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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
RECV(2) Linux Programmer's Manual RECV(2)
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
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()
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.
recv()
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);
recvmsg()
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 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. 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 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 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)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-03-13 RECV(2)
Pages that refer to this page: getpeername(2), recvmmsg(2), select(2), select_tut(2), send(2), socket(2), socketcall(2), syscalls(2), cmsg(3), getifaddrs(3), getnameinfo(3), if_nameindex(3), rtime(3), sockatmark(3), ddp(7), ip(7), netlink(7), packet(7), raw(7), sctp(7), signal(7), signal-safety(7), socket(7), tcp(7), udp(7), unix(7)