libc/sys/recv.2

#include <sys/types.h>
#include <sys/socket.h>

cc = recv(s, buf, len, flags)
int cc, s;
char *buf;
int len, flags;

cc = recvfrom(s, buf, len, flags, from, fromlen)
int cc, s;
char *buf;
int len, flags;
struct sockaddr *from;
int *fromlen;

cc = recvmsg(s, msg, flags)
int cc, 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
it is in a connected state or not.

If
 from is non-zero, 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 identitical to
 recfrom with a zero-valued
 fromlen parameter.
As it is redundant, it may not be supported in future releases.

The length of the message is returned in
 cc . 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
 ioctl (2)) in which case a
 cc of -1 is returned with the external variable errno
set to EWOULDBLOCK.

The
 select (2) call may be used to determine when more data arrives.

The
 flags argument to a recv call is formed by
 or 'ing one or more of the values,


#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 */


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.
The
 msg_iov and
 msg_iovlen describe the scatter gather locations, as described in
 read (2).  msg_control , which has length
 msg_controllen . This is 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 uio provided immediately after an
 accept (), 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
SCM_RIGHTS.

 msg_flags 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.

 "RETURN VALUE"
These calls return the number of bytes received, or -1
if an error occurred.
 ERRORS
The calls fail if:
 20
[EBADF]
The argument s is an invalid descriptor.
 20
[ENOTSOCK]
The argument s is not a socket.
 20
[EWOULDBLOCK]
The socket is marked non-blocking and the receive operation
would block.
 20
[EINTR]
The receive was interrupted by delivery of a signal before
any data was available for the receive.
 20
[EFAULT]
The data was specified to be received into a non-existent
or protected part of the process address space.
 SEE ALSO
fcntl(2), read(2), send(2), select(2), getsockopt(2), socket(2)