NAME

recvfrom - receive a message from a socket

SYNOPSIS

#include <sys/socket.h>

ssize_t recvfrom(int socket, void *buffer, size_t length, int flags,
             struct sockaddr *address, socklen_t *address_len);

DESCRIPTION

The recvfrom() function receives a message from a connection-mode or connectionless-mode socket. It is normally used with connectionless-mode sockets because it permits the application to retrieve the source address of received data.

The function takes the following arguments:

socket

Specifies the socket file descriptor.

buffer

Points to the buffer where the message should be stored.

length

Specifies the length in bytes of the buffer pointed to by the buffer argument.

flags

Specifies the type of message reception. Values of this argument are formed by logically OR'ing zero or more of the following values:

MSG_PEEK

Peeks at an incoming message. The data is treated as unread and the next recvfrom() or similar function will still return this data.

MSG_OOB

Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific.

MSG_WAITALL

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, if the connection is terminated, if MSG_PEEK was specified, or if an error is pending for the socket.

address

A null pointer, or points to a sockaddr structure in which the sending address is to be stored. The length and format of the address depend on the address family of the socket.

address_len

Specifies the length of the sockaddr structure pointed to by the address argument.

The recvfrom() function returns the length of the message written to the buffer pointed to by the buffer argument. For message-based sockets such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message must be read in a single operation. If a message is too long to fit in the supplied buffer, and MSG_PEEK is not set in the flags argument, the excess bytes are discarded. For stream-based sockets such as SOCK_STREAM, message boundaries are ignored. In this case, data is returned to the user as soon as it becomes available, and no data is discarded.

If the MSG_WAITALL flag is not set, data will be returned only up to the end of the first message.

Not all protocols provide the source address for messages. If the address argument is not a null pointer and the protocol provides the source address of messages, the source address of the received message is stored in the sockaddr structure pointed to by the address argument, and the length of this address is stored in the object pointed to by the address_len argument.

If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address will be truncated.

If the address argument is not a null pointer and the protocol does not provide the source address of messages, the the value stored in the object pointed to by address is unspecified.

If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, recvfrom() blocks until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor, recvfrom() fails and sets errno to [EAGAIN] or [EWOULDBLOCK].

RETURN VALUE

Upon successful completion, recvfrom() returns the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recvfrom() returns 0. Otherwise the function returns -1 and sets errno to indicate the error.

ERRORS

The recvfrom() function will fail if:

[EAGAIN] or [EWOULDBLOCK]

The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band data is available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to await out-of-band data.

[EBADF]

The socket argument is not a valid file descriptor.

[ECONNRESET]

A connection was forcibly closed by a peer.

[EFAULT]

The buffer, address or address_len parameter can not be accessed or written.

[EINTR]

A signal interrupted recvfrom() before any data was available.

[EINVAL]

The MSG_OOB flag is set and no out-of-band data is available.

[ENOTCONN]

A receive is attempted on a connection-mode socket that is not connected.

[ENOTSOCK]

The socket argument does not refer to a socket.

[EOPNOTSUPP]

The specified flags are not supported for this socket type.

[ETIMEDOUT]

The connection timed out during connection establishment, or due to a transmission timeout on active connection.

The recvfrom() function may fail if:

[EIO]

An I/O error occurred while reading from or writing to the file system.

[ENOBUFS]

Insufficient resources were available in the system to perform the operation.

[ENOMEM]

Insufficient memory was available to fulfill the request.

[ENOSR]

There were insufficient STREAMS resources available for the operation to complete.

APPLICATION USAGE

The select() and poll() functions can be used to determine when data is available to be received.

SEE ALSO

poll(), read(), recv(), recvmsg(), select() send(), sendmsg(), sendto(), shutdown(), socket(), write(), <sys/socket.h>.