Traditional Mode - Red Hat ENTERPRISE LINUX 3 - USING CPP Using Instructions

Using cpp, the c preprocessor

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Table Of Contents

Table of Contents

Traditional (pre-standard) C preprocessing is rather different from the preprocessing speciﬁed by the

standard. When GCC is given the

cessor. We do not guarantee that GCC's behavior under

preprocessor exactly.

Traditional mode exists only for backward compatibility. We have no plans to augment it in any way

nor will we change it except to ﬁx catastrophic bugs. You should be aware that modern C libraries

often have header ﬁles which are incompatible with traditional mode.

This is a list of the differences. It may not be complete, and may not correspond exactly to the behavior

of either GCC or a true traditional preprocessor.

Traditional macro expansion pays no attention to single-quote or double-quote characters; macro

•

argument symbols are replaced by the argument values even when they appear within apparent

string or character constants.

Traditionally, it is permissible for a macro expansion to end in the middle of a string or character

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constant. The constant continues into the text surrounding the macro call.

However, the end of the line terminates a string or character constant, with no error. (This is a kluge.

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Traditional mode is commonly used to preprocess things which are not C, and have a different

comment syntax. Single apostrophes often appear in comments. This kluge prevents the traditional

preprocessor from issuing errors on such comments.)

Preprocessing directives are recognized in traditional C only when their leading

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ﬁrst column. There can be no whitespace between the beginning of the line and the

In traditional C, a comment is equivalent to no text at all. (In ISO C, a comment counts as whites-

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pace.) It can be used sort of the same way that

together.

Traditional C does not have the concept of a preprocessing number.

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A macro is not suppressed within its own deﬁnition, in traditional C. Thus, any macro that is used

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recursively inevitably causes an error.

The

and

operators are not available in traditional C.

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In traditional C, the text at the end of a macro expansion can run together with the text after the

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macro call, to produce a single token. This is impossible in ISO C.

None of the GNU extensions to the preprocessor are available in traditional mode, with the excep-

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tion of a partial implementation of assertions, and those may be removed in the future.

A true traditional C preprocessor does not recognize

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and

#elif

#error

Traditional mode is text-based, not token-based, and comments are stripped after macro expansion.

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Therefore,

can be used to paste tokens together provided that there is no whitespace between

/**/

it and the tokens to be pasted.

Traditional mode preserves the amount and form of whitespace provided by the user. Hard tabs

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remain hard tabs. This can be useful, e.g. if you are preprocessing a Makeﬁle (which we do not

encourage).

You can request warnings about features that did not exist, or worked differently, in traditional C with

the

-Wtraditional

-traditional

even in traditional mode, but not

option. This works only if you do not specify