recv
Prev  Section 2  Next

Name

recv, recvfrom, recvmsg — receive a message from a socket

Synopsis

#include <sys/types.h>
#include <sys/socket.h>
ssize_t recv( int   s,
  void *  buf,
  size_t   len,
  int   flags);
ssize_t recvfrom( int   s,
  void *  buf,
  size_t   len,
  int   flags,
  struct sockaddr *  from,
  socklen_t *  fromlen);
ssize_t recvmsg( int   s,
  struct msghdr *  msg,
  int   flags);

DESCRIPTION

The recvfrom() and 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.

If from is not NULL, and the underlying protocol provides the source address, this source address is filled in. The argument fromlen is a value-result parameter, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there.

The recv() call is normally used only on a connected socket (see connect(2)) and is identical to recvfrom() with a NULL from parameter.

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.

If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is non-blocking (see fcntl(2)), in which case the value −1 is returned and the external variable errno set to EAGAIN. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.

The select(2) or poll(2) call may be used to determine when more data arrives.

The flags argument to a recv() call is formed by OR'ing 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)). This flag is useful for the same reasons as the O_CLOEXEC flag of open(2).

MSG_DONTWAIT

Enables non-blocking operation; if the operation would block, EAGAIN is returned (this can also be enabled using the O_NONBLOCK with the F_SETFL fcntl(2)).

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 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.

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 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.

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;
    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_origin is the origin code of where the error originated. The other fields are protocol-specific. The macro 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 sa_family 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 cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE 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

Return the real length of the packet, even when it was longer than the passed buffer. Only valid for packet sockets.

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.

The recvmsg() call uses a msghdr structure to minimize the number of directly supplied parameters. This structure has the following form, as defined 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 */
  socklen_t   msg_controllen;
/* ancillary data buffer len */
  int   msg_flags;
/* flags on received message */
};

Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as a null pointer if no names are desired or required. 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 ancillary 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 sequence.

The messages are of the form:

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[]; */
};

Ancillary data should only be accessed by the macros defined in cmsg(3).

As an example, Linux uses this auxiliary data mechanism to pass extended errors, IP options or file descriptors over Unix sockets.

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 were 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 were received.

MSG_ERRQUEUE

indicates that no data was received but an extended error from the socket error queue.

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.

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.

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.

EBADF

The argument s is an invalid 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 were available.

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 argument s does not refer to a socket.

CONFORMING TO

4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.

POSIX.1-2001 only describes the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.

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 flags argument is int in 4.x BSD, but unsigned int in libc4 and libc5. The len argument is int in 4.x BSD, but size_t in libc4 and libc5. The fromlen argument is int * in 4.x BSD, libc4 and libc5. The present socklen_t * was invented by POSIX. See also accept(2).

According to POSIX.1-2001, the msg_controllen field of the msghdr structure should be typed as socklen_t, but glibc currently (2.4) types it as size_t.

EXAMPLE

An example of the use of recvfrom() is shown in getaddrinfo(3).

SEE ALSO

fcntl(2), getsockopt(2), read(2), select(2), shutdown(2), socket(2), cmsg(3), sockatmark(3)

COLOPHON

This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.


  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 <faithcs.unc.edu>
Modified Tue Oct 22 17:45:19 1996 by Eric S. Raymond <esrthyrsus.com>
Modified 1998,1999 by Andi Kleen
2001-06-19 corrected SO_EE_OFFENDER, bug report by James Hawtin
Prev  Up  Next
reboot  Home  remap_file_pages