DOS Environment Variables

[Rob X Cowan]

> Does anyone know how to permanently change a DOS environment variable from
> within an executing program?  PUTENV() changes/creates a variable but it
> is only in effect while the program is running.  As soon as I go back to
> DOS, the environment is restored to its original variables.  I'm using
> Turbo C.
> 
> Thanks for any help,
> Rick Schatzman

     The following code demonstrates how to access the global environment
string.  It's not the only way, nor the best, but it'll get you started.
masterenvstring is a pointer to the start of the environment list, in which
each variable is an ASCIIZ string of the format <NAME>=<VALUE>.  The list
is terminated by a double null.  masterenvsize is the size of the Memory
Control block that is allocated for the environment string; I didn't put
any code to deal with its manipulation, but you'll have to deal with it if
you plan on adding entries or you risk a system halt by COMMAND.COM.. fun.

/*
 *  Demonstrates traversal of master environment string
 *  	Rob S. Cowan  9105.07
 */

#include <dos.h>

char *env_seekend(char far *);

void main(void)
{
	union REGS		in, out;
   struct SREGS	segs;
   char far		  *masterenvstr;
   int				masterenvsize;

   /*
    *  I don't remember if 2E is a supported function or not.. Nothing
    *   must be linked ahead of COMMAND.COM, or that address will be
    *   returned.
    */
   in.x.ax = 0x352E;					/*  Get PSP of COMMAND.COM (sortof)  */
	intdosx(&in, &out, &segs);

   /*  Get env asciiz string : seg stored at offset 2C of PSP  */
	masterenvstr = MK_FP(*(unsigned *)MK_FP(segs.es, 0x2C), 0);

   /*  Get size of env block from MCB preceding it  */
	masterenvsize = (*(unsigned *)MK_FP(FP_SEG(masterenvstr) - 1, 3)) << 4;

   /*
    *  You must take care to accommodate the memory requirements of
    *   the environment string.  If you wish to add entries you will
    *   have to allocate/deallocate it with the MCB that precedes the
    *   environment.
    */
}

/*
 *  Environment strings are a list of consecutive ASCIIZ strings
 *   terminated by a null entry.  This function will traverse to the
 *   end of this list and return the address of the null entry.
 */
char *env_seekend(char far *envtrav)
{
	for(;;)
   {
   	if (!*envtrav++)
      	if (!*envtrav)
         	return envtrav;
   }
}

Prost,
-Rob
robc at cup.portal.com