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NAME
recv, recvfrom, recvmsg - receive a message from a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/socket.h>
ssize_t recv(int sockfd, void buf[.len], size_t len,
int flags);
ssize_t recvfrom(int sockfd, void buf[restrict .len], size_t len,
int flags,
struct sockaddr *_Nullable restrict src_addr,
socklen_t *_Nullable restrict addrlen);
ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
DESCRIPTION
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 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 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 SO_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 as well as sequenced-packet (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 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. For further information on the use
of ancillary data in various socket domains, see unix(7) and ip(7).
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.
MSG_CMSG_CLOEXEC (since Linux 2.6.23)
indicates that MSG_CMSG_CLOEXEC was specified in the flags argument of recvmsg().
RETURN VALUE
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.
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 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.
VERSIONS
According to POSIX.1, the msg_controllen field of the msghdr structure should be typed as
socklen_t, and the msg_iovlen field should be typed as int, but glibc currently types both
as size_t.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD).
POSIX.1 describes only the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.
NOTES
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.
See recvmmsg(2) for information about a Linux-specific system call that can be used to
receive multiple datagrams in a single call.
EXAMPLES
An example of the use of recvfrom() is shown in getaddrinfo(3).
SEE ALSO
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)