Provided by: freebsd-manpages_12.2-1_all 
NAME
recv, recvfrom, recvmsg, recvmmsg — receive message(s) from a socket
LIBRARY
Standard C Library (libc, -lc)
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 * restrict from,
socklen_t * restrict fromlen);
ssize_t
recvmsg(int s, struct msghdr *msg, int flags);
ssize_t
recvmmsg(int s, struct mmsghdr * restrict msgvec, size_t vlen, int flags,
const struct timespec * restrict timeout);
DESCRIPTION
The recvfrom(), recvmsg(), and 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.
If from is not a null pointer and the socket is not connection-oriented, the source address of the
message is filled in. The fromlen argument is a value-result argument, 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 recv() function is normally used only on a connected socket (see connect(2)) and is identical to
recvfrom() with a null pointer passed as its from argument.
The recvmmsg() function is used to receive multiple messages at a call. Their number is supplied by
vlen. The messages are placed in the buffers described by msgvec vector, after reception. The size of
each received message is placed in the msg_len field of each element of the vector. If 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 ppoll(2) system call is used to implement the timeout mechanism, before first receive
is performed.
The recv(), recvfrom() and recvmsg() return the length of the message on successful completion, whereas
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
socket(2)).
If no messages are available at the socket, the receive call waits for a message to arrive, unless the
socket is non-blocking (see fcntl(2)) in which case the value -1 is returned and the global variable
errno is set to EAGAIN. The receive calls except 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 SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2). The
recvmmsg() function implements this behaviour for each message in the vector.
The select(2) system call may be used to determine when more data arrives.
The flags argument to a recv() function is formed by or'ing one or more of the values:
MSG_OOB process out-of-band data
MSG_PEEK peek at incoming message
MSG_WAITALL wait for full request or error
MSG_DONTWAIT do not block
MSG_CMSG_CLOEXEC set received fds close-on-exec
MSG_WAITFORONE do not block after receiving the first message (only for recvmmsg() )
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 ANSI X3.159-1989 (“ANSI C89”) or
ISO/IEC 9899:1999 (“ISO C99”) compilation mode. The MSG_WAITFORONE flag sets MSG_DONTWAIT after the
first message has been received. This flag is only relevant for recvmmsg().
The recvmsg() system call uses a msghdr structure to minimize the number of directly supplied arguments.
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. The msg_iov and msg_iovlen arguments
describe scatter gather locations, as discussed in read(2). The msg_control argument, 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[]; */
};
As an example, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain
user-connection-request data by requesting a recvmsg() with no data buffer provided immediately after an
accept() system call.
With AF_UNIX domain sockets, ancillary data can be used to pass file descriptors and process credentials.
See unix(4) for details.
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.
The recvmmsg() system call uses the mmsghdr structure, defined as follows in the <sys/socket.h> header:
struct mmsghdr {
struct msghdr msg_hdr; /* message header */
ssize_t msg_len; /* message length */
};
On data reception the msg_len field is updated to the length of the received message.
RETURN VALUES
These calls except recvmmsg() return the number of bytes received. recvmmsg() returns the number of
messages received. A value of -1 is returned if an error occurred.
ERRORS
The calls fail if:
[EBADF] The argument s is an invalid descriptor.
[ECONNRESET] The remote socket end is forcibly closed.
[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.
[EMSGSIZE] The 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, any
pending data can be returned by another call to recvmsg().
[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.
SEE ALSO
fcntl(2), getsockopt(2), read(2), select(2), socket(2), CMSG_DATA(3), unix(4)
HISTORY
The recv() function appeared in 4.2BSD. The recvmmsg() function appeared in FreeBSD 11.0.
Debian August 19, 2018 RECV(2)