C program code for Windows XP:

#include <stdio.h>
#include <stdlib.h>
 
int main()
{
   char ch;
 
   printf("Do you want to shutdown your computer now (y/n)\n");
   scanf("%c", &ch);
 
   if (ch == 'y' || ch == 'Y')
      system("C:\\WINDOWS\\System32\\shutdown -s");
 
   return 0;
}

C program code for Windows 7:

#include <stdio.h>
#include <stdlib.h>
 
int main()
{
   system("C:\\WINDOWS\\System32\\shutdown /s");
 
   return 0;
}
To shutdown immediately use "C:\\WINDOWS\\System32\\ shutdown /s /t 0". To restart use /r instead of /s.

C program code for Ubuntu Linux:

#include <stdio.h>
 
int main() {
  system("shutdown -P now");
  return 0;
} 
You need to be logged in as root user for above program to execute otherwise you will get the message shutdown: Need to be root, now specifies that you want to shutdown immediately. '-P' option specifies you want to power off your machine. You can specify minutes as:
shutdown -P "number of minutes"