None.
Undefines a macro defined by the compiler or by the -D compiler option.
Many macros are predefined by the compiler; see Compiler predefined macros for those that can be undefined (that is, are not protected). The compiler configuration file also uses the -D option to predefine several macro names for specific invocation commands; for details, see the configuration file for your system.
The -U option is not equivalent to the #undef preprocessor directive. It cannot undefine names defined in the source by the #define preprocessor directive. It can only undefine names defined by the compiler or by the -D option.
The -Uname option has a higher precedence than the -Dname option.
None.
xlc myprogram.c -U__unix