Q48793: exec() and spawn() with P_OVERLAY Ignore Max Allocation

Article: Q48793
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 22-JUL-2001

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The information in this article applies to:

- The C Run-Time (CRT), included with:
   - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax 
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++, versions 1.0, 1.5 
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SUMMARY
=======

The Microsoft C Run-time functions exec() and spawn() with P_OVERLAY ignore the
maximum memory allocation information in the .EXE file header. If you exec or
spawn with P_OVERLAY, a program whose maximum allocation is set equal to the
minimum allocation (through EXEMOD or the LINK option /CP:1), the program is
allocated the full 64K DGROUP.

MORE INFORMATION
================

The effective maximum allocation for any exec'ed (or spawn'ed with P_OVERLAY)
program is 0xffff (whatever MS-DOS has available), regardless of the maximum
allocation number in the program's .EXE header. When run from the MS-DOS prompt,
the maximum allocation information of the same program is honored.

This behavior occurs because the exec() and spawn() functions with P_OVERLAY do
not invoke COMMAND.COM, which looks at the minalloc and maxalloc fields in the
.EXE file header. This is expected behavior for exec() and spawn() with
P_OVERLAY. This is documented in the C run-time reference documentation for the
exec() functions.

Alternatives
------------

Instead of calling a program with exec() or spawn() with P_OVERLAY, call
COMMAND.COM with the desired child program as an argument using exec() or
spawn() with P_OVERLAY, as follows:

     spawnl(P_OVERLAY,"c:\\command.com","command.com",
            "/cchild.exe",NULL);

COMMAND.COM then loads your application as if it was from the MS-DOS prompt.
Note, however, that the COMMAND.COM shell itself requires approximately 4K.
Also, consecutive exec() or spawn() with P_OVERLAY calls using this method run
additional shells of COMMAND.COM, accumulating one 4K shell per generation, as
illustrated below:

         +------+    +------+    +------+    +------+
         |  P1  |--> |__P2__|--> |__P3__|--> |__P4__|
         +------+    | .COM |    |_.COM_|    |_.COM_|
                     +------+    | .COM |    |_.COM_|
                                 +------+    | .COM |
                                             +------+

Another option is to call your program with the system() function, but this does
not overlay the child process.

Additional query words: 1.00 1.50 6.00 6.00a 6.00ax 7.00 kbinf execl execle execlp execlpe execcv execve execvp execvpe spawnl spawnle spawnlp spawnlpe spawnv spawnve spawnvp spawnvpe

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Keywords          : kb16bitonly 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : winnt:

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