Tribblix: manual page: recv.3socket

RECV(3SOCKET) Sockets Library Functions RECV(3SOCKET)

NAME

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

SYNOPSIS

cc [ flag... ] file... -lsocket -lnsl [ library... ]
#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 *buf, size_t len, int flags,
struct sockaddr *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(3SOCKET).

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(3SOCKET).

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 poll(2), select(3C), and port_get(3C) functions 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.

MSG_CMSG_CLOEXEC
When receiving the SCM_RIGHTS ancillary data, all
such file descriptors should be marked with the
close-on-exec, FD_CLOEXEC flag. These file
descriptors will be closed on successful execution of
the exec(2) family of functions.

MSG_CMSG_CLOFORK
When receiving the SCM_RIGHTS ancillary data, all
such file descriptors should be marked with the
close-on-fork, FD_CLOFORK flag. These file
descriptors will be closed in any children created
with the fork(2) family of functions.

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

In addition to the errors documented below, an asynchronous error
generated by the underlying socket protocol may be returned. For the
full list of errors, please see the corresponding socket protocol
manual page. For example, for a list of TCP errors, please see
tcp(4P).

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}.

EINVAL
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.