4.4BSD/usr/share/man/cat2/recvmsg.0
RECV(2) BSD Programmer's Manual RECV(2)
NNAAMMEE
rreeccvv, rreeccvvffrroomm, rreeccvvmmssgg - receive a message from a socket
SSYYNNOOPPSSIISS
##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
##iinncclluuddee <<ssyyss//ssoocckkeett..hh>>
_i_n_t
rreeccvv(_i_n_t _s, _v_o_i_d _*_b_u_f, _i_n_t _l_e_n, _i_n_t _f_l_a_g_s);
_i_n_t
rreeccvvffrroomm(_i_n_t _s, _v_o_i_d _*_b_u_f, _i_n_t _l_e_n, _i_n_t _f_l_a_g_s, _s_t_r_u_c_t _s_o_c_k_a_d_d_r _*_f_r_o_m,
_i_n_t _*_f_r_o_m_l_e_n);
_i_n_t
rreeccvvmmssgg(_i_n_t _s, _s_t_r_u_c_t _m_s_g_h_d_r _*_m_s_g, _i_n_t _f_l_a_g_s);
DDEESSCCRRIIPPTTIIOONN
RReeccvvffrroomm() and rreeccvvmmssgg() 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 _f_r_o_m is non-nil, and the socket is not connection-oriented, the source
address of the message is filled in. _F_r_o_m_l_e_n is a value-result parame-
ter, initialized to the size of the buffer associated with _f_r_o_m, and mod-
ified on return to indicate the actual size of the address stored there.
The rreeccvv() call is normally used only on a _c_o_n_n_e_c_t_e_d socket (see
connect(2)) and is identical to rreeccvvffrroomm() with a nil _f_r_o_m parameter.
As it is redundant, it may not be supported in future releases.
All three routines return the length of the message on successful comple-
tion. 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 re-
ceived from (see socket(2)).
If no messages are available at the socket, the receive call waits 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 _e_r_r_n_o 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; this behavior is affected by the socket-level options
SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).
The select(2) call may be used to determine when more data arrive.
The _f_l_a_g_s argument to a recv call is formed by _o_r _A_p _i_n_g one or more of
the values:
MSG_OOB process out-of-band data
MSG_PEEK peek at incoming message
MSG_WAITALL wait for full request or error
The MSG_OOB 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. The MSG_PEEK 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. The MSG_WAITALL 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 re-
turned.
The rreeccvvmmssgg() call uses a _m_s_g_h_d_r structure to minimize the number of di-
rectly supplied parameters. This structure has the following form, as
defined in <_s_y_s_/_s_o_c_k_e_t_._h>:
struct msghdr {
caddr_t msg_name; /* optional address */
u_int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
u_int msg_iovlen; /* # elements in msg_iov */
caddr_t msg_control; /* ancillary data, see below */
u_int msg_controllen; /* ancillary data buffer len */
int msg_flags; /* flags on received message */
};
Here _m_s_g___n_a_m_e and _m_s_g___n_a_m_e_l_e_n specify the destination address if the
socket is unconnected; _m_s_g___n_a_m_e may be given as a null pointer if no
names are desired or required. _M_s_g___i_o_v and _m_s_g___i_o_v_l_e_n describe scatter
gather locations, as discussed in read(2). _M_s_g___c_o_n_t_r_o_l, which has length
_m_s_g___c_o_n_t_r_o_l_l_e_n, points to a buffer for other protocol control related
messages or other miscellaneous ancillary data. The messages are of the
form:
struct cmsghdr {
u_int cmsg_len; /* data byte count, including hdr */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by
u_char cmsg_data[]; */
};
As an example, one could use this to learn of changes in the data-stream
in XNS/SPP, or in ISO, to obtain user-connection-request data by request-
ing a recvmsg with no data buffer provided immediately after an aacccceepptt()
call.
Open file descriptors are now passed as ancillary data for AF_UNIX domain
sockets, with _c_m_s_g___l_e_v_e_l set to SOL_SOCKET and _c_m_s_g___t_y_p_e set to
SCM_RIGHTS.
The _m_s_g___f_l_a_g_s field is set on return according to the message received.
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 data-
gram was larger than the buffer supplied. MSG_CTRUNC indicates that some
control data were discarded due to lack of space in the buffer for ancil-
lary data. MSG_OOB is returned to indicate that expedited or out-of-band
data were received.
RREETTUURRNN VVAALLUUEESS
These calls return the number of bytes received, or -1 if an error oc-
curred.
EERRRROORRSS
The calls fail if:
[EBADF] The argument _s is an invalid descriptor.
[ENOTCONN] The socket is assoicated 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 time-
out expired before data were 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 ad-
dress space.
SSEEEE AALLSSOO
fcntl(2), read(2), select(2), getsockopt(2), socket(2)
HHIISSTTOORRYY
The rreeccvv function call appeared in 4.2BSD.
4.3-Reno Berkeley Distribution June 4, 1993 3