RECV(2) — SYSTEM CALLS

NAME

recv, recvfrom, recvmsg − receive a message from a socket

SYNOPSIS

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

int recv(s, buf, len, flags)
int s;
char ∗buf;
int len, flags;

int recvfrom(s, buf, len, flags, from, fromlen)
int s;
char ∗buf;
int len, flags;
struct sockaddr ∗from;
int ∗fromlen;

int recvmsg(s, msg, flags)
int s;
struct msghdr ∗msg;
int flags;

DESCRIPTION

s is a socket created with socket(2). recv, recvfrom, and recvmsg are used to receive messages from another socket. recv() 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 not a NULL pointer, 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. 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 −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 parameter is formed by ORing one or more of the following:

MSG_OOB Read any “out-of-band” data present on the socket, rather than the regular “in-band” data.

MSG_PEEK “Peek” at the data present on the socket; the data is returned, but not consumed, so that a subsequent receive operation will see the same data.

The recvmsg() call uses a msghdr structure to minimize the number of directly supplied parameters. This structure is defined in /usr/include/sys/socket.h, and includes the following members:

caddr_tmsg_name;/∗ optional address ∗/
intmsg_namelen;/∗ size of address ∗/
struct iovec∗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(2V). A buffer to receive any access rights sent along with the message is 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 s is an invalid descriptor.

ENOTSOCK s is a descriptor for a file, not a socket.

EINTR The operation was interrupted by delivery of a signal before any data was available to be received.

EFAULT The data was specified to be received into a non-existent or protected part of the process address space.

EWOULDBLOCK The socket is marked non-blocking and the requested operation would block.

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