recvfrom(2)
NAME
recv, recvfrom, recvmsg - receive a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int recv(int s, void *buf, size_t len, int flags);
int recvfrom(int s, void *buf, size_t len, int flags,
struct sockaddr *from, socklen_t *fromlen);
int recvmsg(int s, struct msghdr *msg, int flags);
DESCRIPTION
The recvfrom and recvmsg calls are used to receive mes
sages 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.
The argument fromlen is a value-result parameter, initial
ized to the size of the buffer associated with from, and
modified on return to indicate the actual size of the
address stored there.
The recv call is normally used only on a connected socket
(see connect(2)) and is identical to recvfrom with a NULL
from parameter.
All three routines return 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) or poll(2) call may be used to determine
when more data arrives.
The flags argument to a recv 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_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 an ancillary message with a type depen
dent on the protocol (for IPv4 IP_RECVERR ). The
user should supply a buffer of sufficient size.
See cmsg(3) for more information on ancillary mes
sages.
The error is supplied in a sock_extended_err 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
{
u_int32_t ee_errno; /* error number */
u_int8_t ee_origin; /* where the error originated */
u_int8_t ee_type; /* type */
u_int8_t ee_code; /* code */
u_int8_t ee_pad;
u_int32_t ee_info; /* additional information */
u_int32_t ee_data; /* other data */
/* More data may follow */
};
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. The macro SOCK_EE_OFFENDER returns a
pointer to the address of the network object where
the error originated from given a pointer to the
ancillary message. 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 an error has been passed, the pend
ing socket error is regenerated based on the next
queued error and will be passed on the next socket
operation.
The recvmsg call uses a msghdr structure to minimize the
number of directly supplied parameters. This structure
has the following form, as defined in <sys/socket.h>:
struct msghdr {
void * msg_name; /* optional address */
socklen_t msg_namelen; /* size of address */
struct iovec * msg_iov; /* scatter/gather array */
size_t msg_iovlen; /* # elements in msg_iov */
void * msg_control; /* ancillary data, see below */
socklen_t msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Here msg_name and msg_namelen specify the destination
address if the socket is unconnected; msg_name may be
given as a null pointer if no names are desired or
required. The fields msg_iov and msg_iovlen describe
scatter-gather locations, as discussed in readv(2). The
field msg_control, which has length msg_controllen, points
to a buffer for other protocol control related messages or
miscellaneous ancillary data. When recvmsg is called,
msg_controllen should contain the length of the available
buffer in msg_control; upon return from a successful call
it will contain the length of the control message
sequence.
The messages are of the form:
struct cmsghdr {
socklen_t cmsg_len; /* data byte count, including hdr */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by
u_char cmsg_data[]; */
};
Ancillary data should only be accessed by the macros
defined in cmsg(3).
As an example, Linux uses this auxiliary data mechanism to
pass extended errors, IP options or file descriptors over
Unix sockets.
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.
ENOTCONN
The socket is associated with a connection-oriented
protocol and has not been connected (see connect(2)
and accept(2)).
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 signal
before any data were available.
EFAULT The receive buffer pointer(s) point outside the
process's address space.
EINVAL Invalid argument passed.
CONFORMING TO
4.4BSD (these function calls first appeared in 4.2BSD).
NOTE
The prototypes given above follow glibc2. The Single Unix
Specification agrees, except that it has return values of
type `ssize_t' (while BSD 4.* and libc4 and libc5 all have
`int'). The flags argument is `int' in BSD 4.*, but
`unsigned int' in libc4 and libc5. The len argument is
`int' in BSD 4.*, but `size_t' in libc4 and libc5. The
fromlen argument is `int *' in BSD 4.*, libc4 and libc5.
The present `socklen_t *' was invented by POSIX. See
also accept(2).
SEE ALSO
fcntl(2), read(2), select(2), getsockopt(2), socket(2),
cmsg(3)
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