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 2002-12-31 "Linux Man Page" "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 " s ", void *" buf ", size_t " len ", int " flags );
.sp
.BI "ssize_t recvfrom(int " s ", void *" buf ", size_t " len ", int " flags , 
.BI "                 struct sockaddr *" from ", socklen_t *" fromlen );
.sp
.BI "ssize_t recvmsg(int " s ", 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 from
is not NULL, and the underlying protocol provides the source address,
this source address is filled in.
.\" (Note: for datagram sockets in both the Unix and Internet domains,
.\" .I from
.\" is filled in.
.\" .I from
.\" 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 fromlen
is a value-result parameter, initialized to the size of the buffer
associated with
.IR from ,
and modified on return to indicate the actual size of the address stored
there.
.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 from
parameter. 
.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 (see
.BR socket (2)).
.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
set to
.BR EAGAIN .
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 
.IR OR 'ing
one or more of the following values:
.TP
.B MSG_DONTWAIT
Enables non-blocking operation; if the operation would block,
.B EAGAIN
is returned (this can also be enabled using the
.B O_NONBLOCK
with the
.B F_SETFL
.BR fcntl (2)).
.TP
.B MSG_ERRQUEUE
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 +0.25i
.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
{
    u_int32_t ee_errno;   /* error number */
    u_int8_t  ee_origin;  /* where the error originated */ 
    u_int8_t  ee_type;    /* type */
    u_int8_t  ee_code;    /* code */
    u_int8_t  ee_pad;
    u_int32_t ee_info;    /* additional information */
    u_int32_t ee_data;    /* other data */  
    /* More data may follow */ 
};

struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);
.fi
.in -0.25i
.IP
.B ee_errno 
contains the errno number of the queued error. 
.B 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 
.B sockaddr 
contains 
.B AF_UNSPEC
and the other fields of the 
.B 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
.B MSG_TRUNC
Return the real length of the packet, even when it was longer than
the passed buffer. Only valid for packet sockets.
.TP
.B MSG_WAITALL
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 parameters.  This
structure has the following form, as defined in
.IR <sys/socket.h> :
.in +0.25i
.nf

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 */
    socklen_t     msg_controllen; /* ancillary data buffer len */
    int           msg_flags;      /* flags on received message */
};
.fi
.in -0.25i
.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 +0.25i
.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
    u_char    cmsg_data[]; */
};
.fi
.in -0.25i
.PP
Ancillary data should only be accessed by the macros defined in 
.BR cmsg (3).
.PP
As an example, Linux uses this auxiliary data mechanism to pass extended
errors, IP options or file descriptors over Unix sockets. 
.PP
The
.I msg_flags
field in the 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
.B EAGAIN
The socket is marked non-blocking and the receive operation
would block, or a receive timeout had been set and the timeout expired
before data was received.
.TP
.B EBADF
The argument
.I s
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.
.TP
.B EINVAL
Invalid argument passed. 
.\" e.g., msg_namelen < 0 for recvmsg() or fromlen < 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 s
does not refer to a socket.
.SH "CONFORMING TO"
4.4BSD (these function calls first appeared in 4.2BSD),
POSIX 1003.1-2001.
.LP
POSIX 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 `ssize_t' (while 4.x BSD and libc4 and libc5 all have `int').
The
.I flags
argument is `int' in 4.x BSD, but `unsigned int' in libc4 and libc5.
The
.I len
argument is `int' in 4.x BSD, but `size_t' in libc4 and libc5.
The
.I fromlen
argument is `int *' in 4.x BSD, libc4 and libc5.
The present  `socklen_t *' 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 (2.4) types it as 
.IR size_t .
.\" FIXME . glibc bug raised 12 Mar 2006
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=2448
.SH "SEE ALSO"
.BR fcntl (2),
.BR getsockopt (2),
.BR read (2),
.BR select (2),
.BR shutdown (2),
.BR socket (2),
.BR cmsg (3)