recvfrom

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);

Status

Implemented

Conformance

IEEE Std 1003.1-2017

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 stored 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 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. For AF_UNIX sockets the source address is provided for datagrams sent from a bound socket; for a datagram from an unbound socket, and for connection-mode sockets, the length of the address is set to 0.

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. 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.

  • 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.

POSIX deviations

If the source address of a datagram from an AF_UNIX socket is longer than the supplied sockaddr structure, the stored address is truncated, and the object pointed to by address_len is set to the length of the full address, as in IEEE Std 1003.1g, not to the length of the stored address as IEEE Std 1003.1-2017 specifies. The behavior is intentional: the application detects the truncation by comparing the returned length with the size of its buffer and can repeat the call with a larger one, which is more useful on a memory-constrained embedded system than silently working with a cut path name.