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
int recvfrom(int s, void *buf, int len struct sockaddr
*from, int *fromlen));
int recvmsg(int s, struct msghdr *msg, unsigned int flags
DESCRIPTION
The 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-
oriented, 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 supported
in future releases.
All three routines return the length of the message on suc-
cessful completion. 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 external variable errno set to EWOULDBLOCK.
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) 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:
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 subse-
quent 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 call uses a msghdr structure to minimize
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 readv(2). Msg_control, which has length
msg_controllen, points to a buffer for other protocol con-
trol 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-
connection-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-oriented
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 signal
before any data were available.
EFAULT The receive buffer pointer(s) point outside the
process's address space.
CONFORMING TO
4.4BSD (these function calls first appeared in 4.2BSD).
SEE ALSO
fcntl(2), read(2), select(2),