NAME

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

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

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

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 *fromlen);

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

DESCRIPTION

The recv(), recvfrom(), and recvmsg() functions are used to receive messages from another socket. The s socket is created with socket(3C).

If from is a non- NULL pointer, the source address of the message is filled in. The value-result parameter fromlen is initialized to the size of the buffer associated with from and modified on return to indicate the actual size of the address stored in the buffer. 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 from which the message is received. See socket(3C).

If no messages are available at the socket, the receive call waits for a message to arrive. If the socket is non-blocking, -1 is returned with the external variable errno set to EWOULDBLOCK. See fcntl(2).

For processes on the same host, recvmsg() can be used to receive a file descriptor from another process, but it cannot receive ancillary data. See libxnet(3LIB).

If a zero-length buffer is specified for a message, an EOF condition results that is indistinguishable from the successful transfer of a file descriptor. For that reason, one or more bytes of data should be provided when recvmsg() passes a file descriptor.

The select(3C) call can be used to determine when more data arrives.

The flags parameter is formed by an OR operation on 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 to allow a subsequent receive operation to see the same data.

MSG_WAITALL: Messages are blocked until the full amount of data requested is returned. The recv() function can return a smaller amount of data if a signal is caught, the connection is terminated, MSG_PEEK is specified, or if an error is pending for the socket.

MSG_DONTWAIT: Pending messages received on the connection are returned. If data is unavailable, the function does not block. This behavior is the equivalent to specifying O_NONBLOCK on the file descriptor of a socket, except that write requests are unaffected.

The recvmsg() function call uses a msghdr structure defined in <sys/socket.h> to minimize the number of directly supplied parameters.

RETURN VALUES

Upon successful completion, these functions return the number of bytes received. Otherwise, they return -1 and set errno to indicate the error.

ERRORS

The recv(), recvfrom(), and recvmsg() functions return errors under the following conditions:


[EBADF]: The s file descriptor is invalid.

[ECONNRESET]: The s argument refers to a connection oriented socket and the connection was forcibly closed by the peer and is no longer valid. I/O can no longer be performed to filedes.

[EINVAL]: The MSG_OOB flag is set and no out-of-band data is available.

[EINTR]: The operation is interrupted by the delivery of a signal before any data is available to be received.

[EIO]: An I/O error occurs while reading from or writing to the file system.

[ENOMEM]: Insufficient user memory is available to complete operation.

[ENOSR]: Insufficient STREAMS resources are available for the operation to complete.

[ENOTSOCK]: s is not a socket.

[ESTALE]: A stale NFS file handle exists.

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

[ECONNREFUSED]: The requested connection was refused by the peer. For connected IPv4 and IPv6 datagram sockets, this indicates that the system received an ICMP “Destination Port Unreachable” message from the peer.

The recv() and recvfrom() functions fail under the following conditions:


[EINVAL]: The len argument overflows a ssize_t.

The recvmsg() function returns errors under the following conditions:


[EINVAL]: The msg_iovlen member of the msghdr structure pointed to by msg is less than or equal to 0, or greater than [IOV_MAX}. See Intro(2) for a definition of [IOV_MAX}.
One of the iov_len values in the msg_iov array member of the msghdr structure pointed to by msg is negative, or the sum of the iov_len values in the msg_iov array overflows a ssize_t.

MT-LEVEL

Safe