NAME
recv
, recvfrom
,
recvmsg
, recvmmsg
—
receive a message from a
socket
SYNOPSIS
#include
<sys/socket.h>
ssize_t
recv
(int
s, void *buf,
size_t len,
int flags);
ssize_t
recvfrom
(int
s, void *buf,
size_t len,
int flags,
struct sockaddr *from,
socklen_t *fromlen);
ssize_t
recvmsg
(int
s, struct msghdr
*msg, int
flags);
int
recvmmsg
(int
s, struct mmsghdr
*mmsg, unsigned int
vlen, int flags,
struct timespec
*timeout);
DESCRIPTION
recv
(),
recvfrom
(),
recvmsg
(), and recvmmsg
()
are used to receive messages from a socket, s.
recv
() is normally used only on a
connected
socket (see connect(2) ). recvfrom
(),
recvmsg
(), and recvmmsg
()
may be used to receive data on a socket whether or not it is
connection-oriented.
recv
()
is identical to
recvfrom
()
with a null from parameter.
If from is non-null 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.
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
.
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) system calls may be used to determine when more data arrive.
The flags argument is the bitwise OR of zero or more of the following values:
MSG_OOB
- process out-of-band data
MSG_PEEK
- peek at incoming message
MSG_WAITALL
- wait for full request or error
MSG_DONTWAIT
- don't block
MSG_CMSG_CLOEXEC
- set the close-on-exec flag on received file descriptors
The 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
MSG_DONTWAIT
flag requests the call to return when
it would block otherwise. If no data is available,
errno is set to EAGAIN
. This
flag is not available in strict ANSI or C99 compilation mode. The
MSG_CMSG_CLOEXEC
requests that any file descriptors
received as ancillary data with
recvmsg
()
and recvmmsg
() (see below) have their close-on-exec
flag set.
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 */ unsigned 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. 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[]; */ };
See CMSG_DATA(3) for how these messages are constructed and decomposed.
Open file descriptors are now passed as ancillary data for
AF_UNIX
domain and
socketpair(2) 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. It will contain zero or more of the following values:
MSG_OOB
- Returned to indicate that expedited or out-of-band data was 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_BCAST
- Indicates that the packet was received as broadcast.
MSG_MCAST
- Indicates that the packet was received as multicast.
The
recvmmsg
()
call uses an array of the mmsghdr structure of length
vlen to group multiple msghdr
structures into a single system call. vlen is capped
at maximum 1024
messages that are received in a
single call. The flags field allows setting
MSG_WAITFORONE
to wait for one
msghdr, and set MSG_DONTWAIT
for all subsequent messages. A provided timeout limits
the time spent in the function but it does not limit the time spent in lower
parts of the kernel.
The mmsghdr structure has the following
form, as defined in
<sys/socket.h>
:
struct mmsghdr { struct msghdr msg_hdr; unsigned int msg_len; };
Here msg_len indicated the number of bytes received for each msg_hdr member.
RETURN VALUES
The recv
(),
recvfrom
(), and recvmsg
()
calls return the number of bytes received, or -1 if an error occurred. The
recvmmsg
() call returns the number of messages
received, or -1 if an error occurred before the first message has been
received.
ERRORS
recv
(),
recvfrom
(), recvmsg
(), and
recvmmsg
() fail if:
- [
EBADF
] - The argument s is an invalid descriptor.
- [
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.
- [
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.
- [
EINTR
] - The receive was interrupted by delivery of a signal before any data were available.
- [
EFAULT
] - The receive buffer pointer(s) point outside the process's address space.
- [
EHOSTUNREACH
] - A socket operation was attempted to an unreachable host.
- [
EHOSTDOWN
] - A socket operation failed because the destination host was down.
- [
ENETDOWN
] - A socket operation encountered a dead network.
- [
ECONNREFUSED
] - The socket is associated with a connection-oriented protocol and the connection was forcefully rejected (see connect(2)).
In addition, recv
() and
recvfrom
() may return the following error:
- [
EINVAL
] - len was larger than
SSIZE_MAX
.
And recvmsg
() and
recvmmsg
() may return one of the following
errors:
- [
EINVAL
] - The sum of the iov_len values in the msg_iov array overflowed an ssize_t.
- [
EMSGSIZE
] - The msg_iovlen member of msg
was less than 0 or larger than
IOV_MAX
. - [
EMSGSIZE
] - The receiving program did not have sufficient free file descriptor slots.
The descriptors are closed and any pending data can be returned by another
call to
recvmsg
().
SEE ALSO
connect(2), fcntl(2), getsockopt(2), poll(2), read(2), select(2), socket(2), socketpair(2), CMSG_DATA(3), sockatmark(3)
STANDARDS
The recv
(),
recvfrom
(), and recvmsg
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”). The MSG_DONTWAIT
,
MSG_BCAST
, and MSG_MCAST
flags are extensions to that specification.
HISTORY
The recv
() function call appeared in
4.1cBSD. The recvmmsg
()
syscall first appeared in Linux 2.6.33 and was added to
OpenBSD 7.2.
CAVEATS
Calling recvmsg
() with a control message
having no or an empty scatter/gather array exposes variations in
implementations. To avoid these, always use an iovec
with at least a one-byte buffer and set msg_iov and an
msg_iovlen to use this vector.