.\" Copyright (c) 1983, 1990 The Regents of the University of California.
.\" %sccs.include.redist.man%
.\" @(#)recv.2 6.8 (Berkeley) %G%
recv, recvfrom, recvmsg \- receive a message from a socket
cc = recv(s, buf, len, flags)
cc = recvfrom(s, buf, len, flags, from, fromlen)
cc = recvmsg(s, msg, flags)
are used to receive messages from a socket,
and may be used to receive data on a socket whether
it is in a connected state or not.
is non-zero, the source address of the message is filled in.
is a value-result parameter, initialized to the size of
the buffer associated with
and modified on return to indicate the actual size of the
call is normally used only on a
As it is redundant, it may not be supported in future releases.
The length of the message is returned in
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
If no messages are available at the socket, the
receive call waits for a message to arrive, unless
the socket is nonblocking (see
of \-1 is returned with the external variable errno
call may be used to determine when more data arrives.
argument to a recv call is formed by
one or more of the values,
.ta \w'#define\ \ 'u +\w'MSG_DONTROUTE\ \ \ 'u +\w'0x\0\0\0\ \ 'u
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_PEEK 0x2 /* peek at incoming message */
#define MSG_DONTROUTE 0x4 /* send without using routing tables */
#define MSG_EOR 0x8 /* data completes record */
#define MSG_TRUNC 0x10 /* data discarded before delivery */
#define MSG_CTRUNC 0x20 /* control data lost before delivery */
structure to minimize the number of directly supplied parameters.
This structure has the following form, as defined in
.ta \w'struct 'u +\w'caddr_t 'u +\w'msg_controllen 'u
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 */
specify the destination address if the socket is unconnected;
may be given as a null pointer if no names are desired or required.
describe the scatter gather locations, as described in
This is a buffer for other protocol control related messages
or other miscellaneous ancillary data.
The messages are of the form:
.ta \w'struct 'u +\w'u_char 'u +\w'msg_controllen 'u
u_int cmsg_len; /* data byte count, including hdr */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
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 uio provided immediately after an
thought of here in the sense of get-next-connection-request without
an implicit connection confirmation.
Open file descriptors are now passed as ancillary data for AF_UNIX
domain sockets, with cmsg_level being SOL_SOCKET and cmsg_type being
is set on return in a way that may include some of the same values specified
for the flags parameter to a recv system call.
The returned values MSG_EOR indicates end-of-record, MSG_TRUNC indicates that
some trailing datagram data was discarded, MSG_CTRUNC indicates that some
control data was discarded due to lack of space.
MSG_OOB is returned to indicate that expedited data was received.
These calls return the number of bytes received, or \-1
The argument \fIs\fP is an invalid descriptor.
The argument \fIs\fP is not a socket.
The socket is marked non-blocking and the receive operation
The receive was interrupted by delivery of a signal before
any data was available for the receive.
The data was specified to be received into a non-existent
or protected part of the process address space.
fcntl(2), read(2), send(2), select(2), getsockopt(2), socket(2)
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