| RECV(2) | System Calls Manual | RECV(2) |
recv, recvfrom,
recvmsg — receive a message
from a socket
Standard C Library (libc, -lc)
#include
<sys/socket.h>
ssize_t
recv(int socket,
void *buffer, size_t length,
int flags);
ssize_t
recvfrom(int socket,
void *restrict buffer, size_t
length, int flags, struct
sockaddr *restrict address, socklen_t *restrict
address_len);
ssize_t
recvmsg(int socket,
struct msghdr *message, int
flags);
The
recvfrom()
and recvmsg() system calls 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 address is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The address_len argument is a value-result argument, initialized to the size of the buffer associated with address, and modified on return to indicate the actual size of the address stored there.
The
recv()
function is normally used only on a
connected
socket (see connect(2) or connectx(2))
and is identical to recvfrom() with a null pointer
passed as its address argument. As it is redundant, it
may not be supported 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 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) system call may be used to determine when more data arrive.
If no messages are available to be received and the peer has performed an orderly shutdown, the value 0 is returned.
The flags argument to a
recv()
function 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 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()
system call uses a msghdr structure to minimize the
number of directly supplied arguments. This structure has the following
form, as defined in
<sys/socket.h>:
struct msghdr {
void *msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* # elements in msg_iov */
void *msg_control; /* ancillary data, see below */
socklen_t 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.
The msg_iov and msg_iovlen arguments describe scatter gather locations, as discussed in read(2). msg_iovlen shall be set to the dimension of this array. In each iovec structure, the iov_base field specifies a storage area and the iov_len field gives its size in bytes. Each storage area indicated by msg_iov is filled with received data in turn until all of the received data is stored or all of the areas have been filled.
The msg_control argument, 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()
system 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 message received. MSG_EOR indicates
end-of-record; the data returned completed a record.
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.
These calls return the number of bytes received, or -1 if an error occurred.
For TCP sockets, the return value 0 means the peer has closed its half side of the connection.
The calls fail if:
EAGAIN]EBADF]ECONNRESET]EFAULT]EINTR]EINVAL]ENOBUFS]ENOTCONN]ENOTSOCK]EOPNOTSUPP]ETIMEDOUT]The recvfrom() call may also fail if:
EINVAL]The recvmsg() call may also fail if:
The recv() function appeared in
4.2BSD.
| March 18, 2015 | Mac OS X 12 |