User Datagram Protocol (UDP) is a simple, unreliable datagram protocol used to support the SOCK_DGRAM abstraction for the Internet protocol family. UDP sockets are connectionless and are normally used with the sendto and recvfrom calls, though the connect call may also be used to fix the destination for future packets (in which case the recv or read or write system calls may be used).

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 may not be connected to a TCP port). Also, broadcast packets may be sent (assuming the underlying network supports this) 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 the SYSPRV or BYPASS privilege for broadcasting to succeed.

A.4 errno Values

errno is an external variable whose value is set whenever an error occurs during a call to any of the Compaq C RTL routines. You can use this value to obtain a more detailed description of the error. errno is not cleared on successful calls, so its value should be checked only when an error has been indicated.

Most calls to the Compaq C RTL routines have one or more returned values. Any error condition is indicated by an otherwise impossible return value. This is almost always --1; the individual routine descriptions specify the details.

All return codes and values from routines are of type int unless otherwise noted. An error number is also made available in the external variable errno , which is not cleared on successful calls. The errno values may be translated to a message, similar to that found in UNIX systems, by using the perror routine. vaxc$errno may also be returned as an error.

Table A-2 lists the errno values.

Table A-2 errno Values

Value	Meaning
EINPROGRESS	Operation now in progress
	An operation that takes a long time to complete, such as connect , was attempted on a nonblocking object.
EALREADY	Operation already in progress
	An operation was attempted on a nonblocking object that already had an operation in progress.
ENOTSOCK	Socket operation on a non-socket
EDESTADDRREQ	Destination address required
	A required address was omitted from an operation on a socket.
EMSGSIZE	Message too long
	A message sent on a socket was larger than the internal message buffer.
EPROTOTYPE	Protocol wrong type for socket
	A protocol was specified that does not support the semantics of the socket type requested. For example, you cannot use the ARPA Internet UDP protocol with type SOCK_STREAM.
ENOPROTOOPT	Protocol not available
	A bad option was specified in a getsockopt or setsocketopt call.
EPROTONOSUPPORT	Protocol not supported
	The protocol has not been configured into the system or no implementation for it exists.
ESOCKTNOSUPPORT	Socket type not supported
	The support for the socket type has not been configured into the system or no implementation for it exists.
EOPNOTSUPP	Error-operation not supported
	For example, trying to accept a connection on a datagram socket.
EPFNOSUPPORT	Protocol family not supported
	The protocol family has not been configured into the system or no implementation for it exists.
EAFNOSUPPORT	Address family not supported by protocol family
	An address incompatible with the requested protocol was used.
EADDRINUSE	Address already in use
	Each address can be used only once.
EADDRNOTAVAIL	Cannot assign requested address
	Normally, results from an attempt to create a socket with an address not on this machine.
ENETDOWN	Network is down
	A socket operation encountered a dead network.
ENETUNREACH	Network is unreachable
	A socket operation was attempted to an unreachable network.
ENETRESET	Network dropped connection on reset
	The host you were connected to crashed and rebooted.
ECONNABORTED	Software caused connection abort
	A connection abort was caused internal to your host machine.
ECONNRESET	Connection reset by peer
	A connection was forcibly closed by a peer. This usually results from the peer executing a shutdown call.
ENOBUFS	No buffer space available
	An operation on a socket or pipe was not performed because the system lacked sufficient buffer space.
EISCONN	Socket is already connected
	A connect request was made on an already connected socket; or, a sendto or sendmsg request on a connected socket specified a destination other than the connected party.
ENTOTCONN	Socket is not connected
	Request to send or receive data was disallowed because the socket is not connected.
ESHUTDOWN	Cannot send after socket shutdown
	A request to send data was disallowed because the socket had already been shut down with a previous shutdown call.
ETOOMANYREFS	Too many references: cannot splice
ETIMEDOUT	Connection timed out
	A connect request failed because the connected party did not properly respond after a period of time. (The timeout period is dependent on the communication protocol.) A connect request or remote file operation failed because the connected party did not properly respond after a period of time that is dependent on the communication protocol.
ECONNREFUSED	Connection refused
	No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.
ELOOP	Too many levels of symbolic links
	A path name lookup involved more than eight symbolic links.
ENAMETOOLONG	File name too long
	A component of a path name exceeded 255 characters, or an entire path name exceeded 1023 characters.
EHOSTDOWN	Host is down
	A socket operation failed because the destination host was down.
EHOSTUNREACH	No route to host
	A socket operation was attempted to an unreachable host.
EVMSERR	OpenVMS system-specific error code that is nontranslatable

A.5 h_errno Values

The external integer h_errno is available only with OpenVMS Version 7.0, and is set only by a 4.4BSD TCP/IP interface. Specifically, The gethostbyname and gethostbyaddr functions require 4.4BSD semantics to set h_errno . The gethostbyname and gethostbyaddr functions indicate an error condition by returning a null pointer and setting the external integer h_errno to indicate the error return status.

When gethostbyname or gethostbyaddr returns an error status, h_errno , which is very similar to errno , can be checked to determine whether the error is the result of a temporary failure or an invalid or unknown host.

Use the herror routine to print the error message describing the failure. If the argument string to herror is not NULL, it is printed, followed by a colon (:) and a space. The error message is printed with a trailing new-line character.

The <netdb.h> header file declares h_errno on a per-thread basis as:

#define h_errno (*decc$h_errno_get_addr())

The <netdb.h> header file also symbolically defines the error code values that h_errno can accept, as follows:

HOST_NOT_FOUND	No such host is known.
TRY_AGAIN	Usually a temporary error that means the local server did not receive a response from an authoritative server. A retry at some later time can succeed.
NO_RECOVERY	An unexpected server failure is encountered. This is a nonrecoverable error.
NO_DATA	The requested name is valid but does not have an IP address; this is not a temporary error. The name is known to the name server but there is no address associated with this name. Another type of request to the name server using this domain name results in an answer; for example, a mail-forwarder registered for this domain.
NO_ADDRESS	No address; look for MX record

Like errno , the value of h_errno is zero at program startup. Checking h_errno is valid only when a failure status is returned by a Compaq C RTL routine that is defined to set it.

A.6 Relationship Between errno and h_errno

Since the routines that set h_errno can also set errno , this section explains the relationship between the two variables.

A given routine failure does not set both h_errno and errno for the same failure. A routine failure sets either h_errno or errno depending on which is appropriate for the failing condition.

For example, consider the following program:

/* Demonstrate relationship between errno and h_errno */ /* from Appendix A of the reference manual */ #include <errno.h> #include <stdio.h> #include <netdb.h> main() { static char hostname[256]; struct hostent *hostentptr; errno = 0; h_errno = 0; if ((hostentptr = gethostbyname("hndy")) == NULL) { printf("unknown host name errno is: %d h_errno is: %d\n", errno, h_errn perror("p_gethostbyname"); herror("h_gethostbyname"); } errno = 0; h_errno = 0; if ((hostentptr = gethostbyname(0)) == NULL) { printf("illegal host name errno is: %d h_errno is: %d\n", errno, h_errn perror("p_gethostbyname"); herror("h_gethostbyname"); } }

In the first call to gethostbyname , the host name parameter "hndy" does not represent a known host. In this case, gethostbyname sets h_errno to host_not_found , but does not set errno .

The call to perror in this example would output:

p_gethostbyname: Error 0

The call to herror in this example would print a message describing the failure:

h_gethostbyname: Unknown

In the second call to gethostbyname , the host name parameter is 0 (zero), an invalid argument. In this case, gethostbyname sets errno to einval , but does not set h_errno .

A call to perror would print a message describing the failure:

p_gethostbyname: Invalid

A call to herror would print:

h_gethostbyname: Error 0

A.7 TCP/IP Interface Enhancements

The Compaq C RTL provides the language bindings for a 4.4BSD-compatible TCP/IP implementation. The underlying 4.4BSD semantics for the specific TCP/IP routines are the responsibility of the TCP/IP back-end vendors.

With 4.4BSD semantics, gethostbyname and gethostbyaddr set the external variable h_errno . Other routines (see Table A-3) deal with 4.4BSD versions of the sockaddr and msghdr data structures. See the <socket.h> header file for a description of these structures.

The TCP/IP backend routines need to know the following:

• Whether gethostbyname and gethostbyaddr should set h_errno .
• What type of sockaddr and msghdr structures the Compaq C RTL is passing them---those defined by 4.4BSD systems, or those defined by pre-4.4BSD systems).

This information is controlled by the _sockaddr_len feature-test macro (see Section 1.5).

Compiling with _sockaddr_len defined will enable the entry points that have 4.4BSD semantics. See Table A-3. Otherwise, entry points expecting pre-4.4BSD semantics are enabled.

Table A-3 4.4BSD Entry Points

__bsd44_accept	__bsd44_gethostbyaddr	__bsd44_recvfrom
__bsd44_bind	__bsd44_gethostbyname	__bsd44_recvmsg
__bsd44_connect	__bsd44_getpeername	__bsd44_sendto
	__bsd44_getsockname	__bsd44_sendmsg

A.8 Summary of Socket Routines

This section groups and briefly describes the socket routines. For complete descriptions, see the routine reference section that follows in this appendix.

A.8.1 Basic Communication Routines

Table A-4 lists the basic communication routines that make up the building blocks of Internet programs.

Table A-4 Basic Communication Routines

Routine	Description
accept	Accepts a connection on a socket.
bind	Binds a name to a socket.
close	Closes a connection and deletes a socket descriptor.
connect	Initiates a connection on a socket.
ioctl	Controls socket operations only.
listen	Sets the maximum limit of outstanding connection requests for a socket.
read	Reads bytes from a file or socket and places them into a buffer.
readv	Not implemented.
recv	Receives bytes from a socket and places them into a buffer.
recvfrom	Receives bytes for a socket from any source.
recvmsg	Receives bytes from a socket and places them into scattered buffers.
select	Allows the polling or checking of a group of sockets.
send	Sends bytes through a socket to a connected peer.
sendmsg	Sends gathered bytes through a socket to any other socket.
sendto	Sends bytes through a socket to any other socket.
shutdown	Shuts down all or part of a bidirectional socket.
socket	Creates an endpoint for communication by returning a socket descriptor.
write	Writes bytes from a buffer to a file or socket.
writev	Not implemented.

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