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@(#)recv.2 6.6 (Berkeley) 02/14/89

C 5 recv, recvfrom, recvmsg - receive a message from a socket

#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
 Recv ,  recvfrom , and
 recvmsg are used to receive messages from a socket.

The 
 recv call is normally used only on a 
 connected socket (see
 connect (2)), while 
 recvfrom and 
 recvmsg 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 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 */



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 */
 int msg_namelen; /* size of address */
 struct iovec *msg_iov; /* scatter/gather array */
 int msg_iovlen; /* # elements in msg_iov */
 caddr_t msg_accrights; /* access rights sent/received */
 int msg_accrightslen;
};



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). A buffer to receive any access rights sent along with the message is specified
in 
 msg_accrights , which has length
 msg_accrightslen . Access rights are currently limited to file descriptors,
which each occupy the size of an
 int . If access rights are not being transferred, the 
 msg_accrights field should be set to NULL.
 "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)