Q51742: Spawned Process Runs Out of Environment Space

Article: Q51742
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc s_quickasm | mspl13_c
Last Modified: 20-DEC-1989

Using spawn to create a child process, then attempting to increase the
environment space of the child (by adding a new environment variable
or expanding an existing environment variable) results in an "Out of
Environment Space" error.

This problem does NOT happen under OS/2.

The following program(s) illustrates this behavior:

/*-----------------------------------------------------------------*/
/*-----------------------------------------------------------------*/
parent.c  This will call the child process (child) with the spawnlp
          function.
/*-----------------------------------------------------------------*/

#include <stdio.h>
#include <process.h>

void main (void);

void main (void)
{
    printf ("In the parent process\n");

    spawnlp (P_WAIT, "child.exe", "child", NULL);

    printf ("\nAnd back to the parent process.\n");
}

/*-----------------------------------------------------------------*/
child.c Called by parent.c, uses the system function to call a batch
        file (BATCH.BAT) to attempt to set a new environment variable.

/*-----------------------------------------------------------------*/
#include <stdio.h>
#include <process.h>
#include <conio.h>
#include <errno.h>

void main (void);

void main (void)
{
    printf ("At child process...\n");
    system ("batch.bat");
    getch ();
}

/*-----------------------------------------------------------------*/
BATCH.BAT  The batch file, which is called by the child process
            (CHILD.EXE). It just shows the environment variables,
            attempts to set another environment variable, then shows
            the environment variables one more time.
/*-----------------------------------------------------------------*/
set
set blah=thisisatestonlyatestsoitdoesnotreallymatter
set
/*----------------------------------------------------------------------*/

Parent will spawn child, which in turn spawns (through system)
BATCH.BAT. The idea is to show that when BATCH.BAT is called, an "Out
of Environment Space" error will be given. Yet, if BATCH.BAT is run
from DOS, no such error is issued.

To work around this problem, set up a dummy environment variable that
is large enough to hold the new environment variable you plan to use
in the spawned process. When the child process is called, you can then
set the dummy variable to null (with a "set dummy="), then you can set
your processes environment variable. Please note that this will change
the dummy environment variable for the child only, not the parent
process.

Under DOS, the child will get only enough space to hold the current
environment variables, while under OS/2, the segment that holds the
environment can usually be expanded.

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