RECV(2) FreeBSD System Calls Manual RECV(2)
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, the SO_TIMESTAMP socket option returns a reception
timestamp for UDP packets.
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.
FreeBSD 13.1-RELEASE-p6 August 19, 2018 FreeBSD 13.1-RELEASE-p6
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