View Source: recvmsg(2) - Waikato Linux Users Group

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!!NAME
recv, recvfrom, recvmsg - receive a message from a socket

!!SYNOPSIS
 __#include <sys/types.h>__
 __#include <sys/socket.h>__

 __int recv(int__ ''s''__, void *__''buf''__, size_t__ ''len''__, int__ ''flags''__);__

 __int recvfrom(int__ ''s''__, void *__''buf''__, size_t__ ''len''__, int__ ''flags''__, struct sockaddr *__''from''__, socklen_t *__''fromlen''__);__

 __int 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 socket is not connection-oriented, the source address of the message 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 (see socket(2)).

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)) 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_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_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.

;__MSG_NOSIGNAL__: This flag turns off raising of [SIGPIPE] on stream sockets when the other end disappears.

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

;__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
 {
  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 *);

__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.

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 destination 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
  u_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.

;__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)).

!!RETURN VALUE
These calls return the number of bytes received, or -1 if an error occurred.

!!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.

;[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).
;[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.
;[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.
;[EINTR]: The receive was interrupted by delivery of a signal before any data were available.
;[EFAULT]: The receive buffer pointer(s) point outside the process's address space.
;[EINVAL]: Invalid argument passed.

!!CONFORMING TO
4.4BSD (these function calls first appeared in 4.2BSD).

!!NOTE
The prototypes given above follow glibc2. The Single Unix Specification agrees, except that it has return values of type `ssize_t' (while BSD 4.* and libc4 and libc5 all  have `int'). The ''flags'' argument is `int' in BSD 4.*, but `unsigned int' in libc4 and libc5. The ''len'' argument is `int' in BSD 4.*, but `size_t' in libc4 and libc5. The ''fromlen'' argument is `int *' in BSD 4.*, libc4 and libc5. The present `socklen_t *' was invented by POSIX. See also accept(2).

!!SEE ALSO
fcntl(2), read(2), select(2), getsockopt(2), socket(2), cmsg(3)
10 pages link to recvmsg(2):
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Last edited on Monday, March 17, 2003 1:24:03 am by PerryLorier
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