NAME
recv
, recvfrom
,
recvmsg
, recvmmsg
—
receive a message from a
socket
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/socket.h>
ssize_t
recv
(int
s, void *buf,
size_t len,
int flags);
ssize_t
recvfrom
(int
s, void * restrict
buf, size_t len,
int flags,
struct sockaddr * restrict
from, socklen_t *
restrict fromlen);
ssize_t
recvmsg
(int
s, struct msghdr
*msg, int
flags);
int
recvmmsg
(int
s, struct mmsghdr
*mmsg, unsigned int
vlen, unsigned int
flags, struct timespec
*timeout);
DESCRIPTION
recvfrom
(),
recvmsg
() and recvmmsg
() are
used to receive messages from a socket, and may be used to receive data on a
socket whether or not it is connection-oriented.
If from is non-nil, and the socket is not connection-oriented, 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
recvmmsg
()
call can be used to receive multiple messages in the same call using an
array of mmsghdr elements with the following form, as
defined in ⟨sys/socket.h⟩:
struct mmsghdr { struct msghdr msg_hdr; /* the message to be sent */ unsigned int msg_len; /* number of bytes received */ };
The msg_len member contains
the number of bytes received for each msg_hdr member.
The array has vlen elements, which is limited to
1024
. If there is an error, a number fewer than
vlen may be returned, and the error may be retrieved
using getsockopt(2) with SO_ERROR
. If the
flag MSG_WAITFORONE
is set in
flags then the
recvmmsg
()
call will wait for one message, and set MSG_DONTWAIT
for the rest. If the timeout parameter is not
NULL
, then recvmmsg
() will
return if that time is exceeded.
The
recv
() call is
normally used only on a
connected
socket (see connect(2)) and is identical to
recvfrom
() with a nil from
parameter. As it is redundant, it may not be supported in future
releases.
recv
(),
recvfrom
() and recvmsg
()
routines return the length of the message on successful completion.
recvmmsg
() returns the number of messages received.
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
fcntl(2)) in which case the value -1 is returned and the external
variable errno set to EAGAIN
.
If no data is available and the remote peer was shut down, 0 is returned.
The receive calls normally return any data available, up to the requested
amount, rather than waiting for receipt of the full amount requested; this
behavior is affected by the socket-level options
SO_RCVLOWAT
and SO_RCVTIMEO
described in
getsockopt(2).
The select(2) or poll(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:
MSG_CMSG_CLOEXEC |
set the close on exec property for passed file descriptors |
MSG_OOB |
process out-of-band data |
MSG_PEEK |
peek at incoming message |
MSG_WAITALL |
wait for full request or error |
MSG_OOB
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. The MSG_PEEK
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. The MSG_WAITALL
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.
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 { void *msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ void *msg_control; /* ancillary data, see below */ socklen_t msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ };
Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as a null pointer if no names are desired or required. If the socket is connected, msg_name and msg_namelen are ignored. msg_iov and msg_iovlen describe scatter gather locations, as discussed in read(2). msg_control, which has length msg_controllen, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form:
struct cmsghdr { socklen_t cmsg_len; /* data byte count, including hdr */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by u_char cmsg_data[]; */ };
accept
()
call.
Open file descriptors are now passed as ancillary data for
AF_LOCAL
domain sockets, with
cmsg_level set to SOL_SOCKET
and cmsg_type set to
SCM_RIGHTS
.
The msg_flags field is set on return
according to the message received. 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 were 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 were received.
RETURN VALUES
The recv
(),
recvfrom
() and recvmsg
()
calls return the number of bytes received, or -1 if an error occurred. For
connected sockets whose remote peer was shut down, 0 is returned when no
more data is available. The recvmmsg
() call returns
the number of messages received, or -1 if an error occurred.
ERRORS
The calls fail if:
- [
EAGAIN
] - The socket is marked non-blocking, and the receive operation would block, or a receive timeout had been set, and the timeout expired before data were received.
- [
EBADF
] - The argument s is an invalid descriptor.
- [
EFAULT
] - The receive buffer pointer(s) point outside the process's address space.
- [
EINTR
] - The receive was interrupted by delivery of a signal before any data were available.
- [
EINVAL
] - The total length of the I/O is more than can be expressed by the ssize_t return value.
- [
ENOBUFS
] - A message was not delivered because it would have overflowed the buffer.
- [
ENOTCONN
] - The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
- [
ENOTSOCK
] - The argument s does not refer to a socket.
recvmsg
() will also fail if:
- [
EMSGSIZE
] - The msg_iovlen member of the
msg structure is less than or equal to 0, or is
greater than {
IOV_MAX
}.
SEE ALSO
fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2)
HISTORY
The recv
() function call appeared in
4.2BSD. The recvmmsg
()
function call appeared in Linux 2.6.32 and NetBSD
7.0.