NAME

recvfrom — receive a message from a socket

SYNOPSIS

#include <sys/socket.h>

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


DESCRIPTION

The recvfrom() function shall receive 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 recvfrom() 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 shall 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

On SOCK_STREAM sockets this requests that the function block until the full amount of data can be returned. The function may return the smaller amount of data if the socket is a message-based socket, 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

Either a null pointer, if address is a null pointer, or a pointer to a socklen_t object which on input specifies the length of the supplied sockaddr structure, and on output specifies the length of the sending address.

The recvfrom() function shall return the length of the message written to the buffer pointed to by the buffer argument. For message-based sockets, such as ^[RS]  SOCK_RAW, SOCK_DGRAM, and SOCK_SEQPACKET, the entire message shall 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 shall be discarded. For stream-based sockets, such as SOCK_STREAM, message boundaries shall be ignored. In this case, data shall be returned to the user as soon as it becomes available, and no data shall be discarded.

If the MSG_WAITALL flag is not set, data shall 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 shall be stored in the sockaddr structure pointed to by the address argument, and the length of this address shall be 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 shall be truncated.

If the address argument is not a null pointer and the protocol does not provide the source address of messages, 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() shall block until a message arrives or a timeout occurs (see SO_RCVTIMEO in 2.10.16 Use of Options). If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor, recvfrom() shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].

RETURN VALUE

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

ERRORS

The recvfrom() function shall 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. See also SO_RCVTIMEO in 2.10.16 Use of Options .

[EBADF]

The socket argument is not a valid file descriptor.

[ECONNRESET]

A connection was forcibly closed by a peer.

[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.

EXAMPLES

None.

APPLICATION USAGE

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

For AF_UNIX sockets, it is recommended that address points to a buffer of length greater than sizeof(struct sockaddr_un) which has been initialized with null bytes. That way, even if the implementation supports the use of all bytes of sun_path without a terminating null byte, the larger buffer guarantees that the sun_path member can then be passed to other interfaces that expect a null-terminated string. If no truncation occurred based on the input value of address_len, it is unspecified whether the returned address_len will be sizeof(struct sockaddr_un), or merely a value at least as large as offsetof(struct sockaddr_un, sun_path) plus the number of non-null bytes stored in sun_path.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

poll, pselect, read, recv, recvmsg, send, sendmsg, sendto, setsockopt, shutdown, socket, write

XBD <sys/socket.h>

CHANGE HISTORY

First released in Issue 6. Derived from the XNS, Issue 5.2 specification.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0503 [464] is applied.

Issue 8

Austin Group Defect 561 is applied, adding a paragraph about sun_path to APPLICATION USAGE.

Austin Group Defect 1429 is applied, clarifying the behavior on timeout by adding references to 2.10.16 Use of Options.

Austin Group Defect 1565 is applied, changing the description of address_len.