RECV(2) System Calls RECV(2)
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, size_t len, unsigned int flags);
int recvfrom (int s, void *buf, size_t len, unsigned int flags, struct
sockaddr *from, int *fromlen);
int recvmsg (int s, struct msghdr *msg, unsigned 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 connec‐
tion-oriented.
If from is non-NULL, 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 con
nect(2)) and is identical to recvfrom with a NULL from parameter. As
it is redundant, it may not be supported in future releases.
All three routines return the length of the message on successful com‐
pletion. If a message is too long to fit in the supplied buffer, ex‐
cess 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) 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 opera‐
tion to return data from the beginning of the receive queue without re‐
moving 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 dis‐
connect 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 di‐
rectly 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-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 do‐
main 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 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 buf‐
fer 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 oc‐
curred.
ERRORS
The calls fail if:
EBADF The argument s is an invalid descriptor.
ENOTCONN
The socket is associated with a connection-oriented pro‐
tocol and has not been connected (see connect(2) and ac
cept(2)).
ENOTSOCK
The argument s does not refer to a socket.
EAGAIN The socket is marked non-blocking, and the receive opera‐
tion 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 be‐
fore any data were available.
EFAULT The receive buffer pointer(s) point outside the process's
address space.
CONVORMANCE
The GNO prototypes of these routines differ slightly from that of
4.4BSD.
SEE ALSO
fcntl(2), read(2), select(2), getsockopt(2), socket(2)
HISTORY
The recvmsg function call appeared in 4.2BSD. The first appearance in
GNO was in v2.0.5.
GNO 16 January 1997 RECV(2)
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