Q: How can I determine which identifiers are safe for me to use and which are reserved?
A: Namespace management can be a sticky issue. The problem--which isn't always obvious--is that you don't want to pick identifiers already in use by the implementation, such that you get ``multiply defined'' errors or--even worse--quietly replace one of the implementation's symbols and break everything. You also want some guarantee that later releases won't usurp names you're legitimately using. [footnote] (Few things are more frustrating than taking a debugged, working, production program, recompiling it under a new release of a compiler, and having the build fail due to namespace or other problems.) Therefore, the ANSI/ISO C Standard contains rather elaborate definitions carving out distinct namespace subsets for the user and the implementation.
To make sense of ANSI's rules, and before we can say whether a given identifier is reserved, we must understand three attributes of the identifier: its scope, namespace, and linkage.
There are four kinds of scope (regions over which an identifier's declaration is in effect) in C: function, file, block, and prototype. (The fourth one exists only in the parameter lists of function prototype declarations; see also question 11.5.)
There are four different kinds of namespaces, for:
Another set of names (though not termed a ``namespace'' by the Standard) consists of preprocessor macros; these are all expanded before the compiler gets around to considering the four formal namespaces.
The standard defines three kinds of ``linkage'': external, internal, and none. For our purposes, external linkage means global, non-static variables and functions (across all source files), internal linkage means static variables and functions with file scope, and ``no linkage'' refers to local variables, and also things like typedef names and enumeration constants.
The rules, paraphrased from ANSI Sec. 188.8.131.52, are:
Rules 3 and 4 are additionally complicated by the fact that several sets of macro names and standard library identifiers are reserved for ``future directions'' that is, later revisions of the Standard may define new names matching certain patterns.
Here is a list of the patterns which are reserved for ``future directions'' associared with each standard header:
[TABLE GOES HERE](The notation [A-Z] means ``any uppercase letter''; similarly, [a-z] and [0-9] indicate lower-case letters and digits. The notation * means ``anything.'' For example, the pattern for <stdlib.h> says that all external identifiers beginning with the letters str followed by a lower-case letter are reserved.)
What do the above rules really mean? If you want to be on the safe side:
In fact, the preceding subparagraphs are overly conservative. If you wish, you may remember the following exceptions:
However, before making use of any of these exceptions, recognize that some of them are pretty risky (especially exceptions 3 and 5, since you could accidentally #include the relevant header file at a later time, perhaps through a chain of nested #include files), and others (especially the ones labeled 1,2) represent sort of a ``no man's land'' between the user namespaces and the namespaces reserved to the implementation.
One reason for providing these exceptions is to allow the implementors of various add-in libraries a way to declare their own internal or ``hidden'' identifiers. If you make use of any of the exceptions, you won't clash with any identifiers defined by the Standard, but you might clash with something defined by a third-party library you're using. (If, on the other hand, you're the one who's implementing an add-on library, you're welcome to make use of them, if necessary, and if you're careful.)
(It is generally safe to make use of exception 4 to give function parameters or local variables names matching standard library routines or ``future directions'' patterns. For example, ``string'' is a common--and legal--name for a parameter or local variable.)
Additional links: Stan Brown's comprehensive list of reserved identifiers
ISO Sec. 184.108.40.206, Sec. 220.127.116.11, Sec. 18.104.22.168, Sec. 7.1.3, Sec. 7.13
Rationale Sec. 22.214.171.124
H&S Sec. 2.5 pp. 21-3, Sec. 4.2.1 p. 67, Sec. 4.2.4 pp. 69-70, Sec. 4.2.7 p. 78, Sec. 10.1 p. 284