multiple addition of header file even after checking - regex

Question

I want to add in a .C file incase its not present. Using Perl
MY CODE SNIPPET
my $flag = 0;
my $pos = 0;
open(FILE, $input) or die $!;
my #lines = <FILE>;
foreach(#lines)
{
$pos++;
#checks for #include where it can add stdint.h
if ($_ =~ (m/#include/))
{
#prevents multiple addition for each header file
if($flag == 0)
{
#checks whether stdint already present or not
unless($_ =~ m/#include <stdint.h>/ )
{
splice #lines,$pos,0,"#include <stdint.h>"."\n";
$flag = 1;
}
}
}
}
But my code is adding stdint.h everytime it runs which means multiple addition for every run.
whats wrong with the code
unless($_ =~ m/#include <stdint.h>/){
doesn't work even if i use
unless($_ =~ m/<stdint.h>/){

Answer 1

Imagine you have this C file:
#include <stdio.h>
#include <stdint.h>
int main(int argc, char ** argv) {
return 0;
}
What is supposed to happen when this goes through your script?
Nothing, because is already included
What actually happens though? This is where learning to use the Perl debugger or simply tracing by hand is really useful.
flag and pos are initialized to 0. The first line in the file is #include <stdio.h> which is not #include <stdint.h>, so your code immediately assumes the file is missing and adds it.
So, in your above code you insert #include <stdint.h> on the first include that is not it, regardless of whether or not it is actually there later in the file or before, which will always be any other include file.
What you should actually do is gather all of the include lines in an Array, then search for the file matching <stdint.h> only adding it if it isn't contained in the complete list.

Answer 2

Here is a way to do it:
open(my $FILE, '<', $input) or die $!;
my #lines = <$FILE>;
my $flag = 0;
my $pos = 0;
my $insert_pos = 1; #add stdin even if there're no other include
foreach(#lines) {
$pos++;
if (/#include/){
$insert_pos = $pos;
if (/#include <stdint.h>/) {
$insert_pos = 0;
last;
}
}
}
if ($insert_pos) {
splice #lines, $insert_pos, 0, "#include <stdint.h>"."\n";
}

Answer 3

This is an awful thing to be doing to a C project.
What you have coded adds #include <stdint.h> right after the first #include line, and has no effect on files that don't #include anything.
However, if you want to "edit" a file using Perl, then you should use Tie::File
The code in your question would look like this
use strict;
use warnings;
use Tie::File;
my ($input) = #ARGV;
tie my #c_file, 'Tie::File', $input or die qq{Unable to open C file "$input": $!};
for my $i (0 .. $#c_file) {
next unless $c_file[$i] =~ /#include/;
splice #c_file, $i, 0, '#include <stdint.h>';
last;
}