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====== The GCC pre-processor ======

GCC compiler pre-processes source code in a first stage before performing the actual compiling. Pre-processor macros are used to define variables, functions and to conditionally enable/disable code.

This article provides a brief overview about how the GCC pre-compiler works.



===== The GCC -E option =====

The -E option tells GCC to invoke the pre-processor and to display the resulting output to stdout. The GCC man page says the following about this option : 

<code>
-E  Stop after the preprocessing stage; do not run the 
compiler proper. The output is in the form of preprocessed 
source code, which is sent to the standard output. Input files
which don't require preprocessing are ignored.

</code>

===== Example using GCC -E =====

In this example, we will illustrate how to use the GCC pre-processor using the -E option. Consider the following C file :

<code C>
int main() {

#ifdef FEATURE
This code is enabled when FEATURE is defined
#endif

return 0;
}
</code>

If we run the following command :
<code>
gcc -E test.c
</code>

The result will be : 

<code>
# 1 "test.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.c"
int main() {





return 0;
}
</code>

However, if we define FEATURE and invoke gcc as follows :
<code>
gcc -E -DFEATURE test.c
</code>

The resulting output is will be :

<code>
# 1 "test.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.c"
int main() {


This code is enabled when 1 is defined


return 0;
}

</code> 

The output from GCC -E is the result of interpreting the pre-processor macros (ifdef), in this case, the pre-processor disable and enabled code block according the whether the constant FEATURE is defined.

===== References =====

  * [[http://gcc.gnu.org/onlinedocs/cpp/|C Pre-Processor (CPP)]]
  * [[http://linux.die.net/man/1/gcc|man gcc]]
  * [[http://predef.sourceforge.net/index.php|Pre-defined C/C++ Compiler Macros]]
  * [[wp>C preprocessor]]

{{tag>coding unix}}


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