Feature-test macros provide a means for writing portable programs. They ensure that the Compaq C RTL symbolic names used by a program do not clash with the symbolic names supplied by the implementation.

The Compaq C Run-time Library header files are coded to support the use of a number of feature-test macros. When an application defines a feature-test macro, the Compaq C RTL header files supply the symbols and prototypes defined by that feature-test macro and nothing else. If a program does not define such a macro, the Compaq C RTL header files define symbols without restriction.

The feature-test macros supported by the Compaq C Run-time Library fall into three broad categories for controlling the visibility of symbols in header files according to the following:

• Standards
• Multiple-version support
• Compatibility

The Compaq C Run-time Library implements parts of the following standards:

• X/Open CAE Specification, System Interfaces and Headers, Issue 4, Version 2, also known as XPG4 V2.
• X/Open CAE Specification, System Interfaces and Headers, Issue 4, also known as XPG4.
• Standard for Information Technology - Portable Operating System Interface (POSIX) - Part 1: System Application Program Interface (API)---Amendment 2: Threads Extension [C Language], also known as POSIX 1003.1c-1995 or IEEE 1003.1c-1995.
• ISO/IEC 9945-2:1993 - Information Technology - Portable Operating System Interface (POSIX) - Part 2: Shell and Utilities, also known as ISO POSIX-2.
• ISO/IEC 9945-1:1990 - Information Technology - Portable Operating System Interface (POSIX) - Part 1: System Application Programming Interface (API) (C Language), also known as ISO POSIX-1.
• ISO/IEC 9899:1990-1994 - Programming Languages - C, Amendment 1: Integrity, also known as ISO C, Amendment 1.
• ISO/IEC 9899:1990 - Programming Languages - C, also known as ISO C. The normative part is the same as X3.159-1989, American National Standard for Information Systems - Programming Language C, also known as ANSI C.

You can define a feature-test macro to select each standard. You can do this either with a #define preprocessor directive in your C source before the inclusion of any header file, or with the /DEFINE qualifier on the CC command line.

Table 1-4 lists and describes the Compaq C RTL feature-test macros that control standards support.

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Table 1-4 Feature Test Macros - Standards

Macro Name	Standard Selected	Other Standards Implied	Description
_XOPEN_SOURCE_EXTENDED	XPG4 V2	XPG4, ISO POSIX-2, ISO POSIX-1, ANSI C	Makes visible XPG4-extended features, including traditional UNIX-based interfaces not previously adopted by X/Open.
_XOPEN_SOURCE	XPG4	ISO POSIX-2, ISO POSIX-1, ANSI C	Makes visible XPG4 standard symbols and causes _POSIX_C_SOURCE to be set to 2 if it is not already defined with a value greater than 2. 1, 2
_POSIX_C_SOURCE ==199506	IEEE 1003.1c-1995	ISO POSIX-2, ISO POSIX-1, ANSI C	Header files defined by ANSI C make visible those symbols required by IEEE 1003.1c-1995.
_POSIX_C_SOURCE == 2	ISO POSIX-2	ISO POSIX-1, ANSI C	Header files defined by ANSI C make visible those symbols required by ISO POSIX-2 plus those required by ISO POSIX-1.
_POSIX_C_SOURCE == 1	ISO POSIX-1	ANSI C	Header files defined by ANSI C make visible those symbols required by ISO POSIX-1.
__STDC_VERSION__ ==199409	ISO C amdt 1	ANSI C	Makes ISO C amendment 1 symbols visible.
_ANSI_C_SOURCE	ANSI C	---	Makes ANSI C standard symbols visible.

Features not defined by one of the previously named standards are considered Compaq C extensions and are selected by not defining any standards-related, feature-test macros.

If you do not explicitly define feature test macros to control header file definitions, you implicitly include all defined symbols as well as Compaq C extensions.

1.5.3 Interactions with the /STANDARD Qualifier

The /STANDARD qualifier selects the dialect of the C language supported.

With the exception of /STANDARD=ANSI89 and /STANDARD=ISOC94, the selection of C dialect and the selection of Compaq C RTL APIs to use are independent choices. All other values for /STANDARD cause the entire set of APIs to be available, including extensions.

Specifying /STANDARD=ANSI89 restricts the default API set to the ANSI C set. In this case, to select a broader set of APIs, you must also specify the appropriate feature-test macro. To select the ANSI C dialect and all APIs, including extensions, undefine __HIDE_FORBIDDEN_NAMES before including any header file.

Compiling with /STANDARD=ISOC94 sets __STDC_VERSION__ to 199409. Conflicts that arise when compiling with both XPG4 and ISO C Amendment 1 resolve in favor of ISO C Amendment 1. XPG4 extensions to ISO C Amendment 1 are selected by defining _XOPEN_SOURCE .

The following examples help clarify these rules:

• The fdopen function is an ISO POSIX-1 extension to <stdio.h> . Therefore, <stdio.h> defines fdopen only if one or more of the following is true:
  • The program including it is not compiled in strict ANSI C mode (/STANDARD=ANSI89).
  • _POSIX_C_SOURCE is defined as 1 or greater.
  • _XOPEN_SOURCE is defined.
  • _XOPEN_SOURCE_EXTENDED is defined.
• The popen function is an ISO POSIX-2 extension to <stdio.h> . Therefore, <stdio.h> defines popen only if one or more of the following is true:
  • The program including it is not compiled in strict ANSI C mode (/STANDARD=ANSI89).
  • _POSIX_C_SOURCE is defined as 2 or greater.
  • _XOPEN_SOURCE is defined.
  • _XOPEN_SOURCE_EXTENDED is defined.
• The getw function is an X/Open extension to <stdio.h> . Therefore, <stdio.h> defines getw only if one or more of the following is true:
  • The program is not compiled in strict ANSI C mode (/STANDARD=ANSI89).
  • _XOPEN_SOURCE is defined.
  • _XOPEN_SOURCE_EXTENDED is defined.
• The X/Open Extended symbolic constants _SC_PAGESIZE, _SC_PAGE_SIZE, _SC_ATEXIT_MAX, and _SC_IOV_MAX were added to <unistd.h> to support the sysconf function. However, these constants are not defined by _POSIX_C_SOURCE .
  The <unistd.h> header file defines these constants only if a program does not define _POSIX_C_SOURCE and does define _XOPEN_SOURCE_EXTENDED .
  If _POSIX_C_SOURCE is defined, these constants are not visible in <unistd.h> . Note that _POSIX_C_SOURCE is defined only for programs compiled in strict ANSI C mode.
• The fgetname function is a Compaq C RTL extension to <stdio.h> . Therefore, <stdio.h> defines fgetname only if the program is not compiled in strict ANSI C mode (/STANDARD=ANSI89).
• The macro _PTHREAD_KEYS_MAX is defined by POSIX 1003.1c-1995. This macro is made visible in <limits.h> when compiling for this standard with _POSIX_C_SOURCE == 199506 defined, or by default when compiling without any standards-defining, feature-test macros.
• The macro WCHAR_MAX defined in <wchar.h> is required by ISO C Amendment 1 but not by XPG4. Therefore:
  • Compiling for ISO C Amendment 1 makes this symbol visible, but compiling for XPG4 compliance does not.
  • Compiling for both ISO C Amendment 1 and XPG4 makes this symbol visible.
  
  Similarly, the functions wcsftime and wcstok in <wchar.h> are defined slightly differently by the ISO C Amendment 1 and XPG4:
  • Compiling for ISO C Amendment 1 makes the ISO C Amendment 1 prototypes visible.
  • Compiling for XPG4 compliance makes the XPG4 prototypes visible.
  • Compiling for both ISO C Amendment 1 and XPG4 selects the ISO C prototypes because conflicts resulting from this mode of compilation resolve in favor of ISO C.
  • Compiling without any standard selecting feature test macros makes ISO C Amendment 1 features visible.
  
  So in this example, compiling with no standard-selecting feature-test macros makes WCHAR_MAX and the ISO C Amendment 1 prototypes for wcsftime and wcstok visible.
• The wcswidth and wcwidth functions are XPG4 extensions to ISO C Amendment 1. Their prototypes are in <wchar.h> .
  These symbols are visible if:
  • Compiling for XPG4 compliance by defining _XOPEN_SOURCE or _XOPEN_SOURCE_EXTENDED.
  • Compiling for DEC C Version 4.0 compatibility or on pre-OpenVMS Version 7.0 systems.
  • Compiling with no standard selecting feature test macros.
  • Compiling for both ISO C Amendment 1 and XPG4 compilance because these symbols are XPG4 extensions to ISO C Amendment 1.
  
  Compiling for strict ISO C Amendment 1 does not make them visible.

By default, the header files enable APIs in the Compaq C RTL provided by the version of the operating system on which the compilation occurs. This is accomplished by the predefined setting of the __VMS_VER macro, as described in the Compaq C User's Guide for OpenVMS Systems. For example, compiling on OpenVMS Version 6.2 causes only Compaq C RTL APIs from Version 6.2 and earlier to be made available.

Another example of the use of the __VMS_VER macro is support for the 64-bit versions of Compaq C RTL functions available with OpenVMS Alpha Version 7.0 and higher. In all header files, functions that provide 64-bit support are conditionalized so that they are visible only if __VMS_VER indicates a version of OpenVMS that is greater than or equal to 7.0.

To target an older version of the operating system, do the following:

1. Define a logical DECC$SHR to point to the old version of DECC$SHR. The compiler uses a table from DECC$SHR to perform routine name prefixing.
2. Define __VMS_VER appropriately, either with the /DEFINE qualifier or with a combination of the #undef and #define preprocessor directives. With /DEFINE, you may need to disable the warning regarding redefinition of a predefined macro.

Targeting a newer version of the operating system might not always be possible. For some versions, you can expect that the new DECC$SHR.EXE will require new features of the operating system that are not present. For such versions, the defining if the logical DECC$SHR in Step 1 would cause the compilation to fail.

1.5.5 Compatibility Modes

The following predefined macros are used to select header-file compatibility with previous versions of DEC C) or the OpenVMS operating system:

• _DECC_V4_SOURCE
• _VMS_V6_SOURCE

There are two types of incompatibilities that can be controlled in the header files:

• To conform to standards, some changes are source-code incompatible but binary compatible. To select DEC C Version 4.0 source compatibility, use the _DECC_V4_SOURCE macro.
• Other changes to conform to standards introduce a binary or run-time incompatibility.
  In general, programs that recompile get new behaviors. In these cases, use the _VMS_V6_SOURCE feature test macro to retain previous behaviors.
  However, for the exit , kill , and wait functions, the OpenVMS Version 7.0 changes to make these routines ISO POSIX-1 compliant were considered too incompatible to become the default. Therefore, in these cases the default behavior is the same as on pre-OpenVMS Version 7.0 systems. To access the versions of these routines that comply with ISO POSIX-1, use the _POSIX_EXIT feature test macro.

The following examples help clarify the use of these macros:

• To conform to the ISO POSIX-1 standard, typedefs for the following have been added to <types.h> :

  dev_t
  gid_t
  ino_t
  mode_t
  nlink_t
  off_t
  pid_t
  size_t
  ssize_t
  uid_t

  
  Previous development environments using a version of DEC C earlier than Version 5.2 may have compensated for the lack of these typedef s in <types.h> by adding them to another module. If this is the case on your system, then compiling with the <types.h> provided with DEC C Version 5.2 might cause compilation errors.
  To maintain your current environment and include the DEC C Version 5.2 <types.h> , then compile with _DECC_V4_SOURCE defined. This will omit incompatible references from the DEC C Version 5.2 headers. In <types.h> , for example, the previously listed typedefs will not be visible.
• As of OpenVMS Version 7.0, the Compaq C RTL getuid and geteuid functions are defined to return an OpenVMS UIC (user identification code) that contains both the group and member portions of the UIC. In previous versions of the DEC C RTL, these functions returned only the member number from the UIC code.
  Note that the prototypes for getuid and geteuid in <unistd.h> (as required by the ISO POSIX-1 standard) and in <unixlib.h> (for Compaq C RTL compatibility) have not changed. By default, newly compiled programs that call getuid and geteuid get the new definitions. That is, these functions will return an OpenVMS UIC.
  To let programs retain the pre-OpenVMS Version 7.0 behavior of getuid and geteuid , compile with the _VMS_V6_SOURCE feature-test macro defined.
• As of OpenVMS Version 7.0, the Compaq C RTL exit function is defined with ISO POSIX-1 semantics. As a result, the input status argument to exit takes a number between 0 and 255. (Prior to this, exit could take an OpenVMS condition code in its status parameter.)
  By default, the behavior for exit on OpenVMS systems is the same as before --- exit accepts an OpenVMS condition code. To enable the ISO POSIX-1 compatible exit function, compile with the _POSIX_EXIT feature-test macro defined.

The following feature-test macros are used to control the curses and socket subsets of the Compaq C RTL library:

• _BSD44_CURSES
  This macro selects the curses package from the 4.4BSD Berkeley Software Distribution.
• _VMS_CURSES
  This macro selects a curses package based on the VAX C compiler. This is the default curses package.
• _SOCKADDR_LEN
  This macro is used to select 4.4BSD-compatible and XPG4 V2-compatible socket interfaces. These interfaces require support in your underlying TCP/IP software. Contact your TCP/IP vendor to inquire if the version of TCP/IP software you run supports 4.4BSD sockets.

Strict XPG4 V2 compliance requires the 4.4BSD-compatible socket interface. Therefore, if _XOPEN_SOURCE_EXTENDED is defined on OpenVMS Version 7.0 or higher, _SOCKADDR_LEN is defined to be 1.

The following examples help clarify the use of these macros:

• Symbolic constants like AE, AL, AS, AM, BC, which represent pointers to termcap fields used by the BSD curses package, are only visible in <curses.h> if _BSD44_CURSES is defined.
• The <socket.h> header file defines a 4.4BSD sockaddr structure only if _SOCKADDR_LEN or _XOPEN_SOURCE_EXTENDED is defined. Otherwise, <socket.h> defines a pre-4.4BSD sockaddr structure. If _SOCKADDR_LEN is defined and _XOPEN_SOURCE_EXTENDED is not defined,
  The <socket.h> header file also defines an osockaddr structure, which is a 4.3BSD sockaddr structure to be used for compatibility purposes. Since XPG4 V2 does not define an osockaddr structure, it is not visible in _XOPEN_SOURCE_EXTENDED mode.

After you learn how to link with the Compaq C RTL and call Compaq C functions and macros, you can use the Compaq C RTL for its primary purpose: input/output (I/O).

Since every system has different methods of I/O, familiarize yourself with the OpenVMS-specific methods of file access. In this way, you will be equipped to predict functional differences when porting your source program from one operating system to another.

Figure 1-2 shows the I/O methods available with the Compaq C RTL. The OpenVMS system services communicate directly with the OpenVMS operating system, so they are closest to the operating system. The OpenVMS Record Management Services (RMS) functions use the system services, which manipulate the operating system. The Compaq C Standard I/O and UNIX I/O functions and macros use the RMS functions. Since the Compaq C RTL Standard I/O and UNIX I/O functions and macros must go through several layers of function calls before the system is manipulated, they are furthest from the operating system.

Figure 1-2 I/O Interface from C Programs

The C programming language was developed on the UNIX operating system, and the Standard I/O functions were designed to provide a convenient method of I/O that would be powerful enough to be efficient for most applications, and also be portable so that the functions could be used on any system running C language compilers.

The Compaq C RTL adds functionality to this original specification. Since, as implemented in the Compaq C RTL, the Standard I/O functions recognize line terminators, the Compaq C RTL Standard I/O functions are particularly useful for text manipulation. The Compaq C RTL also implements some of the Standard I/O functions as preprocessor defined macros.

In a similar manner, the UNIX I/O functions originally were designed to provide a more direct access to the UNIX operating systems. These functions were meant to use a numeric file descriptor to represent a file. A UNIX system represents all peripheral devices as files to provide a uniform method of access.

The Compaq C RTL adds functionality to the original specification. The UNIX I/O functions, as implemented in Compaq C, are particularly useful for manipulating binary data. The Compaq C RTL also implements some of the UNIX I/O functions as preprocessor defined macros.

The Compaq C RTL includes the Standard I/O functions that should exist on all C compilers, and also the UNIX I/O functions to maintain compatibility with as many other implementations of C as possible. However, both Standard I/O and UNIX I/O use RMS to access files. To understand how the Standard I/O and UNIX I/O functions manipulate RMS formatted files, learn the fundamentals of RMS. See Section 1.6.1 for more information about Standard I/O and UNIX I/O in relationship to RMS files. For an introduction to RMS, see the Guide to OpenVMS File Applications.

Before deciding which method is appropriate for you, first ask this question: Are you concerned with UNIX compatibility or with developing code that will run solely under the OpenVMS operating system?

• If UNIX compatibility is important, you probably want to use the highest levels of I/O---Standard I/O and UNIX I/O---because that level is largely independent of the operating system. Also, the highest level is easier to learn quickly, an important consideration if you are a new programmer.
• If UNIX compatibility is not important to you or if you require the sophisticated file processing that the Standard I/O and UNIX I/O methods do not provide, you might find RMS desirable.

If you are writing system-level software, you may need to access the OpenVMS operating system directly through calls to system services. For example, you may need to access a user-written device driver directly through the Queue I/O Request System Service ($QIO). To do this, use the OpenVMS level of I/O; this level is recommended if you are an experienced OpenVMS programmer. For examples of programs that call OpenVMS system services, see the Compaq C User's Guide for OpenVMS Systems.

You may never use the RMS or the OpenVMS system services. The Standard I/O and UNIX I/O functions are efficient enough for a large number of applications. Figure 1-3 shows the dependency of the Standard I/O and the UNIX I/O functions on RMS, and the various methods of I/O available to you.

Figure 1-3 Mapping Standard I/O and UNIX I/O to RMS

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