man-pages/man2/recv.2

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.\"     $Id: recv.2,v 1.3 1999/05/13 11:33:38 freitag Exp $
.\"
.\" Modified Sat Jul 24 00:22:20 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998,1999 by Andi Kleen
.\" 2001-06-19 corrected SO_EE_OFFENDER, bug report by James Hawtin
.\"
.TH RECV 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
recv, recvfrom, recvmsg \- receive a message from a socket
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.PP
.BI "ssize_t recv(int " sockfd ", void *" buf ", size_t " len ", int " flags );
.BI "ssize_t recvfrom(int " sockfd ", void *restrict " buf ", size_t " len \
", int " flags ,
.BI "                 struct sockaddr *restrict " src_addr ,
.BI "                 socklen_t *restrict " addrlen );
.BI "ssize_t recvmsg(int " sockfd ", struct msghdr *" msg ", int " flags );
.fi
.SH DESCRIPTION
The
.BR recv (),
.BR recvfrom (),
and
.BR 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.
.PP
The only difference between
.BR recv ()
and
.BR read (2)
is the presence of
.IR flags .
With a zero
.I flags
argument,
.BR recv ()
is generally equivalent to
.BR read (2)
(but see NOTES).
Also, the following call
.PP
    recv(sockfd, buf, len, flags);
.PP
is equivalent to
.PP
    recvfrom(sockfd, buf, len, flags, NULL, NULL);
.PP
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.
.PP
If no messages are available at the socket, the receive calls wait for a
message to arrive, unless the socket is nonblocking (see
.BR fcntl (2)),
in which case the value \-1 is returned and
.I errno
is set to
.BR 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.
.PP
An application can use
.BR select (2),
.BR poll (2),
or
.BR epoll (7)
to determine when more data arrives on a socket.
.SS The flags argument
The
.I flags
argument is formed by ORing one or more of the following values:
.TP
.BR 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
.B SCM_RIGHTS
operation (described in
.BR unix (7)).
This flag is useful for the same reasons as the
.B O_CLOEXEC
flag of
.BR open (2).
.TP
.BR MSG_DONTWAIT " (since Linux 2.2)"
Enables nonblocking operation; if the operation would block,
the call fails with the error
.BR EAGAIN " or " EWOULDBLOCK .
This provides similar behavior to setting the
.B O_NONBLOCK
flag (via the
.BR fcntl (2)
.B F_SETFL
operation), but differs in that
.B MSG_DONTWAIT
is a per-call option, whereas
.B O_NONBLOCK
is a setting on the open file description (see
.BR 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.
.TP
.BR 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
.BR IP_RECVERR ).
The user should supply a buffer of sufficient size.
See
.BR cmsg (3)
and
.BR ip (7)
for more information.
The payload of the original packet that caused the error
is passed as normal data via
.IR msg_iovec .
The original destination address of the datagram that caused the error
is supplied via
.IR msg_name .
.IP
The error is supplied in a
.I sock_extended_err
structure:
.IP
.in +4n
.EX
#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
.in
.IP
.I ee_errno
contains the
.I errno
number of the queued error.
.I ee_origin
is the origin code of where the error originated.
The other fields are protocol-specific.
The macro
.B 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
.I sa_family
member of the
.I sockaddr
contains
.B AF_UNSPEC
and the other fields of the
.I sockaddr
are undefined.
The payload of the packet that caused the error is passed as normal data.
.IP
For local errors, no address is passed (this
can be checked with the
.I cmsg_len
member of the
.IR cmsghdr ).
For error receives,
the
.B MSG_ERRQUEUE
flag is set in the
.IR 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.
.TP
.B 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.
.TP
.B 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.
.TP
.BR MSG_TRUNC " (since Linux 2.2)"
For raw
.RB ( AF_PACKET ),
Internet datagram (since Linux 2.4.27/2.6.8),
netlink (since Linux 2.6.22), and UNIX datagram
.\" commit 9f6f9af7694ede6314bed281eec74d588ba9474f
(since Linux 3.4) sockets:
return the real length of the packet or datagram,
even when it was longer than the passed buffer.
.IP
For use with Internet stream sockets, see
.BR tcp (7).
.TP
.BR 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.
.\"
.SS recvfrom()
.BR recvfrom ()
places the received message into the buffer
.IR buf .
The caller must specify the size of the buffer in
.IR len .
.PP
If
.I 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
.IR src_addr .
.\" (Note: for datagram sockets in both the UNIX and Internet domains,
.\" .I src_addr
.\" is filled in.
.\" .I src_addr
.\" is also filled in for stream sockets in the UNIX domain, but is not
.\" filled in for stream sockets in the Internet domain.)
.\" [The above notes on AF_UNIX and AF_INET sockets apply as at
.\" Kernel 2.4.18. (MTK, 22 Jul 02)]
In this case,
.I addrlen
is a value-result argument.
Before the call,
it should be initialized to the size of the buffer associated with
.IR src_addr .
Upon return,
.I 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,
.I addrlen
will return a value greater than was supplied to the call.
.PP
If the caller is not interested in the source address,
.I src_addr
and
.I addrlen
should be specified as NULL.
.\"
.SS recv()
The
.BR recv ()
call is normally used only on a
.I connected
socket (see
.BR connect (2)).
It is equivalent to the call:
.PP
    recvfrom(fd, buf, len, flags, NULL, 0);
.\"
.SS recvmsg()
The
.BR recvmsg ()
call uses a
.I msghdr
structure to minimize the number of directly supplied arguments.
This structure is defined as follows in
.IR <sys/socket.h> :
.PP
.in +4n
.EX
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 */
};
.EE
.in
.PP
The
.I 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
.I msg_namelen
to the size of this buffer before this call;
upon return from a successful call,
.I msg_namelen
will contain the length of the returned address.
If the application does not need to know the source address,
.I msg_name
can be specified as NULL.
.PP
The fields
.I msg_iov
and
.I msg_iovlen
describe scatter-gather locations, as discussed in
.BR readv (2).
.PP
The field
.IR msg_control ,
which has length
.IR msg_controllen ,
points to a buffer for other protocol control-related messages or
miscellaneous ancillary data.
When
.BR recvmsg ()
is called,
.I msg_controllen
should contain the length of the available buffer in
.IR msg_control ;
upon return from a successful call it will contain the length
of the control message sequence.
.PP
The messages are of the form:
.PP
.in +4n
.EX
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[]; */
};
.EE
.in
.PP
Ancillary data should be accessed only by the macros defined in
.BR cmsg (3).
.PP
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
.BR unix (7)
and
.BR ip (7).
.PP
The
.I msg_flags
field in the
.I msghdr
is set on return of
.BR recvmsg ().
It can contain several flags:
.TP
.B MSG_EOR
indicates end-of-record; the data returned completed a record (generally
used with sockets of type
.BR SOCK_SEQPACKET ).
.TP
.B MSG_TRUNC
indicates that the trailing portion of a datagram was discarded because the
datagram was larger than the buffer supplied.
.TP
.B MSG_CTRUNC
indicates that some control data was discarded due to lack of space in the
buffer for ancillary data.
.TP
.B MSG_OOB
is returned to indicate that expedited or out-of-band data was received.
.TP
.B MSG_ERRQUEUE
indicates that no data was received but an extended error from the socket
error queue.
.SH RETURN VALUE
These calls return the number of bytes received, or \-1
if an error occurred.
In the event of an error,
.I errno
is set to indicate the error.
.PP
When a stream socket peer has performed an orderly shutdown,
the return value will be 0 (the traditional "end-of-file" return).
.PP
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.
.PP
The value 0 may also be returned if the requested number of bytes
to receive from a stream socket was 0.
.SH 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.
.TP
.BR EAGAIN " or " EWOULDBLOCK
.\" Actually EAGAIN on Linux
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.
.TP
.B EBADF
The argument
.I sockfd
is an invalid file descriptor.
.TP
.B ECONNREFUSED
A remote host refused to allow the network connection (typically
because it is not running the requested service).
.TP
.B EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.TP
.B EINTR
The receive was interrupted by delivery of a signal before
any data was available; see
.BR signal (7).
.TP
.B EINVAL
Invalid argument passed.
.\" e.g., msg_namelen < 0 for recvmsg() or addrlen < 0 for recvfrom()
.TP
.B ENOMEM
Could not allocate memory for
.BR recvmsg ().
.TP
.B ENOTCONN
The socket is associated with a connection-oriented protocol
and has not been connected (see
.BR connect (2)
and
.BR accept (2)).
.TP
.B ENOTSOCK
The file descriptor
.I sockfd
does not refer to a socket.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008,
4.4BSD (these interfaces first appeared in 4.2BSD).
.PP
POSIX.1 describes only the
.BR MSG_OOB ,
.BR MSG_PEEK ,
and
.B MSG_WAITALL
flags.
.SH NOTES
If a zero-length datagram is pending,
.BR read (2)
and
.BR recv ()
with a
.I flags
argument of zero provide different behavior.
In this circumstance,
.BR read (2)
has no effect (the datagram remains pending), while
.BR recv ()
consumes the pending datagram.
.PP
The
.I socklen_t
type was invented by POSIX.
See also
.BR accept (2).
.PP
According to POSIX.1,
.\" POSIX.1-2001, POSIX.1-2008
the
.I msg_controllen
field of the
.I msghdr
structure should be typed as
.IR socklen_t ,
and the
.I msg_iovlen
field should be typed as
.IR int ,
but glibc currently types both as
.IR size_t .
.\" glibc bug for msg_controllen raised 12 Mar 2006
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
.\" The problem is an underlying kernel issue: the size of the
.\" __kernel_size_t type used to type these fields varies
.\" across architectures, but socklen_t is always 32 bits,
.\" as (at least with GCC) is int.
.PP
See
.BR recvmmsg (2)
for information about a Linux-specific system call
that can be used to receive multiple datagrams in a single call.
.SH EXAMPLES
An example of the use of
.BR recvfrom ()
is shown in
.BR getaddrinfo (3).
.SH SEE ALSO
.BR fcntl (2),
.BR getsockopt (2),
.BR read (2),
.BR recvmmsg (2),
.BR select (2),
.BR shutdown (2),
.BR socket (2),
.BR cmsg (3),
.BR sockatmark (3),
.BR ip (7),
.BR ipv6 (7),
.BR socket (7),
.BR tcp (7),
.BR udp (7),
.BR unix (7)