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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 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-oriented,
the source address of the message 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_NOSIGNAL
- This flag turns off raising of SIGPIPE
on stream sockets when the other end disappears.
- 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 3struct 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)
).
Return Value
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.
- ECONNREFUSED
- A remote host refused to allow the network connection (typically because
it is not running the requested service).
- 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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