NAME
recv, recvfrom, recvmsg - receive a message from a socket
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
ssize_t recv(int sockfd, void *buf, size_t len, int flags);
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
struct sockaddr *src_addr, socklen_t *addrlen);
ssize_t recvmsg(int sockfd, 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 src_addr is not NULL, and the underlying protocol provides the
source address, this source address is filled in. When src_addr is
NULL, nothing is filled in; in this case, addrlen is not used, and
should also be NULL. The argument addrlen is a value-result argument,
which the caller should initialize before the call to the size of the
buffer associated with src_addr, and modified on return to indicate the
actual size of the source address. The returned address is truncated
if the buffer provided is too small; in this case, addrlen will return
a value greater than was supplied to the call.
The recv() call is normally used only on a connected socket (see
connect(2)) and is identical to recvfrom() with a NULL src_addr
argument.
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
is set to EAGAIN or EWOULDBLOCK. 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_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)
Set the close-on-exec flag for the file descriptor received via
a Unix domain file descriptor using the SCM_RIGHTS operation
(described in unix(7)). This flag is useful for the same
reasons as the O_CLOEXEC flag of open(2).
MSG_DONTWAIT (since Linux 2.2)
Enables nonblocking operation; if the operation would block, the
call fails with the error EAGAIN or EWOULDBLOCK (this can also
be enabled using the O_NONBLOCK flag with the F_SETFL fcntl(2)).
MSG_ERRQUEUE (since Linux 2.2)
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
{
uint32_t ee_errno; /* error number */
uint8_t ee_origin; /* where the error originated */
uint8_t ee_type; /* type */
uint8_t ee_code; /* code */
uint8_t ee_pad; /* padding */
uint32_t ee_info; /* additional information */
uint32_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 (since Linux 2.2)
For raw (AF_PACKET), Internet datagram (since Linux
2.4.27/2.6.8), and netlink (since Linux 2.6.22) sockets: return
the real length of the packet or datagram, even when it was
longer than the passed buffer. Not implemented for Unix domain
(unix(7)) sockets.
For use with Internet stream sockets, see tcp(7).
MSG_WAITALL (since Linux 2.2)
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 arguments. This structure is defined as follows in
<sys/socket.h>:
struct iovec { /* Scatter/gather array items */
void *iov_base; /* Starting address */
size_t iov_len; /* Number of bytes to transfer */
};
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
unsigned 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.
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.
EAGAIN or EWOULDBLOCK
The socket is marked nonblocking and the receive operation would
block, or a receive timeout had been set and the timeout expired
before data was received. POSIX.1-2001 allows either error to
be returned for this case, and does not require these constants
to have the same value, so a portable application should check
for both possibilities.
EBADF The argument sockfd 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; see signal(7).
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 sockfd does not refer to a socket.
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 addrlen
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.
EXAMPLE
An example of the use of recvfrom() is shown in getaddrinfo(3).
SEE ALSO
fcntl(2), getsockopt(2), read(2), select(2), shutdown(2), socket(2),
cmsg(3), sockatmark(3), socket(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.