Description

   To understand how the exec functions operate, consider how
   the OpenVMS system calls any VSI C program, as shown in the
   following syntax:

   int main (int argc, char *argv[], char *envp[]);

   The identifier argc is the argument count; argv is an array
   of argument strings. The first member of the array (argv[0])
   contains the name of the image. The arguments are placed in
   subsequent elements of the array. The last element of the array
   is always the NULL pointer.

   An exec function calls a child process in the same way that
   the run-time system calls any other VSI C program. The exec
   functions pass the name of the image to be activated in the
   child; this value is placed in argv[0]. However, the functions
   differ in the way they pass arguments and environment information
   to the child:

   o  Arguments can be passed in separate character strings (execl,
      execle, and execlp) or in an array of character strings
      (execv, execve, and execvp).

   o  The environment can be explicitly passed in an array (execle
      and execve) or taken from the parent's environment (execl,
      execv, execlp, and execvp).

   If vfork was called before invoking an exec function, then when
   the exec function completes, control is returned to the parent
   process at the point of the vfork call. If vfork was not called,
   the exec function waits until the child has completed execution
   and then exits the parent process. See vfork.