C preprocessor: Difference between revisions
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The C preprocessor is the first step in the process of translating C/C++ source code into a binary.
▲The C preprocessor is the first step in the process of translating C/C++ source code into a binary (preprocessing -> compiling -> linking). In trivial environments, the preprocessor is used only for '''#include'''ing header files, and providing "header guards" to avoid multiple inclusions.
=== General ===▼
The preprocessor handles ''preprocessor directives'', which are lines that begin with '' '#' ''. Really old compiler versions demanded that the '' '#' '' be placed in column 1, modern versions of C and C++ allow preprocessor directives to begin in any column, as long as the first non-whitespace character of the line is '' '#' ''.
Lines with preprocessor directives can be "continued" by placing a backslash ('\') as the last character of the line.
The most familiar use of the preprocessor is to include header files (containing function declarations, definition of constants etc.):
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Another possible use is "templating" a piece of code that keeps recurring in more than one source file but could not be put into a seperate function. This way, you could still reduce redundancy by keeping the shared code in a single file and merely ''#include''ing it where needed. This, however, is a pretty ugly construct and should be avoided if possible.
The preprocessor can ''define'' tokens. It is good custom to write these tokens in ALL CAPS. (See pt. 2 as for why.)
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Most compilers also allow the definition of preprocessor tokens on the command line, e.g. the "-D MYTOKEN" option for [[GCC]].
The preprocessor can ''conditionally'' select which parts of source code to compile, depending on whether a given token is defined or not (see above).
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Note that such ''#if'' / ''#ifdef'' / ''#ifndef'' - ''#endif'' sections can be nested.
Non-trivial projects face the problem that a header file includes other header files in turn. Let's say both ''abc.h'' and ''def.h'' both include ''xyz.h''. Should you ''#include'' both ''abc.h'' and ''def.h'' in your source, you will likely end up with warnings and errors about redefinitions etc.
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</source>
Preprocessor tokens can also be assigned a ''value''.
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Such code can easily be re-enabled temporarily with no more effort than replacing the "0" with a "1". Source comments as to why you disabled code this way are in order.
Using the ''#undef'' directive, a preprocessor token can be undefined. This is useful for trickier setups where you might want to redefine a token to a different value: Redefinitions generate a warning message, undefinitions of undefined tokens don't.
This should not be constructed as an advice to ''always'' use ''#undef'' before a ''#define''. Those warnings might actually be pointing to a real problem in your logic. Use ''#undef'' with care.
The preprocessor provides
▲The preprocessor provides the tokens ''__FILE__'', ''__LINE__'' and ''__func__'', which are automatically defined to the appropriate values - something very useful when constructing error or tracing messages. Note that some obsolete compilers might balk at ''__func__''.
{| {
! Preprocessor Token
! Explanation
|-
| __FILE__ || Holds the name of the current source file being compiled (as a string).
|-
| __LINE__ || Holds the current line being compiled (as an integer).
|-
| __DATE__ || Holds the date when the compilation process began (a string with the format "Mmm dd yyyy").
|-
| __TIME__ || Same as the previous, but the time (a string with the format "hh:mm:ss").
|-
| __cplusplus || When defined, the value indicates that C++ compilation is active. When the compiler is (fully) compliant to the standards, the value should be >= 199711L.
|-
| __STDC__ || When defined, the value indicates that the compiler is (fully) compliant with the ANSI C standard.
|-
| __func__ || Holds the name of the function it is used within (as a string).
|}
Different compilers may define extra preprocessor tokens. Visual C++ for example may define _MSC_VER __cplusplus_cli. See the link section below for more information.
=== assert() ===▼
Assertions are used to catch situations which should never happen, even under error circumstances. If the condition given in the parantheses does not evaluate to "true", a diagnosis is printed which contains source file name, line number, and (since C99) name of the current function; the program then calls abort().
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#endif
</source>
There is a number of counter-intuitive consequences of macros and macro expanding design.▼
== See also ==
=== Articles ===
* [[C]]
=== External Links ===
▲
* [http://gcc.gnu.org/onlinedocs/cpp/ The GNU C preprocessor manual
* [http://msdn.microsoft.com/en-us/library/b0084kay(VS.80).aspx VC++ preprocessor information]
[[Category:C]]
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