lib/libsys/recv.2

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.Dd May 17, 2025
.Dt RECV 2
.Os
.Sh NAME
.Nm recv ,
.Nm recvfrom ,
.Nm recvmsg ,
.Nm recvmmsg
.Nd receive message(s) from a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/socket.h
.Ft ssize_t
.Fn recv "int s" "void *buf" "size_t len" "int flags"
.Ft ssize_t
.Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen"
.Ft ssize_t
.Fn recvmsg "int s" "struct msghdr *msg" "int flags"
.Ft ssize_t
.Fn recvmmsg "int s" "struct mmsghdr * restrict msgvec" "size_t vlen" "int flags" "const struct timespec * restrict timeout"
.Sh DESCRIPTION
The
.Fn recvfrom ,
.Fn recvmsg ,
and
.Fn recvmmsg
system calls
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.
.Pp
If
.Fa from
is not a null pointer
and the socket is not connection-oriented,
the source address of the message is filled in.
The
.Fa fromlen
argument
is a value-result argument, initialized to the size of
the buffer associated with
.Fa from ,
and modified on return to indicate the actual size of the
address stored there.
.Pp
The
.Fn recv
function is normally used only on a
.Em connected
socket (see
.Xr connect 2 )
and is identical to
.Fn recvfrom
with a
null pointer passed as its
.Fa from
argument.
.Pp
The
.Fn recvmmsg
function is used to receive multiple
messages at a call.
Their number is supplied by
.Fa vlen .
The messages are placed in the buffers described by
.Fa msgvec
vector, after reception.
The size of each received message is placed in the
.Fa msg_len
field of each element of the vector.
If
.Fa timeout
is NULL the call blocks until the data is available for each
supplied message buffer.
Otherwise it waits for data for the specified amount of time.
If the timeout expired and there is no data received,
a value 0 is returned.
The
.Xr ppoll 2
system call is used to implement the timeout mechanism,
before first receive is performed.
.Pp
The
.Fn recv ,
.Fn recvfrom
and
.Fn recvmsg
return the length of the message on successful
completion, whereas
.Fn recvmmsg
returns the number of received messages.
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
.Xr socket 2 ) .
.Pp
If no messages are available at the socket, the
receive call waits for a message to arrive, unless
the socket is non-blocking (see
.Xr fcntl 2 )
in which case the value
\-1 is returned and the global variable
.Va errno
is set to
.Er EAGAIN .
The receive calls except
.Fn recvmmsg
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
.Dv SO_RCVLOWAT
and
.Dv SO_RCVTIMEO
described in
.Xr getsockopt 2 .
The
.Fn recvmmsg
function implements this behaviour for each message in the vector.
.Pp
The
.Xr select 2
system call may be used to determine when more data arrives.
.Pp
The
.Fa flags
argument to a
.Fn recv
function is formed by
.Em or Ap ing
one or more of the values:
.Bl -column ".Dv MSG_CMSG_CLOEXEC" -offset indent
.It Dv MSG_OOB Ta process out-of-band data
.It Dv MSG_PEEK Ta peek at incoming message
.It Dv MSG_TRUNC Ta return real packet or datagram length
.It Dv MSG_WAITALL Ta wait for full request or error
.It Dv MSG_DONTWAIT Ta do not block
.It Dv MSG_CMSG_CLOEXEC Ta set received fds close-on-exec
.It Dv MSG_CMSG_CLOFORK Ta set received fds close-on-fork
.It Dv MSG_WAITFORONE Ta do not block after receiving the first message
(only for
.Fn recvmmsg
)
.El
.Pp
The
.Dv 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
.Dv 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
.Dv MSG_TRUNC
flag causes the receive operation to return the full length of the packet
or datagram even if larger than provided buffer. The flag is supported
on SOCK_DGRAM sockets for
.Dv AF_INET
,
.Dv AF_INET6
and
.Dv AF_UNIX
families.
The
.Dv 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
.Dv MSG_DONTWAIT
flag requests the call to return when it would block otherwise.
If no data is available,
.Va errno
is set to
.Er EAGAIN .
This flag is not available in
.St -ansiC
or
.St -isoC-99
compilation mode.
The
.Dv MSG_WAITFORONE
flag sets MSG_DONTWAIT after the first message has been received.
This flag is only relevant for
.Fn recvmmsg .
.Pp
The
.Fn recvmsg
system call uses a
.Fa msghdr
structure to minimize the number of directly supplied arguments.
This structure has the following form, as defined in
.In sys/socket.h :
.Bd -literal
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 */
};
.Ed
.Pp
Here
.Fa msg_name
and
.Fa msg_namelen
specify the source address if the socket is unconnected;
.Fa msg_name
may be given as a null pointer if no names are desired or required.
The
.Fa msg_iov
and
.Fa msg_iovlen
arguments
describe scatter gather locations, as discussed in
.Xr read 2 .
The
.Fa msg_control
argument,
which has length
.Fa msg_controllen ,
points to a buffer for other protocol control related messages
or other miscellaneous ancillary data.
The messages are of the form:
.Bd -literal
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[]; */
};
.Ed
.Pp
As an example, the SO_TIMESTAMP socket option returns a reception
timestamp for UDP packets.
.Pp
With
.Dv AF_UNIX
domain sockets, ancillary data can be used to pass file descriptors and
process credentials.
See
.Xr unix 4
for details.
.Pp
The
.Fa msg_flags
field is set on return according to the message received.
.Dv MSG_EOR
indicates end-of-record;
the data returned completed a record (generally used with sockets of type
.Dv SOCK_SEQPACKET ) .
.Dv MSG_TRUNC
indicates that
the trailing portion of a datagram was discarded because the datagram
was larger than the buffer supplied.
.Dv MSG_CTRUNC
indicates that some
control data were discarded due to lack of space in the buffer
for ancillary data.
.Dv MSG_OOB
is returned to indicate that expedited or out-of-band data were received.
.Pp
The
.Fn recvmmsg
system call uses the
.Fa mmsghdr
structure, defined as follows in the
.In sys/socket.h
header:
.Bd -literal
struct mmsghdr {
	struct msghdr	 msg_hdr;	/* message header */
	ssize_t		 msg_len;	/* message length */
};
.Ed
.Pp
On data reception the
.Fa msg_len
field is updated to the length of the received message.
.Sh RETURN VALUES
On successful completion, the
.Fn recv ,
.Fn recvfrom ,
and
.Fn recvmsg
functions return the number of bytes received, while the
.Fn recvmmsg
function returns the number of messages received.
If no messages are available to be received and the peer has
performed an orderly shutdown, 0 is returned.
Otherwise, -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The calls fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa s
is an invalid descriptor.
.It Bq Er ECONNRESET
The remote socket end is forcibly closed.
.It Bq Er ENOTCONN
The socket is associated with a connection-oriented protocol
and has not been connected (see
.Xr connect 2
and
.Xr accept 2 ) .
.It Bq Er ENOTSOCK
The argument
.Fa s
does not refer to a socket.
.It Bq Er EMFILE
The
.Fn recvmsg
system call
was used to receive rights (file descriptors) that were in flight on the
connection.
However, the receiving program did not have enough free file
descriptor slots to accept them.
In this case the descriptors are closed, with pending data either discarded
in the case of the unreliable datagram protocol or preserved in the case of a
reliable protocol.
The pending data can be retrieved with another call to
.Fn recvmsg .
.It Bq Er EMSGSIZE
The
.Fa msg_iovlen
member of the
.Fa msghdr
structure pointed to by
.Fa msg
is less than or equal to 0, or is greater than
.Va IOV_MAX .
.It Bq Er 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.
.It Bq Er EINTR
The receive was interrupted by delivery of a signal before
any data were available.
.It Bq Er EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr CMSG_DATA 3 ,
.Xr unix 4
.Sh HISTORY
The
.Fn recv
function appeared in
.Bx 4.2 .
The
.Fn recvmmsg
function appeared in
.Fx 11.0 .