recv
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NAME
recv, recvfrom, recvmsg - receive a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int recv(int s, void *buf, int len, unsigned int flags);
int recvfrom(int s, void *buf, int len, unsigned int flags
struct sockaddr *from, int *fromlen);
int recvmsg(int s, struct msghdr *msg, unsigned int
flags);
DESCRIPTION
WARNING: This is a BSD man page. As of Linux 0.99.11,
recvmsg was not implemented.
Recvfrom and recvmsg 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 non-nil, and the socket is not connection-ori-
ented, 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.
The recv call is normally used only on a connected socket
(see connect(2)) and is identical to recvfrom with a nil
from parameter. As it is redundant, it may not be sup-
ported in future releases.
All three routines return the length of the message on
successful completion. If a message is too long to fit in
the supplied buffer, excess bytes may be discarded depend-
ing 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 EWOULD-
BLOCK. The receive calls normally return any data avail-
able, 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) call may be used to determine when more data
arrive.
The flags argument to a recv call 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 satis-
fied. 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 call uses a msghdr structure to mini-
mize the number of directly supplied parameters.
This structure has the following form, as defined
in sys/socket.h:
struct msghdr {
caddr_t msg_name; /* optional address */
u_int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
u_int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_control; /* ancillary data, see below */
u_int 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. 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 {
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-con-
nection-request data by requesting a recvmsg with no data
buffer provided immediately after an accept 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 mes-
sage 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.
RETURN VALUES
These calls return the number of bytes received, or -1 if
an error occurred.
ERRORS
EBADF The argument s is an invalid descriptor.
ENOTCONN
The socket is associated with a connection-ori-
ented protocol and has not been connected (see
connect(2) and accept(2)).
ENOTSOCK
The argument s does not refer to a socket.
EWOULDBLOCK
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 sig-
nal before any data were available.
EFAULT The receive buffer pointer(s) point outside the
process's address space.
HISTORY
These function calls appeared in BSD 4.2.
SEE ALSO
fcntl(2), read(2), select(2), getsockopt(2), socket(2)
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Copyright (C) 1998
Hurricane Electric.
All Rights Reserved.