preprocessors

Preprocessors are used to make writing code easier by enabling macros and other utilities like splitting a string across several lines using a \
we can see the expansion of macros in c using the -E flag

    gcc -E source_file.c -o output_file.i

this will just run the preprocessor and enable use to see the actual input to our compiler
Great video on c preprocessors

even an empty file

void main() {
}

the preprocessed file looks like this

# 0 "empty.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "empty.c"
void main() {
}

these are the default preprocessor actions, atleast for gcc

but if you include <stdio.h>

#include <stdio.h>
#define PI 3.141459

int main() {
    printf("%d", PI);
    return 0;
}

the resultant preprocessed file is over 700 lines!!! now that is interesting

constants are directly replaced
io.c

#define FIVE 5
int main() {
    int a = FIVE;
    int b = a+4;
    return 0;
}

io_pre.i

# 0 "io.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "io.c"

int main() {
    int a = 5;
    int b = a+4;
    return 0;
}

lua does not have preprocessors it would be interesting project to try to add preprocessing to the lua compiler