1.5 Feature-Test Macros for Header-File Control

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

The DEC 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 DEC 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 DEC C RTL header files define symbols without restriction.

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

1.5.1 Standards Macros

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

1.5.2 Selecting a Standard

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 DEC C RTL feature-test macros that control standards support.

Table 1-4 Feature Test Macros - Standards

macro)
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. 

[1] Where the ISO C Amendment 1 includes symbols not specified by XPG4, defining __STDC_VERSION__ == 199409 and _XOPEN_ SOURCE (or _XOPEN_SOURCE_EXTENDED) selects both ISO C and XPG4 APIs. Conflicts that arise when compiling with both XPG4 and ISO C Amendment 1 resolve in favor of ISO C Amendment 1.

[2] Where XPG4 extends the ISO C Amendment 1, defining _ XOPEN_SOURCE or _XOPEN_SOURCE_EXTENDED selects ISO C APIs as well as the XPG4 extensions available in the header file. This mode of compilation makes XPG4 extensions visible.

Features not defined by one of the previously named standards are considered DEC 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 DEC 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 DEC 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:

1.5.4 Multiple-Version-Support Macro

By default, the header files enable APIs in the DEC 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 DEC C User's Guide for OpenVMS Systems. For example, compiling on OpenVMS Version 6.2 causes only DEC 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 DEC 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:

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

The following examples help clarify the use of these macros:

1.5.6 Curses and Socket Compatibility Macros

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

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:


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