C/C++ forbid variable names which begin with an underscore followed by a capital letter.

The C++ Standard [ISO/IEC 14882-2003], Section 17.4.3.1.2 specifies the following sets of reserved names: "Each name that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use."

The C99 Standard [ISO/IEC 9899:1999] specifies in section 7.1.3 the following sets of reserved names: "All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use."

sun/java2d/loops/ProcessPath.c defines the variable names "_X" and "_X" where "_X" clashes with the preprocessor macro:

#define _X 0x00000080 /* heXadecimal digit */

defined in /usr/include/iso/ctype_iso.h on Solaris.