recvmsg(2) - Waikato Linux Users Group

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.

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).