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 may be 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 send call is formed by or’ing one or more of the values,

#defineMSG_PEEK0x1/∗ peek at incoming message ∗/
#defineMSG_OOB0x2/∗ process out-of-band data ∗/

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_tmsg_name;/∗ optional address ∗/
intmsg_namelen;/∗ size of address ∗/
structiov ∗msg_iov;/∗ scatter/gather array ∗/
intmsg_iovlen;/∗ # elements in msg_iov ∗/
caddr_tmsg_accrights;/∗ access rights sent/received ∗/
intmsg_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). Access rights to be sent along with the message are specified in msg_accrights, which has length msg_accrightslen.

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]

NAME

acct − turn accounting on or off

SYNOPSIS

acct(file)
char ∗file;

DESCRIPTION

The system is prepared to write a record in an accounting file for each process as it te

4th Berkeley Distribution  —  1 August 1985