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Path: utzoo!mnetor!seismo!rutgers!topaz.rutgers.edu!ron
From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.unix.wizards
Subject: Re:  Adding to Environment Pointers
Message-ID: <12864@topaz.rutgers.edu>
Date: Tue, 23-Jun-87 11:10:42 EDT
Article-I.D.: topaz.12864
Posted: Tue Jun 23 11:10:42 1987
Date-Received: Thu, 25-Jun-87 01:16:02 EDT
References: <7950@brl-adm.ARPA>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 86

This one is simple so I'll just include the code:

/*
 *  Environment diddlers
 *
 *  Ron Natalie.
 */
extern char **environ;		    /*  Pointer to environment, set by crt0
				     *  used by execv, execl
				     */
static	int	_set_env = 0;	    /*  When zero, indicates that the environ
				     *  points to the original stack resident
				     *  environment.  We set it to one so that
				     *  we can free memory we malloc'd on
				     *  subsequent calls to setenv.
				     */

/*
 *  setenv - add an element to the environment
 *
 *  var is added to the environment.  It should be a string of
 *  the form "NAME=value".  Note, that the actual string is not
 *  copied, but a pointer is used, so be careful not to overwrite
 *  it before the exec.  Also, this does not check to make sure
 *  one doesn't already exist, so delenv first if you're not sure.
 */
setenv(var)
	char	*var;
{
	char	**e, **n;
	int	count;
 	char	**new;

	/*
         *  Count the number of items in the environment and malloc
	 *  room for all those plus the one we are adding.
	 */
	for(e = environ, count = 0; *e; e++) count++;
	new = (char **)malloc( (count+2)*sizeof (char *));
	/*
	 *  Copy over the points from the old to the new, and add the
	 *  new one.
	 */
	for(e = environ, n = new; count; count--)
		*n++ = *e++;
	*n++ = var;
	*n = 0;
	/*
	 *  If we had allocated this environ from a previous call,
	 *  free it now.
	 */
	if(_set_env) free(environ);
	else _set_env = 1;
	environ = new;
}

/*
 *  delenv - delete an environment variable.
 *
 *  Var should be the name of an environment variable such as "PATH"
 *  Note, that the space is not reclaimed, but this doesn't happen all
 *  that much.
 */
delenv(var)
	char	*var;
{
	char	**e;
	int	len = strlen(var);

	/*
	 *  Look for the entry in the environment.
	 */
	e = environ;
	while(*e)  {
		if(strncmp(var, *e, len) == 0)
			break;
		e++;
	}
	/*
	 *  Move everything after it, including the null terminator
	 *  down one, obliterating the deleted pointer.
	while(*e)  {
		*e = e[1];
		e++;
	}
}