Complying with standard C

Names safe to use

As you can tell by now, the rules regarding when certain names are reserved are complicated. There are, however, four fairly simple rules you can follow to keep from colliding with any ANSI C reserved names:

#include all system headers at the top of your source files (except possibly after a #define of _POSIX_SOURCE or _XOPEN_SOURCE, or both).
Do not define or declare any names that begin with an underscore.
Use an underscore or a capital letter somewhere within the first few characters of all file scope tags and regular names. (But beware of the va_ prefix found in stdarg.h or varargs.h.)
Use a digit or a non-capital letter somewhere within the first few characters of all macro names. (But note that almost all names beginning with an ``E'' are reserved if errno.h is #included.)

As noted earlier, most implementations will continue to add names to the standard headers by default. Therefore, these rules are just a guideline to follow.