file_spec

    The full file specification of a new image to be activated in the
    child process.

 arg0, ...

    A sequence of pointers to null-terminated character strings.

    If the POSIX-1 format is used, at least one argument must be
    present and must point to a string that is the same as the new
    process filename (or its last component). (This pointer can also
    be the NULL pointer, but then execle would accomplish nothing.)
    The last pointer must be the NULL pointer. This is also the
    convention if the compatibility format is used.

 envp

    An array of strings that specifies the program's environment.
    Each string in envp has the following form:

    name = value

    The name can be one of the following names and the value is a
    null-terminated string to be associated with the name:

    o  HOME-Your login directory

    o  TERM-The type of terminal being used

    o  PATH-The default device and directory

    o  USER-The name of the user who initiated the process

    The last element in envp must be the NULL pointer.

    When the operating system executes the program, it places a copy
    of the current environment vector (envp) in the external variable
    environ.