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Receive a message and its header from a socket
#include <sys/types.h>
#include <sys/socket.h>
ssize_t recvmsg( int s,
struct msghdr * msg,
int flags );
- s
- The descriptor for the socket; see
socket().
- msg
- A pointer to a msghdr structure where the function can
store the message header; see below.
- len
- The size of the buffer.
- flags
- A combination formed by ORing one or more of the values:
- MSG_OOB — process out-of-band data.
This flag requests receipt of out-of-band data that wouldn't be
received in the normal data stream.
You can't use this flag with protocols that place expedited data at the
head of the normal data queue.
- MSG_PEEK — peek at the incoming message.
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 — wait for full request or error. This
flag requests that the operation block until the full
request is satisfied. But the call may still return less
data than requested if a signal is caught, if an error or
disconnect occurs, or if the next data to be received is of
a different type than that returned.
libsocket
Use the -l socket option to
qcc
to link against this library.
The recvmsg() routine receives a message from a
socket, s, whether or not it's connection-oriented.
The recvmsg() call uses a msghdr
structure to minimize the number of directly supplied
parameters. This structure, defined in
<sys/socket.h>, has the following form:
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 */
};
The msg_name and msg_namelen
parameters specify the address (source address
for recvmsg(); destination address for
sendmsg())
if the socket is unconnected;
the msg_name parameter may be given
as a null pointer if no names are desired or required.
The msg_iov and msg_iovlen parameters
describe scatter-gather locations, as discussed in
read().
The msg_control parameter, whose length is
determined by msg_controllen, points to a buffer
for other protocol-control related messages or for 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[]; */
};
The msg_flags field is set on return according to the message received:
- MSG_CTRUNC
- Indicates that some control data was discarded due to lack of
space in the buffer for ancillary data.
- MSG_EOR
- Indicates end-of-record; the data returned completed a record.
- MSG_OOB
- Indicates that expedited or out-of-band data was received.
- MSG_TRUNC
- Indicates that the trailing portion of a datagram was discarded
because the datagram was larger than the buffer supplied.
The number of bytes received, or -1 if an error occurs (errno is set).
- ENOMEM
- Not enough memory.
POSIX 1003.1
Safety: | |
Cancellation point |
Yes |
Interrupt handler |
No |
Signal handler |
No |
Thread |
Yes |
recv(),
recvfrom(),
sendmsg()
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