The socket to receive the message from
msg
Received message
vec
Input s/g array for message data
num
Size of input s/g array
size
Number of bytes to read
flags
Message flags (MSG_DONTWAIT, etc...)
DESCRIPTION
On return the msg structure contains the scatter/gather array passed in the vec argument. The array is modified so that it consists of the
unfilled portion of the original array.
The returned value is the total number of bytes received, or an error.
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 KERNEL_RECVMSG(9)
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RECV(2) System Calls Manual RECV(2)NAME
recv, recvfrom, recvmsg - receive a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
cc = recv(s, buf, len, flags)
int cc, s;
char *buf;
int len, flags;
cc = recvfrom(s, buf, len, flags, from, fromlen)
int cc, s;
char *buf;
int len, flags;
struct sockaddr *from;
int *fromlen;
cc = recvmsg(s, msg, flags)
int cc, s;
struct msghdr msg[];
int flags;
DESCRIPTION
Recv, recvfrom, and recvmsg are used to receive messages from a socket.
The recv call is normally used only on a connected socket (see connect(2)), while recvfrom and recvmsg may be used to receive data on a
socket whether it is in a connected state or not.
If from is non-zero, the source address of the message is filled in. Fromlen is a value-result parameter, initialized to the size of the
buffer associated with from, and modified on return to indicate the actual size of the address stored there. The length of the message is
returned in cc. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the
message is received from (see socket(2)).
If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see ioctl(2))
in which case a cc of -1 is returned with the external variable errno set to EWOULDBLOCK.
The select(2) call may be used to determine when more data arrives.
The flags argument to a recv call is formed by or'ing one or more of the values,
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_PEEK 0x2 /* peek at incoming message */
The recvmsg call uses a msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as
defined in <sys/socket.h>:
struct msghdr {
caddr_t msg_name; /* optional address */
int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_accrights; /* access rights sent/received */
int msg_accrightslen;
};
Here msg_name and msg_namelen specify the destination address if the socket is unconnected; msg_name may be given as a null pointer if no
names are desired or required. The msg_iov and msg_iovlen describe the scatter gather locations, as described in read(2). A buffer to
receive any access rights sent along with the message is specified in msg_accrights, which has length msg_accrightslen. Access rights are
currently limited to file descriptors, which each occupy the size of an int.
RETURN VALUE
These calls return the number of bytes received, or -1 if an error occurred.
ERRORS
The calls fail if:
[EBADF] The argument s is an invalid descriptor.
[ENOTSOCK] The argument s is not a socket.
[EWOULDBLOCK] The socket is marked non-blocking and the receive operation would block.
[EINTR] The receive was interrupted by delivery of a signal before any data was available for the receive.
[EFAULT] The data was specified to be received into a non-existent or protected part of the process address space.
SEE ALSO fcntl(2), read(2), send(2), select(2), getsockopt(2), socket(2)4.2 Berkeley Distribution May 23, 1986 RECV(2)