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RECV(2) BSD System Calls Manual RECV(2)
NAME
recv, recvfrom, recvmsg -- receive a message from a socket
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
ssize_t
recv(int socket, void *buffer, size_t length, int flags);
ssize_t
recvfrom(int socket, void *restrict buffer, size_t length, int flags,
struct sockaddr *restrict address, socklen_t *restrict address_len);
ssize_t
recvmsg(int socket, struct msghdr *message, int flags);
DESCRIPTION
The recvfrom() and recvmsg() 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 address is not a null pointer and the socket is not connection-ori-ented, connection-oriented,
ented, the source address of the message is filled in. The address_len
argument is a value-result argument, initialized to the size of the
buffer associated with address, 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 address argument. As it is redundant, it may not be supported in
future releases.
All three routines return the length of the message on successful comple-tion. completion.
tion. 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. 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) system call may be used to determine when more data arrive.
If no messages are available to be received and the peer has performed an
orderly shutdown, the value 0 is returned.
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
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 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 destination 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 ancil-lary ancillary
lary data. The messages are of the form:
struct cmsghdr {
u_int 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 request-ing requesting
ing a recvmsg() with no data buffer provided immediately after an
accept() system call.
Open file descriptors are now passed as ancillary data for AF_UNIX 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 indi-cates indicates
cates 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 outof-band
of-band data were received.
RETURN VALUES
These calls return the number of bytes received, or -1 if an error
occurred.
For TCP sockets, the return value 0 means the peer has closed its half
side of the connection.
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 socket is an invalid descriptor.
[ECONNRESET] The connection is closed by the peer during a receive
attempt on a socket.
[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] MSG_OOB is set, but no out-of-band data is available.
[ENOBUFS] An attempt to allocate a memory buffer fails.
[ENOTCONN] The socket is associated with a connection-oriented
protocol and has not been connected (see connect(2)
and accept(2)).
[ENOTSOCK] The argument socket does not refer to a socket.
[EOPNOTSUPP] The type and/or protocol of socket do not support the
option(s) specified in flags.
[ETIMEDOUT] The connection timed out.
The recvfrom() call may also fail if:
[EINVAL] The total of the iov_len values overflows a ssize_t.
The recvmsg() call may also fail if:
[EMSGSIZE] The requested message size is invalid.
[ENOMEM] Insufficient memory is available.
SEE ALSO
fcntl(2), getsockopt(2), read(2), select(2), socket(2)
HISTORY
The recv() function appeared in 4.2BSD.
BSD May 15, 2006 BSD
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