recv, recvfrom, recvmsg — receive a message from a socket
#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.
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_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)).
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.
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_TRUNC
Return the real length of the packet, even when it was longer than the passed buffer. Only valid for packet sockets.
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.
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.
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.
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.
The argument s
is an invalid
descriptor.
A remote host refused to allow the network connection (typically because it is not running the requested service).
The receive buffer pointer(s) point outside the process's address space.
The receive was interrupted by delivery of a signal before any data were available.
Invalid argument passed.
Could not allocate memory for recvmsg
().
The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
The argument s
does not refer to a
socket.
4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.
POSIX.1-2001 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
4.x BSD and libc4 and libc5 all have int
). The flags
argument is int
in 4.x BSD, but
unsigned int in libc4
and libc5. The len
argument is int
in
4.x BSD, but size_t
in libc4 and libc5. The fromlen
argument is
int * in 4.x BSD,
libc4 and libc5. The present socklen_t * was invented by POSIX.
See also accept(2).
According to POSIX.1-2001, the msg_controllen
field of the
msghdr
structure
should be typed as socklen_t
, but glibc
currently (2.4) types it as size_t
.
fcntl(2), getsockopt(2), read(2), select(2), shutdown(2), socket(2), cmsg(3), sockatmark(3)
|