College of Engineering & CS Wright State University Dayton, Ohio 45435-0001

CEG 333: Introduction to Unix Prabhaker Mateti The execve interface

In simple C++ programs, main() takes no arguments. However, the shell can actually pass any number of arguments between zero and three to main:

1. argc: An integer, number of command-line arguments.
2. argv: An array of cstrings, one per command-line argument, in the order given.
3. env: An array of cstrings, one per environment argument, of the form ENV_VAR_NAME="value".

Note: The final element in both the argv and env arrays is always set to NULL, so it can be used to test for the end of the array.

Warning: an argc starts counting arguments from 1; a value of 1 means argv has one value, the name of the program. But C++ array indexes start from 0, so argv always has values argv[0] ... argv[argc-1]. argv[argc] is the final NULL, not one of the arguments!

A C++ env (outputs every variable/value pair in the environment string):

      #include <cstdio>
      using namespace std;

      int main(int argc, char *argv[], char *env[]) {
          for (int i = 0; env[i]; i++) {
	      printf("%s\n", env[i]);
	  }

	  return 0;
      }