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#include <sys/types.h> #include <sys/socket.h>int recv(s, buf, len, flags) int s; void *buf; int len, flags;
int recvfrom(s, buf, len, flags, from, fromlen) int s; void *buf; int len, flags; struct sockaddr *from; int *fromlen;
int recvmsg(s, msg, flags) int s; struct msghdr *msg; int flags;
The recv( ) call may be used only on a connected socket (see connect(SSC) for more information), while recvfrom( ) and recvmsg( ) may be used to receive data on a socket whether it is in a connected state or not.
If from is non-zero, the source address of the message is filled in. 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 length of the message is returned in cc. 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(SSC).
If no messages are available at the socket, the
receive call waits for a message to arrive.
The flags argument to a send call is formed by or'ing one or more of the values:
#define MSG_OOB 0x1 /* process out-of-band data */ #define MSG_PEEK 0x2 /* peek at incoming message */
The recvmsg( ) call uses a msghdr structure to minimize the number of directly supplied parameters. This structure is defined in the sys/socket.h header file, and includes the following members:
caddr_t msg_name; /* optional address */ int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_control; /* control information sent/received */ int msg_controllen; /* size of control information */ int msg_flags; /* size of control information */
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.
msg_iov
and
msg_iovlen
describe the scatter-gather locations, as described in
readv(S).
A buffer to receive any control information sent along with the
message is specified in
msg_control,
which has length
msg_controllen .
Control 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[]; */
};
Control messages can be stepped through using the CMSG_FIRSTHDR and CMSG_NEXTHDR macros defined in <sys/socket.h>.
The ``msg_flags'' member is set on return according to the message received. 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.