C/C++ and Fortran

Originally posted by: ChristineLi

Are you developing programs consumed by multiple compilers?

If your answer is yes and the IBM XL C/C++ compilers are among your target compilers (either for AIX, Linux for big endian distributions or Linux for little endian distributions), you will be interested in this blog.

There is a quick way to distinguish code you wish to be consumed by XL C/C++ from code you wish to be consumed by other compilers.

Leverage the macros listed in Table 1 to identify the IBM XL C/C++ compiler. Note that besides the ones listed in Table 1, more macros that identify XL C/C++ are predefined, which is covered in the Further information section below.

#ifdef macro-to-identify-XL

your_code_for_XL_C/C++

#endif

Table 1. Macros recommended to use to identify the IBM XL C/C++ compiler

Most of the macros related to the XL C/C++ compiler are predefined and protected, which means that the compiler will issue a warning if you try to undefine or redefine them. You can use the -qshowmacros -E compiler options to view the values of the predefined macros.

Note that these macros contain two underscores (‘__’) as a prefix and a suffix. The use of such two underscores in identifiers is reserved for the compiler's internal use according to the ANSI-C standard.

Further information

1. You may notice that for XL C/C++ for Linux, the macros predefined by the compiler to identify the XL compiler for little endian and big endian distributions are different, requiring migration effort when you move from big endian to little endian. To ease your migration effort, starting from V13.1.2, XL C/C++ for Linux (little endian distrubutions) provides a compiler option -qxlcompatmacros  to define macros __IBMCPP__, __IBMC__, __xlc__, __xlC__, and __xlC_ver__.

Note that the default setting of -qxlcompatmacros changes along with the version of XL C/C++ for Linux, as shown in Table 2:

Table 2: Default setting of option -qxlcompatmacros for XL C/C++ for Linux releases

For more information about compiler behavior for XL C/C++ for Linux, V16.1, see -qxlcompatmacros.

2. Besides IBM XL specific macros, XL C/C++ for Linux for little endian distributions also predefines some Clang related macros, as shown in Table 3. The reason is that XL C/C++ for Linux for little endian distributions combines the Clang front end infrastructure with the optimization technology in the IBM compiler back end. The predefined Clang related macros correctly reflect the architecture.

Table 3. Predefined macros related to Clang for XL C/C++ for Linux for little endian distributions

What’s next?

Are you programming across multiple platforms? Stay tuned for our future blog on tips about differentiating platforms.