recv(2) - Linux manual page (original) (raw)
recv(2) System Calls Manual recv(2)
NAME top
recv, recvfrom, recvmsg - receive a message from a socketLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/socket.h>**
**ssize_t recv(int** _sockfd_**, void** _buf_**[.**_size_**], size_t** _size_**,**
**int** _flags_**);**
**ssize_t recvfrom(int** _sockfd_**, void** _buf_**[restrict .**_size_**], size_t** _size_**,**
**int** _flags_**,**
**struct sockaddr *_Nullable restrict** _srcaddr_**,**
**socklen_t *_Nullable restrict** _addrlen_**);**
**ssize_t recvmsg(int** _sockfd_**, struct msghdr ***_msg_**, int** _flags_**);**DESCRIPTION top
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)](../man2/read.2.html) is the presence of
_flags_. With a zero _flags_ argument, **recv**() is generally equivalent
to [read(2)](../man2/read.2.html) (but see NOTES). Also, the following call
recv(sockfd, buf, size, flags);
is equivalent to
recvfrom(sockfd, buf, size, flags, NULL, NULL);
All three calls return the size 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)](../man2/fcntl.2.html)), in which case the value -1 is returned and _[errno](../man3/errno.3.html)_ 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)](../man2/select.2.html), [poll(2)](../man2/poll.2.html), or [epoll(7)](../man7/epoll.7.html) 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)](../man7/unix.7.html)). This flag is useful for
the same reasons as the **O_CLOEXEC** flag of [open(2)](../man2/open.2.html).
**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)](../man2/fcntl.2.html) **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)](../man2/open.2.html)),
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)](../man3/cmsg.3.html) and [ip(7)](../man7/ip.7.html) for more
information. The payload of the original packet that
caused the error is passed as normal data via _msgiovec_.
The original destination address of the datagram that
caused the error is supplied via _msgname_.
The error is supplied in a _sockextendederr_ 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 *);
_eeerrno_ contains the _[errno](../man3/errno.3.html)_ number of the queued error.
_eeorigin_ 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 _safamily_ 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 _cmsglen_ 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 size of the packet or datagram,
even when it was longer than the passed buffer.
For use with Internet stream sockets, see [tcp(7)](../man7/tcp.7.html).
**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 size.
If _srcaddr_ 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 _srcaddr_. In this case, _addrlen_ is a
value-result argument. Before the call, it should be initialized
to the size of the buffer associated with _srcaddr_. 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, _srcaddr_
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, size, 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 size */
int msg_flags; /* Flags on received message */
};
The _msgname_ field points to a caller-allocated buffer that is
used to return the source address if the socket is unconnected.
The caller should set _msgnamelen_ to the size of this buffer
before this call; upon return from a successful call, _msgnamelen_
will contain the size of the returned address. If the application
does not need to know the source address, _msgname_ can be
specified as NULL.
The fields _msgiov_ and _msgiovlen_ describe scatter-gather
locations, as discussed in [readv(2)](../man2/readv.2.html).
The field _msgcontrol_, which has size _msgcontrollen_, points to a
buffer for other protocol control-related messages or
miscellaneous ancillary data. When **recvmsg**() is called,
_msgcontrollen_ should contain the size of the available buffer in
_msgcontrol_; upon return from a successful call it will contain
the size 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)](../man3/cmsg.3.html).
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)](../man7/unix.7.html) and [ip(7)](../man7/ip.7.html).
The _msgflags_ 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 top
These calls return the number of bytes received, or -1 if an error
occurred. In the event of an error, _[errno](../man3/errno.3.html)_ 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-size 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 top
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)](../man7/signal.7.html).
**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)](../man2/connect.2.html) and
[accept(2)](../man2/accept.2.html)).
**ENOTSOCK**
The file descriptor _sockfd_ does not refer to a socket.VERSIONS top
According to POSIX.1, the _msgcontrollen_ field of the _msghdr_
structure should be typed as _socklent_, and the _msgiovlen_ field
should be typed as _int_, but glibc currently types both as _sizet_.STANDARDS top
POSIX.1-2008.HISTORY top
POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD).
POSIX.1 describes only the **MSG_OOB**, **MSG_PEEK**, and **MSG_WAITALL**
flags.NOTES top
If a zero-size datagram is pending, [read(2)](../man2/read.2.html) and **recv**() with a
_flags_ argument of zero provide different behavior. In this
circumstance, [read(2)](../man2/read.2.html) has no effect (the datagram remains
pending), while **recv**() consumes the pending datagram.
See [recvmmsg(2)](../man2/recvmmsg.2.html) for information about a Linux-specific system call
that can be used to receive multiple datagrams in a single call.EXAMPLES top
An example of the use of **recvfrom**() is shown in [getaddrinfo(3)](../man3/getaddrinfo.3.html).SEE ALSO top
[fcntl(2)](../man2/fcntl.2.html), [getsockopt(2)](../man2/getsockopt.2.html), [read(2)](../man2/read.2.html), [recvmmsg(2)](../man2/recvmmsg.2.html), [select(2)](../man2/select.2.html),
[shutdown(2)](../man2/shutdown.2.html), [socket(2)](../man2/socket.2.html), [cmsg(3)](../man3/cmsg.3.html), [sockatmark(3)](../man3/sockatmark.3.html), [ip(7)](../man7/ip.7.html), [ipv6(7)](../man7/ipv6.7.html),
[socket(7)](../man7/socket.7.html), [tcp(7)](../man7/tcp.7.html), [udp(7)](../man7/udp.7.html), [unix(7)](../man7/unix.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 recv(2)
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