RECV(2)

NAME
recv, recvfrom, recvmsg - receive a message from a socket

SYNOPSIS
#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:

[EBADF]
    The argument s is an invalid descriptor.

[ENOTSOCK]
    The argument s is not a socket.

[EWOULDBLOCK]
    The socket is marked non-blocking and the receive operation would block.

[EINTR]
    The receive was interrupted by delivery of a signal before any data was available for the receive.

[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)