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
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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
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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
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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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