path: root/man2/recv.2

.\" Copyright (c) 1983, 1990, 1991 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     $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 2011-09-16 "Linux" "Linux Programmer's Manual"
.SH NAME
recv, recvfrom, recvmsg \- receive a message from a socket
.SH SYNOPSIS
.\" .B #include <sys/uio.h>
.\" .br
.nf
.B #include <sys/types.h>
.br
.B #include <sys/socket.h>
.sp
.BI "ssize_t recv(int " sockfd ", void *" buf ", size_t " len ", int " flags );
.sp
.BI "ssize_t recvfrom(int " sockfd ", void *" buf ", size_t " len ", int " flags ,
.BI "                 struct sockaddr *" src_addr ", socklen_t *" addrlen );
.sp
.BI "ssize_t recvmsg(int " sockfd ", struct msghdr *" msg ", int " flags );
.fi
.SH DESCRIPTION
The
.BR recvfrom ()
and
.BR recvmsg ()
calls are used to receive messages from a socket, and may be used
to receive data on a socket whether or not it is connection-oriented.
.PP
If
.I src_addr
is not NULL, and the underlying protocol provides the source address,
this source address is filled in.
When
.I src_addr
is NULL, nothing is filled in; in this case,
.I addrlen
is not used, and should also be NULL.
.\" (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)]
The argument
.I addrlen
is a value-result argument,
which the caller should initialize before the call to
the size of the buffer associated with
.IR src_addr ,
and modified on return to indicate 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
The
.BR recv ()
call is normally used only on a
.I connected
socket (see
.BR connect (2))
and is identical to
.BR recvfrom ()
with a NULL
.I src_addr
argument.
.PP
All three routines 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 the external variable
.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
The
.BR select (2)
or
.BR poll (2)
call may be used to determine when more data arrives.
.PP
The
.I flags
argument to a
.BR recv ()
call 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 can also be enabled using the
.B O_NONBLOCK
flag with the
.B F_SETFL
.BR fcntl (2)).
.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
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
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.

The error is supplied in a
.I sock_extended_err
structure:
.in +4n
.nf

#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 *);
.fi
.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 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
.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
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),
and netlink (since Linux 2.6.22) sockets:
return the real length of the packet or datagram,
even when it was longer than the passed buffer.
Not implemented for UNIX domain
.RB ( unix (7))
sockets.

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.
.PP
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> :
.in +4n
.nf

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 */
};
.fi
.in
.PP
Here
.I msg_name
and
.I msg_namelen
specify the source address if the socket is unconnected;
.I msg_name
may be given as a NULL pointer if no names are desired or required.
The fields
.I msg_iov
and
.I msg_iovlen
describe scatter-gather locations, as discussed in
.BR readv (2).
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:
.in +4n
.nf

struct cmsghdr {
    socklen_t     cmsg_len;     /* data byte count, including hdr */
    int           cmsg_level;   /* originating protocol */
    int           cmsg_type;    /* protocol-specific type */
/* followed by
    unsigned char cmsg_data[]; */
};
.fi
.in
.PP
Ancillary data should only be accessed 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.
.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 were 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 were 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.
The return value will be 0 when the
peer has performed an orderly shutdown.
.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.1-2001 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 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 were 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 argument
.I sockfd
does not refer to a socket.
.SH "CONFORMING TO"
4.4BSD (these function calls first appeared in 4.2BSD),
POSIX.1-2001.
.LP
POSIX.1-2001 only describes the
.BR MSG_OOB ,
.BR MSG_PEEK ,
and
.B MSG_WAITALL
flags.
.SH NOTES
The prototypes given above follow glibc2.
The Single UNIX Specification agrees, except that it has return values
of type \fIssize_t\fP (while 4.x BSD and libc4 and libc5 all have \fIint\fP).
The
.I flags
argument is \fIint\fP in 4.x BSD, but \fIunsigned int\fP in libc4 and libc5.
The
.I len
argument is \fIint\fP in 4.x BSD, but \fIsize_t\fP in libc4 and libc5.
The
.I addrlen
argument is \fIint\ *\fP in 4.x BSD, libc4 and libc5.
The present  \fIsocklen_t\ *\fP was invented by POSIX.
See also
.BR accept (2).

According to POSIX.1-2001, the
.I msg_controllen
field of the
.I msghdr
structure should be typed as
.IR socklen_t ,
but glibc currently types it as
.IR size_t .
.\" glibc bug 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 this field varies
.\" across architectures, but socklen_t is always 32 bits.
.SH EXAMPLE
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 select (2),
.BR shutdown (2),
.BR socket (2),
.BR cmsg (3),
.BR sockatmark (3),
.BR socket (7)