recv, recvfrom, recvmsg - receive a message from a socket
#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);
The
recv(),
recvfrom(), and
recvmsg() calls are used to
receive messages from a socket. They may be used to receive data on both
connectionless and connection-oriented sockets. This page first describes
common features of all three system calls, and then describes the differences
between the calls.
The only difference between
recv() and
read(2) is the presence of
flags. With a zero
flags argument,
recv() is generally
equivalent to
read(2) (but see NOTES). Also, the following call
recv(sockfd, buf, len, flags);
is equivalent to
recvfrom(sockfd, buf, len, flags, NULL, NULL);
All three calls 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.
An application can use
select(2),
poll(2), or
epoll(7) to
determine when more data arrives on a socket.
The
flags argument is formed by ORing 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 provides
similar behavior to setting the O_NONBLOCK flag (via the
fcntl(2) F_SETFL operation), but differs in that
MSG_DONTWAIT is a per-call option, whereas O_NONBLOCK is a
setting on the open file description (see open(2)), which will
affect all threads in the calling process and as well as other processes
that hold file descriptors referring to the same open file
description.
- 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.
- 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 flag 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),
netlink (since Linux 2.6.22), and UNIX datagram (since Linux 3.4) sockets:
return the real length of the packet or datagram, even when it was longer
than the passed buffer.
- 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. This flag has no
effect for datagram sockets.
recvfrom() places the received message into the buffer
buf. The
caller must specify the size of the buffer in
len.
If
src_addr is not NULL, and the underlying protocol provides the source
address of the message, that source address is placed in the buffer pointed to
by
src_addr. In this case,
addrlen is a value-result argument.
Before the call, it should be initialized to the size of the buffer associated
with
src_addr. Upon return,
addrlen is updated to contain 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.
If the caller is not interested in the source address,
src_addr and
addrlen should be specified as NULL.
The
recv() call is normally used only on a
connected socket (see
connect(2)). It is equivalent to the call:
recvfrom(fd, buf, len, flags, NULL, 0);
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 */
size_t msg_controllen; /* Ancillary data buffer len */
int msg_flags; /* Flags on received message */
};
The
msg_name field points to a caller-allocated buffer that is used to
return the source address if the socket is unconnected. The caller should set
msg_namelen to the size of this buffer before this call; upon return
from a successful call,
msg_namelen will contain the length of the
returned address. If the application does not need to know the source address,
msg_name can be specified as NULL.
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 {
size_t cmsg_len; /* Data byte count, including header
(type is socklen_t in POSIX) */
int cmsg_level; /* Originating protocol */
int cmsg_type; /* Protocol-specific type */
/* followed by
unsigned char cmsg_data[]; */
};
Ancillary data should be accessed only by the macros defined in
cmsg(3).
As an example, Linux uses this ancillary data mechanism to pass extended errors,
IP options, or file descriptors over UNIX domain 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 was 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 was
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. In
the event of an error,
errno is set to indicate the error.
When a stream socket peer has performed an orderly shutdown, the return value
will be 0 (the traditional "end-of-file" return).
Datagram sockets in various domains (e.g., the UNIX and Internet domains) permit
zero-length datagrams. When such a datagram is received, the return value is
0.
The value 0 may also be returned if the requested number of bytes to receive
from a stream socket was 0.
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 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 file 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 was
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 file descriptor sockfd does not refer to a socket.
POSIX.1-2001, POSIX.1-2008, 4.4BSD (these interfaces first appeared in 4.2BSD).
POSIX.1 describes only the
MSG_OOB,
MSG_PEEK, and
MSG_WAITALL flags.
If a zero-length datagram is pending,
read(2) and
recv() with a
flags argument of zero provide different behavior. In this
circumstance,
read(2) has no effect (the datagram remains pending),
while
recv() consumes the pending datagram.
The
socklen_t type was invented by POSIX. See also
accept(2).
According to POSIX.1, the
msg_controllen field of the
msghdr
structure should be typed as
socklen_t, but glibc currently types it as
size_t.
See
recvmmsg(2) for information about a Linux-specific system call that
can be used to receive multiple datagrams in a single call.
An example of the use of
recvfrom() is shown in
getaddrinfo(3).
fcntl(2),
getsockopt(2),
read(2),
recvmmsg(2),
select(2),
shutdown(2),
socket(2),
cmsg(3),
sockatmark(3),
ip(7),
ipv6(7),
socket(7),
tcp(7),
udp(7),
unix(7)