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 :
-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.
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 :
int main() { #ifdef FEATURE This code is enabled when FEATURE is defined #endif return 0; }
If we run the following command :
gcc -E test.c
The result will be :
# 1 "test.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.c"
int main() {
return 0;
}
However, if we define FEATURE and invoke gcc as follows :
gcc -E -DFEATURE test.c
The resulting output is will be :
# 1 "test.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.c"
int main() {
This code is enabled when 1 is defined
return 0;
}
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.
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