Pipe C Program input to system() function fuinput - c

Question

I have a simple C Wrapper program to run a bash script in an elevated context. It sits behind a FastCGI wrapper and allows some service hooks to call my C Program which will then run my bash script as root. I am well aware of the security issues and my web server only allows a single IP address to call CGI-BIN scripts. I own both machines so there is little to no security risk at all. I am a complete noob a C and have literally copied a snippet off the internet.
So far this has worked fine:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{
setuid( 0 );
return system( "./myscript.sh" );
}
However my program now needs to take in POST data from FastCGI. The CGI spec says it passes all raw POST data in via STDIN. What I would like is to be able to directly pipe this raw POST data from the STDIN of my C Program/Wrapper into my script. I've tried the following which didn't work:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{
setuid( 0 );
dup2(1, 0);
return system( "./myscript.sh" );
}
The script works perfectly fine when using normal piping (eg. echo "Hey" | ./myscript.sh) however I am lost for how to pipe the STDIN of my C Program to the STDIN of my script.

Answer 1

Expanding on what Barmar commented above. Here is your modified C program:
/* foo.c*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{
setuid( 0 );
return system( "wc -l");
}
The wc -l command will wait for input from your STDIN.
Now, compile and run the above program. You will see that it waits for the input and when you end the input using a Ctrl + D, you will see that it prints the number of lines:
/tmp> ./foo
Hello world
This is an input
2

Answer 2

Try this to add a newline at the end of the stdin:
int main()
{
setuid( 0 );
return system( "(cat; echo '') | ./myscript.sh" );
}