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HP OpenVMS systems documentation |
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This chapter describes how to use the Sockets API functions.
3.1 Internet Protocols
The IP (Internet Protocol) family is a collection of protocols on the Transport layer that use the Internet address format. The two basic Internet protocols are:
The TCP/IP protocol suite has been extended beyond the basic 32-bit addressing capabilities of IPv4. With the new IPv6 protocol, the address size is increased to 128 bits. The basic syntax of socket functions remains the same, with extensions to the basic sockets API and advanced sockets application programming interfaces.
The following sections describe the basic TCP and UDP protocols,
including the extensions provided for IPv6.
3.1.1 TCP Sockets
TCP provides reliable, flow-controlled, two-way transmission of data. A byte-stream protocol used to support the SOCK_STREAM abstraction, TCP uses standard IP address formats and provides a per-host collection of port addresses. Thus, each address consists of an internet address specifying the host and network, with a specific TCP port on the host identifying the peer entity.
Sockets using TCP are either active or passive, as described in Table 3-1.
| Socket Type | Description |
|---|---|
| Active |
Initiates connections to passive sockets. By default, TCP sockets are
active.
Active sockets use the connect() function to initiate connections. |
| Passive |
Listens for connection requests from active sockets. To create a
passive socket, use the
bind()
function and then the
listen()
function.
Passive sockets use the accept() function to accept incoming connections. If the server is running on a multihomed system, you can specify wildcard addressing. Wildcard addressing allows a single server to provide service to clients on multiple networks. (See Section 3.1.1.1.) |
When a server is running on a host that has more than one network interface installed, you can use wildcard addressing to configure it to accept incoming connections on all the interfaces.
The wildcard address is the any-interface choice. You specify this address by setting the IP address in the socket address structure to INADDR_ANY (for IPv4) or in6addr_any (for IPv6) before calling the bind() function.
To create a socket that listens to all hosts on any network interface, perform these steps:
The address assigned to the socket is the address associated with the network interface through which packets from the peer are being transmitted and received. This address corresponds to the peer entity's network.
TCP supports the setting of socket options with the setsockopt() function and the checking of current option settings with the getsockopt() function. Under most circumstances, TCP sends data when it is presented. When outstanding data has not been acknowledged, TCP gathers small amounts of output and sends it in a single packet when an acknowledgment is received.
For some clients, such as Microsoft Windows systems, which send a stream of mouse events that receive no replies, this packetization can cause significant delays. TCP/IP Services provides the TCP_NODELAY option (defined in the TCP.H header file) to manage this problem. Refer to Table A-2 for more information about setting this option. Note that this solution may cause an increase in network traffic. |
UDP is a protocol that supports the SOCK_DGRAM abstraction for the internet protocol family. UDP sockets are connectionless and are normally used with the sendto() and recvfrom() functions. You can also use the connect() function to establish the destination address for future datagrams; then you use the read() , write() , send() , or recv() function to transmit or receive datagrams.
UDP address formats are identical to those used by TCP. In particular,
UDP provides a port identifier in addition to the normal internet
address format. Note that the UDP port space is separate from the TCP
port space (for example, a UDP port cannot be connected to a TCP port).
Also, you can send broadcast packets (assuming the underlying network
supports broadcasting) by using a reserved broadcast address. This
address is network-interface dependent. The
SO_BROADCAST
option must be set on the socket, and the process must have SYSPRV,
BYPASS, or OPER privilege for broadcasting to succeed.
3.2 Structures
This section describes, in alphabetical order, the structures you supply as arguments to the various Sockets API functions. Table 3-2 lists these structures.
| Structure | Description |
|---|---|
| addrinfo | This structure describes the type of socket, the address family, and the protocol. |
| cmsghdr | This structure describes ancillary data objects transferred by the sendmsg() and recvmsg() functions. |
| hostent | This structure holds a canonical host name, alias names, a host address type, the length of the address, and a pointer to a list of host addresses. This structure is a parameter value for host name and address lookup functions. |
| in_addr | This structure holds a 32-bit IPv4 address stored in network byte order. |
| in6_addr | This structure holds a 128-bit IPv6 address stored in network byte order as an array of sixteen 8-bit elements. |
| iovec | This structure holds the beginning address and length of an I/O buffer. |
| linger | This structure holds option information for the close function. |
| msghdr | This structure holds the protocol address, the size of the protocol address, a scatter-and-gather array, the number of elements in the scatter-and-gather array, ancillary data, the length of the ancillary data, and returned flags. The structure is a parameter of the recvmsg() and sendmsg() functions. |
| netent | This structure holds a network name, a list of aliases associated with the network, and the network number. |
| protoent | This structure describes a protocol. |
| servent | This structure describes a network service. |
| sockaddr | The socket functions use this generic socket address structure to function with any of the supported protocol families. |
| sockaddr_in | This IPv4 socket address structure holds the length of the structure, the address family, either a TCP or a UDP port number, and a 32-bit IPv4 address stored in network byte order. The structure has a fixed length of 16 bytes. |
| sockaddr_in6 | This IPv6 socket address structure holds the length of the structure, the address family, the transport layer port number, a priority and flow label, and a 128-bit IPv6 address. The structure has a fixed length of 28 bytes. |
| timeval | This structure holds a time interval specified in seconds and microseconds. |
The addrinfo structure is defined in the NETDB.H header file, and consists of the following components:
struct addrinfo {
int ai_flags; /* input flags */
int ai_family; /* protofamily for socket */
int ai_socktype; /* socket type */
int ai_protocol; /* protocol for socket */
size_t ai_addrlen; /* length of socket-address */
char *ai_canonname; /* service location canonical name */
struct sockaddr *ai_addr; /* socket-address for socket */
struct addrinfo *ai_next; /* pointer to next in list */
};
|
The cmsghdr structure describes ancillary data objects transferred by the sendmsg and recvmsg functions.
The msg_control member of the msghdr data structure points to the ancillary data that are contained in a cmsghdr structure. Typically, only one data object is passed in a cmsghdr structure. However, the IPv6 advanced sockets API enables the sendmsg and recvmsg functions to pass multiple objects. For information about using raw IPv6 sockets, see Section 3.6.1.
The data structure is defined in the SOCKET.H header file.
The cmsghdr data structure consists of the following components
struct cmsghdr {
socklen_t cmsg_len; /* #bytes, including this header */
int cmsg_level; /* originating protocol */
int cmsg_type; /* protocol-specific type */
/* followed by unsigned char cmsg_data[]; */
};
|
The hostent structure, defined in the NETDB.H header file, holds a host name, a list of aliases associated with the network, and the network's number as specified in an internet address from the hosts database.
The hostent structure definition is as follows:
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses from name server */
};
#define h_addr h_addr_list[0] /* address, for backward compatibility */
|
The hostent structure members are as follows:
The in_addr structure, defined in the IN.H header file, holds an IPv4 address. The address format can be any of the supported internet address notation formats.
The in_addr structure definition is as follows:
struct in_addr {
union {
struct { u_char s_b1,s_b2,s_b3,s_b4; } S_un_b;
struct { u_short s_w1,s_w2; } S_un_w;
u_long S_addr;
} S_un;
#define s_addr S_un.S_addr /* can be used for most tcp & ip code */
#define s_host S_un.S_un_b.s_b2 /* host on imp */
#define s_net S_un.S_un_b.s_b1 /* network */
#define s_imp S_un.S_un_w.s_w2 /* imp */
#define s_impno S_un.S_un_b.s_b4 /* imp # */
#define s_lh S_un.S_un_b.s_b3 /* logical host */
};
|
The in6_addr structure, defined in the IN6.H header file, holds an IPv6 address. The address format can be any of the supported internet address notation formats. The address is stored in network byte order as an array of sixteen 8-bit elements.
The in6_addr structure definition is as follows:
struct in6_addr {
u_int8_t s6_addr[16]
}
|
A wildcard address, defined in network byte order, has the following forms:
A loopback address, defined in network byte order, has the following forms:
The iovec structure holds one scatter-and-gather buffer. Multiple scatter-and-gather buffer descriptors are stored as an array of iovec elements.
The iovec structure definition is defined in the SOCKET.H header file.
The iovec structure definition is as follows:
struct iovec {
char *iov_base;
int iov_len;
}
|
The iovec structure members are as follows:
The linger structure, defined in the SOCKET.H header file, specifies the setting or resetting of the socket option for the time interval that the socket lingers for data. The linger structure is supported only by connection-based (SOCK_STREAM) sockets.
The linger structure definition is as follows:
struct linger {
int l_onoff; /* option on/off */
int l_linger; /* linger time */
};
|
The linger structure members are as follows:
The msghdr structure specifies the buffer parameter for the recvmsg and sendmsg I/O functions. The structure allows you to specify an array of scatter and gather buffers. The recvmsg function scatters the data to several user receive buffers, and the sendmsg function gathers data from several user transmit buffers before being transmitted.
The SOCKET.H header file defines the following structures for BSD Versions 4.3 and 4.4:
The omsghdr structure definition for use with BSD Version 4.3 is as follows:
struct omsghdr {
char *msg_name; /* protocol address */
int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* number of elements in msg_iov */
char *msg_accrights; /* access rights sent/received */
int msg_accrightslen; /* length of access rights buffer */
};
|
The omsghdr structure members are as follows:
The msghdr structure definition for use with BSD Version 4.4 is as follows:
struct msghdr {
void *msg_name; /* protocol address */
int msg_namelen; /* size of address */
struct iovec *msg_iov; /* scatter/gather array */
int msg_iovlen; /* number of elements in msg_iov */
void *msg_control; /* ancillary data; must be aligned
for a cmsghdr structure */
int msg_controllen; /* length of ancillary data buffer */
int msg_flags; /* flags on received message */
};
|
The msghdr structure members are as follows:
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