recv.2 (2010 09)

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recv(2) recv(2)

NAME

recv(), recvfrom(), recvmsg() - receive a message from a socket

SYNOPSIS

#include <sys/socket.h>

int recv(int s, void *buf, int len, int flags);

int recvfrom(

int s,

void *buf,

int len,

int flags,

void *from,

int *fromlen

);

int recvmsg(int s, struct msghdr msg[], int flags);

UNIX 03 Only (X/Open Sockets)

ssize_t recv(int s, void *buf, size_t len, int flags);

ssize_t recvfrom(

int s,

void *__restrict buf,

size_t len,

int flags,

struct sockaddr *__restrict from,

socklen_t *__restrict fromlen

);

ssize_t recvmsg(int s, struct msghdr *msg, int flags);

Obsolescent UNIX 95 Only (X/Open Sockets)

ssize_t recvfrom(

int s,

void *buf,

size_t len,

int flags,

struct sockaddr *from,

size_t *fromlen

);

DESCRIPTION

The recv(), recvfrom(), and recvmsg() system calls are used to receive messages from a socket.

s is a socket descriptor from which messages are received.

buf is a pointer to the buffer into which the messages are placed.

len is the maximum number of bytes that can ﬁt in the buffer referenced by buf.

If the socket uses connection-based communications, such as a SOCK_STREAM socket, these calls can

only be used after the connection has been established (see connect (2)). For connectionless sockets such

as SOCK_DGRAM, these calls can be used whether a connection has been speciﬁed or not.

recvfrom() operates in the same manner as recv() except that it is able to return the address of the

socket from which the message was sent. For connected datagram sockets, recvfrom() simply returns

the same address as getpeername() (see getpeername (2)). For stream sockets, recvfrom()

retrieves data in the same manner as recv(), but does not return the socket address of the sender. If

from is nonzero, the source address of the message is placed in the socket address structure pointed to by

from. fromlen is a value-result parameter, initialized to the size of the structure associated with from,

and modiﬁed on return to indicate the actual size of the address stored there. If the memory pointed to

by from is not large enough to contain the entire address, only the ﬁrst fromlen bytes of the address are

returned.

For message-based sockets such as SOCK_DGRAM, the entire message must be read in a single opera-

tion. If a message is too long to ﬁt in the supplied buffer, the excess bytes are discarded. For stream-

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (6 pages)

PAGE 1
recv(2) recv(2) NAME recv(), recvfrom(), recvmsg() - receive a message from a socket SYNOPSIS #include
PAGE 2
recv(2) recv(2) based sockets such as SOCK_STREAM, there is no concept of message boundaries. In this case, data is returned to the user as soon as it becomes available, and no data is discarded. See the AF_CCITT Only subsection below for a list of the exceptions to this behavior for connections in the address family AF_CCITT. recvmsg() performs the same action as recv(), but scatters the read data into the buffers specified in the msghdr structure (see X/Open Sockets Only below).
PAGE 3
recv(2) recv(2) • If the remote side of a connection-based socket has performed an orderly shutdown and there is no more data to read (the socket has reached the end of its data stream), recv() returns 0. • If the remote side of a connection-based socket has reset the connection, recv() returns −1 and sets errno to [ECONNRESET]. • If an error occurs, recv() returns −1 and errno is set to indicate the error. If the O_NDELAY flag is set using fcntl() (defined in
PAGE 4
recv(2) recv(2) • The MSG_OOB flag is supported. • The MSG_PEEK flag is supported; the two flags can be combined. • If a message is received that is larger than the user-controlled maximum message size, the X.25 subsystem RESETs the circuit, discards the data, and sends the out-of-band event OOB_VC_MESSAGE_TOO_BIG to the socket.
PAGE 5
recv(2) recv(2) The flags parameter accepts a new value, MSG_WAITALL, which requests that the function block until the full amount of data requested can be returned. The function may return a smaller amount of data if a signal is caught, the connection is terminated, or an error is pending for the socket. On successful completion of recvmsg(), the msg_flags member of the message header is the bitwiseinclusive OR of all of the following flags that indicate conditions detected for the received message.
PAGE 6
recv(2) recv(2) [ENOSR] Buffers could not be allocated for the message that was to be created due to insufficient STREAMS memory resources. [ENOTCONN] Receive on a SOCK_STREAM socket that is not yet connected. [ENOTSOCK] The argument s is a valid file descriptor, but it is not a socket. [EOPNOTSUPP] The MSG_OOB flag was set for a UDP SOCK_DGRAM message-based socket, or MSG_OOB or MSG_PEEK was set for any AF_UNIX socket. The MSG_OOB flag is supported only for stream-based TCP SOCK_STREAM sockets.