recv(2) System Calls Manual recv(2)
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 mes-
sages from a socket. They may be used to receive data on both connec-
tionless and connection-oriented sockets. This page first describes
common features of all three system calls, and then describes the dif-
ferences 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 comple-
tion. 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 val-
ues:
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 rea-
sons 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 fc-
ntl(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_ori-
gin 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 un-
defined. 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 opera-
tion.
MSG_OOB
This flag requests receipt of out-of-band data that would not be
received in the normal data stream. Some protocols place expe-
dited 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 re-
quest is satisfied. However, the call may still return less
data than requested if a signal is caught, an error or discon-
nect 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 ad-
drlen should be specified as NULL.
recv()
The recv() call is normally used only on a connected socket (see con-
nect(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 an-
cillary 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 se-
quence.
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 ex-
tended errors, IP options, or file descriptors over UNIX domain sock-
ets. 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 argu-
ment of recvmsg().
RETURN VALUE
These calls return the number of bytes received, or -1 if an error oc-
curred. 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 do-
mains) 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. Addi-
tional errors may be generated and returned from the underlying proto-
col 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 re-
turned 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 ad-
dress 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() con-
sumes 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)
Linux man-pages 6.7 2024-02-18 recv(2)
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