recv() - Unix, Linux System Call
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NAME
recv, recvfrom, recvmsg - receive a message from a socket
SYNOPSIS
#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);
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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 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
ORing one or more of the following values:
Tag | Description |
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)).
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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. |
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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 *);
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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.
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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.
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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.
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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.
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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 */
};
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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.
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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[]; */
};
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Ancillary data should only be accessed by the macros defined in
cmsg(3).
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As an example, Linux uses this auxiliary data mechanism to pass extended
errors, IP options or file descriptors over Unix sockets.
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The
msg_flags field in the msghdr is set on return of
recvmsg(). It can contain several flags:
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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.
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MSG_CTRUNC | |
indicates that some control data were discarded due to lack of space in the
buffer for ancillary data.
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MSG_OOB | |
is returned to indicate that expedited or out-of-band data were received.
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MSG_ERRQUEUE | |
indicates that no data was received but an extended error from the socket
error queue.
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RETURN VALUE
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.
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.
Tag | Description |
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).
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EFAULT |
The receive buffer pointer(s) point outside the processs
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.
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CONFORMING TO
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.
NOTES
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. SEE ALSO
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