RECV(2) BSD System Calls Manual RECV(2)
NAME
recv, recvfrom, recvmsg -- receive a message from a socket
SYNOPSIS
##include <>
##include <>
ssizet
recv(int s, void *buf, sizet len, int flags);
ssizet
recvfrom(int s, void *buf, sizet len, int flags, struct sockaddr *from,
socklent *fromlen);
ssizet
recvmsg(int s, struct msghdr *msg, int flags);
DESCRIPTION
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 parame-
ter, initialized to the size of the buffer associated with from, and mod-
ified 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.
On successful completion, all three routines return the number of message
bytes read. 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
SORCVLOWAT and SORCVTIMEO 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:
MSGOB process out-of-band data
MSGPEK peek at incoming message
MSGWAITAL wait for full request or error
The MSGOB 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 MSGPEK 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 MSGWAITAL 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 :
struct msghdr {
caddrt msgname; /* optional address */
socklent msgnamelen; /* size of address */
struct iovec *msgiov; /* scatter/gather array */
uint msgiovlen; /* # elements in msgiov */
caddrt msgcontrol; /* ancillary data, see below */
socklent msgcontrollen; /* ancillary data buffer len */
int msgflags; /* flags on received message */
};
Here msgname and msgnamelen specify the source address if the socket is
unconnected; msgname may be given as a null pointer if no names are
desired or required. Msgiov and msgiovlen describe scatter gather
locations, as discussed in read(2). Msgcontrol, which has length
msgcontrollen, points to a buffer for other protocol control related
messages or other miscellaneous ancillary data. The messages are of the
form:
struct cmsghdr {
uint cmsglen; /* data byte count, including hdr */
int cmsglevel; /* originating protocol */
int cmsgtype; /* protocol-specific type */
/* followed by
uchar cmsgdata[]; */
};
As an example, one could use this to learn of changes in the data-stream
in XNS/SP, or in ISO, to obtain user-connection-request data by request-
ing a recvmsg with no data buffer provided immediately after an accept()
call.
Open file descriptors are now passed as ancillary data for AFUNIX domain
sockets, with cmsglevel set to SOLSOCKET and cmsgtype set to
SCMRIGHTS.
The msgflags field is set on return according to the message received.
MSGEOR indicates end-of-record; the data returned completed a record
(generally used with sockets of type SOCKSEQPACKET). MSGTRUNC indi-
cates that the trailing portion of a datagram was discarded because the
datagram was larger than the buffer supplied. MSGCTRUNC indicates that
some control data were discarded due to lack of space in the buffer for
ancillary data. MSGOB 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.
ERORS
The calls fail if:
[EBADF] The argument s is an invalid descriptor.
[ENOTCON] 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.
[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), read(2), select(2), getsockopt(2), socket(2)
HISTORY
The recv() function call appeared in 4.2BSD.
4.3-Reno Berkeley Distribution February 21, 19944.3-Reno Berkeley Distribution
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