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NAME
recvfrom - receive a message from a socket
SYNOPSIS
#include <network.h>
int recvfrom(int s, void *buf, int len, unsigned int flags
struct sockaddr *from, int *fromlen);
DESCRIPTION
The recvfrom call is used to receive messages from a socket,
and may be used to receive data on a socket whether or not it
is connection-oriented.
If from is not NULL, and the socket is not connection-ori
ented, the source address of the message is filled in.
Fromlen is a value-result parameter, initialized to the
size of the buffer associated with from, and modified on
return to indicate the actual size of the address stored
there.
The routine returns the length of the message on
successful completion. If a message is too long to fit in
the supplied buffer, excess bytes may be discarded depend
ing on the type of socket the message is received from
(see socket(2)).
If no messages are available at the socket, the receive
calls wait for a message to arrive, unless the socket is
nonblocking (see fcntl(2)) in which case the value -1 is
returned and the external variable errno set to EAGAIN.
The receive calls normally return any data available, up
to the requested amount, rather than waiting for receipt
of the full amount requested.
The select(2) call may be used to determine
when more data arrives.
The flags argument to a recvfrom call is formed by OR'ing one
or more of the following values:
MSG_OOB
This flag requests receipt of out-of-band data that
would not be received in the normal data stream.
Some protocols place expedited data at the head of
the normal data queue, and thus this flag cannot be
used with such protocols.
MSG_PEEK
This flag causes the receive operation to return
data from the beginning of the receive queue with
out removing that data from the queue. Thus, a
subsequent receive call will return the same data.
MSG_WAITALL
This flag requests that the operation block until
the full request is satisfied. However, the call
may still return less data than requested if a sig
nal is caught, an error or disconnect occurs, or
the next data to be received is of a different type
than that returned.
MSG_ERRQUEUE
Receive packet from the error queue
MSG_NOSIGNAL
This flag turns off raising of SIGPIPE on stream
sockets when the other end disappears.
MSG_ERRQUEUE
This flag specifies that queued errors should be
received from the socket error queue. The error is
passed in a ancilliary message with a type depen
dent on the protocol (for IP IP_RECVERR). The
error is supplied in a sock_extended_error struc
ture:
#define SO_EE_ORIGIN_NONE 0
#define SO_EE_ORIGIN_LOCAL 1
#define SO_EE_ORIGIN_ICMP 2
#define SO_EE_ORIGIN_ICMP6 3
struct sock_extended_err
{
__u32 ee_errno; /* error number */
__u8 ee_origin; /* where the error originated */
__u8 ee_type; /* type */
__u8 ee_code; /* code */
__u8 ee_pad;
__u32 ee_info; /* additional information */
__u32 ee_data; /* other data */
};
struct sockaddr *SOCK_EE_OFFENDER(struct sock_extended_err *);
ee_errno contains the errno number of the queued
error. ee_origin is the origin code of where the
error originated. The other fields are protocol
specific. SOCK_EE_OFFENDER returns a pointer to
the address of the network object where the error
originated from. If this address is not known, the
sa_family member of the sockaddr contains AF_UNSPEC
and the other fields of the sockaddr are undefined.
The payload of the packet that caused the error is
passed as normal data.
For local errors, no address is passed (this can be
checked with the cmsg_len member of the cmsghdr).
For error receives, the MSG_ERRQUEUE is set in the
msghdr. After a error has been passed, the pending
socket error is regenerated based on the next
queued error and will be passed on the next socket
operation.
The msg_flags field is set on return according to the mes
sage received. MSG_EOR indicates end-of-record; the data
returned completed a record (generally used with sockets
of type SOCK_SEQPACKET). MSG_TRUNC indicates that the
trailing portion of a datagram was discarded because the
datagram was larger than the buffer supplied. MSG_CTRUNC
indicates that some control data were discarded due to
lack of space in the buffer for ancillary data. MSG_OOB
is returned to indicate that expedited or out-of-band data
were received. MSG_ERRQUEUE indicates that no data was
received but an extended error from the socket error
queue.
RETURN VALUES
These calls return the number of bytes received, or -1 if
an error occurred.
ERRORS
These are some standard errors generated by the socket
layer. Additional errors may be generated and returned
from the underlying protocol modules; see their manual
pages.
EBADF The argument s is an invalid descriptor.
ENOTSOCK
The argument s does not refer to a socket.
EAGAIN The socket is marked non-blocking and the receive
operation would block, or a receive timeout had
been set and the timeout expired before data was
received.
EINTR The receive was interrupted by delivery of a sig
nal before any data were available.
EINVAL Invalid argument passed.
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