The recv(), recvfrom(), and recvmsg() calls are used to receive
messages from a socket. They may be used to receive data on both
connectionless and connection-oriented sockets. This page first
describes common features of all three system calls, and then
describes the differences between the calls.
The only difference between recv() and read(2) is the presence of
flags. With a zero flags argument, recv() is generally
equivalent to read(2) (but see NOTES). Also, the following call
recv(sockfd, buf, len, flags);
is equivalent to
recvfrom(sockfd, buf, len, flags, NULL, NULL);
All three calls return the length of the message on successful
completion. 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.
If no messages are available at the socket, the receive calls
wait for a message to arrive, unless the socket is nonblocking
(see fcntl(2)), in which case the value -1 is returned and errno
is set to EAGAIN or EWOULDBLOCK. The receive calls normally
return any data available, up to the requested amount, rather
than waiting for receipt of the full amount requested.
An application can use select(2), poll(2), or epoll(7) to
determine when more data arrives on a socket.
The flags argument
The flags argument is formed by ORing one or more of the
following values:
MSG_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)
Set the close-on-exec flag for the file descriptor
received via a UNIX domain file descriptor using the
SCM_RIGHTS operation (described in unix(7)). This flag is
useful for the same reasons as the O_CLOEXEC flag of
open(2).
MSG_DONTWAIT (since Linux 2.2)
Enables nonblocking operation; if the operation would
block, the call fails with the error EAGAIN or
EWOULDBLOCK. This provides similar behavior to setting
the O_NONBLOCK flag (via the fcntl(2) F_SETFL operation),
but differs in that MSG_DONTWAIT is a per-call option,
whereas O_NONBLOCK is a setting on the open file
description (see open(2)), which will affect all threads
in the calling process and as well as other processes that
hold file descriptors referring to the same open file
description.
MSG_ERRQUEUE (since Linux 2.2)
This flag specifies that queued errors should be received
from the socket error queue. The error is passed in an
ancillary message with a type dependent on the protocol
(for IPv4 IP_RECVERR). The user should supply a buffer of
sufficient size. See cmsg(3) and ip(7) for more
information. The payload of the original packet that
caused the error is passed as normal data via msg_iovec.
The original destination address of the datagram that
caused the error is supplied via msg_name.
The error is supplied in a sock_extended_err structure:
#define SO_EE_ORIGIN_NONE 0
#define SO_EE_ORIGIN_LOCAL 1
#define SO_EE_ORIGIN_ICMP 2
#define SO_EE_ORIGIN_ICMP6 3
struct sock_extended_err
{
uint32_t ee_errno; /* Error number */
uint8_t ee_origin; /* Where the error originated */
uint8_t ee_type; /* Type */
uint8_t ee_code; /* Code */
uint8_t ee_pad; /* Padding */
uint32_t ee_info; /* Additional information */
uint32_t ee_data; /* Other data */
/* More data may follow */
};
struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
ee_errno contains the errno number of the queued error.
ee_origin is the origin code of where the error
originated. The other fields are protocol-specific. The
macro SO_EE_OFFENDER returns a pointer to the address of
the network object where the error originated from given a
pointer to the ancillary message. If this address is not
known, the sa_family member of the sockaddr contains
AF_UNSPEC and the other fields of the sockaddr are
undefined. The payload of the packet that caused the
error is passed as normal data.
For local errors, no address is passed (this can be
checked with the cmsg_len member of the cmsghdr). For
error receives, the MSG_ERRQUEUE flag is set in the
msghdr. After an error has been passed, the pending
socket error is regenerated based on the next queued error
and will be passed on the next socket operation.
MSG_OOB
This flag requests receipt of out-of-band data that would
not be received in the normal data stream. Some protocols
place expedited data at the head of the normal data queue,
and thus this flag cannot be used with such protocols.
MSG_PEEK
This flag causes the receive operation to return data from
the beginning of the receive queue without removing that
data from the queue. Thus, a subsequent receive call will
return the same data.
MSG_TRUNC (since Linux 2.2)
For raw (AF_PACKET), Internet datagram (since Linux
2.4.27/2.6.8), netlink (since Linux 2.6.22), and UNIX
datagram (since Linux 3.4) sockets: return the real length
of the packet or datagram, even when it was longer than
the passed buffer.
For use with Internet stream sockets, see tcp(7).
MSG_WAITALL (since Linux 2.2)
This flag requests that the operation block until the full
request is satisfied. However, the call may still return
less data than requested if a signal is caught, an error
or disconnect occurs, or the next data to be received is
of a different type than that returned. This flag has no
effect for datagram sockets.
recvfrom()
recvfrom() places the received message into the buffer buf. The
caller must specify the size of the buffer in len.
If src_addr is not NULL, and the underlying protocol provides the
source address of the message, that source address is placed in
the buffer pointed to by src_addr. In this case, addrlen is a
value-result argument. Before the call, it should be initialized
to the size of the buffer associated with src_addr. Upon return,
addrlen is updated to contain the actual size of the source
address. The returned address is truncated if the buffer
provided is too small; in this case, addrlen will return a value
greater than was supplied to the call.
If the caller is not interested in the source address, src_addr
and addrlen should be specified as NULL.
recv()
The recv() call is normally used only on a connected socket (see
connect(2)). It is equivalent to the call:
recvfrom(fd, buf, len, flags, NULL, 0);
recvmsg()
The recvmsg() call uses a msghdr structure to minimize the number
of directly supplied arguments. This structure is defined as
follows in <sys/socket.h>:
struct iovec { /* Scatter/gather array items */
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
struct msghdr {
void *msg_name; /* Optional address */
socklen_t msg_namelen; /* Size of address */
struct iovec *msg_iov; /* Scatter/gather array */
size_t msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* Ancillary data, see below */
size_t msg_controllen; /* Ancillary data buffer len */
int msg_flags; /* Flags on received message */
};
The msg_name field points to a caller-allocated buffer that is
used to return the source address if the socket is unconnected.
The caller should set msg_namelen to the size of this buffer
before this call; upon return from a successful call, msg_namelen
will contain the length of the returned address. If the
application does not need to know the source address, msg_name
can be specified as NULL.
The fields msg_iov and msg_iovlen describe scatter-gather
locations, as discussed in readv(2).
The field msg_control, which has length msg_controllen, points to
a buffer for other protocol control-related messages or
miscellaneous ancillary data. When recvmsg() is called,
msg_controllen should contain the length of the available buffer
in msg_control; upon return from a successful call it will
contain the length of the control message sequence.
The messages are of the form:
struct cmsghdr {
size_t cmsg_len; /* Data byte count, including header
(type is socklen_t in POSIX) */
int cmsg_level; /* Originating protocol */
int cmsg_type; /* Protocol-specific type */
/* followed by
unsigned char cmsg_data[]; */
};
Ancillary data should be accessed only by the macros defined in
cmsg(3).
As an example, Linux uses this ancillary data mechanism to pass
extended errors, IP options, or file descriptors over UNIX domain
sockets. For further information on the use of ancillary data in
various socket domains, see unix(7) and ip(7).
The msg_flags field in the msghdr is set on return of recvmsg().
It can contain several flags:
MSG_EOR
indicates end-of-record; the data returned completed a
record (generally used with sockets of type
SOCK_SEQPACKET).
MSG_TRUNC
indicates that the trailing portion of a datagram was
discarded because the datagram was larger than the buffer
supplied.
MSG_CTRUNC
indicates that some control data was discarded due to lack
of space in the buffer for ancillary data.
MSG_OOB
is returned to indicate that expedited or out-of-band data
was received.
MSG_ERRQUEUE
indicates that no data was received but an extended error
from the socket error queue.
