Originální popis anglicky:
recv, recvfrom, recvmsg - receive a message from a socket
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/socket.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);
The
recvfrom and
recvmsg calls are 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 underlying protocol provides the source
address, this source address is filled in. The argument
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
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 depending 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 without 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 signal is caught, an error or disconnect occurs, or the
next data to be received is of a different type than that returned.
- MSG_TRUNC
- Return the real length of the packet, even when it was
longer than the passed buffer. Only valid for packet sockets.
- 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 dependent on the protocol (for IPv4 IP_RECVERR). The
user should supply a buffer of sufficient size. See cmsg(3) and
ip(7) for more information. The payload of the original packet that
caused the error is passed as normal data via msg_iovec. The
original destination address of the datagram that caused the error is
supplied via msg_name.
- 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 pending socket error is regenerated based on
the next queued error and will be passed on the next socket operation.
The error is supplied in a sock_extended_err structure:
#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 *SO_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 pending 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 source 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 in the msghdr is set on return of
recvmsg().
It can contain several flags:
- 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.
- MSG_DONTWAIT
- Enables non-blocking operation; if the operation would
block, EAGAIN is returned (this can also be enabled using the
O_NONBLOCK with the F_SETFL fcntl(2)).
These calls return the number of bytes received, or -1 if an error occurred. The
return value will be 0 when the peer has performed an orderly shutdown.
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.
- 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.
- EBADF
- The argument s is an invalid descriptor.
- ECONNREFUSED
- A remote host refused to allow the network connection
(typically because it is not running the requested service).
- EFAULT
- The receive buffer pointer(s) point outside the process's
address space.
- EINTR
- The receive was interrupted by delivery of a signal before
any data were available.
- EINVAL
- Invalid argument passed.
- ENOMEM
- Could not allocate memory for recvmsg.
- 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.
4.4BSD (these function calls first appeared in 4.2BSD), POSIX 1003.1-2001.
POSIX only describes the
MSG_OOB,
MSG_PEEK, and
MSG_WAITALL
flags.
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).
fcntl(2),
getsockopt(2),
read(2),
select(2),
socket(2),
cmsg(3)
