препроцессор C (The C Preprocessor)
Описание (Description)
The C preprocessor, often known as cpp, is a macro processor that
is used automatically by the C compiler to transform your program
before compilation. It is called a macro processor because it
allows you to define macros, which are brief abbreviations for
longer constructs.
The C preprocessor is intended to be used only with C, C++, and
Objective-C source code. In the past, it has been abused as a
general text processor. It will choke on input which does not
obey C's lexical rules. For example, apostrophes will be
interpreted as the beginning of character constants, and cause
errors. Also, you cannot rely on it preserving characteristics
of the input which are not significant to C-family languages. If
a Makefile is preprocessed, all the hard tabs will be removed,
and the Makefile will not work.
Having said that, you can often get away with using cpp on things
which are not C. Other Algol-ish programming languages are often
safe (Ada, etc.) So is assembly, with caution. -traditional-cpp
mode preserves more white space, and is otherwise more
permissive. Many of the problems can be avoided by writing C or
C++ style comments instead of native language comments, and
keeping macros simple.
Wherever possible, you should use a preprocessor geared to the
language you are writing in. Modern versions of the GNU
assembler have macro facilities. Most high level programming
languages have their own conditional compilation and inclusion
mechanism. If all else fails, try a true general text processor,
such as GNU M4.
C preprocessors vary in some details. This manual discusses the
GNU C preprocessor, which provides a small superset of the
features of ISO Standard C. In its default mode, the GNU C
preprocessor does not do a few things required by the standard.
These are features which are rarely, if ever, used, and may cause
surprising changes to the meaning of a program which does not
expect them. To get strict ISO Standard C, you should use the
-std=c90, -std=c99, -std=c11 or -std=c17 options, depending on
which version of the standard you want. To get all the mandatory
diagnostics, you must also use -pedantic.
This manual describes the behavior of the ISO preprocessor. To
minimize gratuitous differences, where the ISO preprocessor's
behavior does not conflict with traditional semantics, the
traditional preprocessor should behave the same way. The various
differences that do exist are detailed in the section Traditional
Mode.
For clarity, unless noted otherwise, references to CPP in this
manual refer to GNU CPP.
