recvmsg
RECV(2) Linux Programmer’s Manual RECV(2)
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 sock-
addr *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 associ-
ated 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 con-
nect(2)) and is identical to recvfrom with a NULL from parameter.
All three routines return the length of the message on successful com-
pletion. If a message is too long to fit in the supplied buffer,
excess bytes may be discarded depending on the type of socket the mes-
sage 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 dif-
ferent type than that returned.
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 nor-
mal 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 opera-
tion.
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 net-
work 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 opera-
tion.
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
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_NON-
BLOCK with the F_SETFL fcntl(2)).
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. Addi-
tional errors may be generated and returned from the underlying proto-
col modules; see their manual pages.
EBADF The argument s is an invalid descriptor.
ECONNREFUSED
A remote host refused to allow the network connection (typi-
cally 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.
ENOMEM Could not allocate memory for recvmsg.
CONFORMING TO
4.4BSD (these function calls first appeared in 4.2BSD), POSIX
1003.1-2001.
POSIX only describes the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.
NOTE
The prototypes given above follow glibc2. The Single Unix Specifica-
tion 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)
Linux Man Page 2002-12-31 RECV(2)
