Q51865: /Zp4 Does Not Work with Standard Stream Handles

Article: Q51865
Product(s): See article
Version(s): 5.10 6.00 6.00a | 5.10 6.00 6.00a
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | S_QUICKC buglist5.10 buglist6.00 buglist6.00a | mspl13_c
Last Modified: 15-JAN-1991

The Microsoft C compiler option /Zp4 (pack structures to four bytes)
does not work correctly with the standard stream handles (stdin,
stdout, stdaux, stderr, stdprn) when compiled under the large model.
This happens with all optimizing compiler versions up to and including
C 6.00a and QuickC compiler versions 2.00, 2.01, 2.50, and 2.51.

To work around this problem, compile the STDIO.H header file with
packing set to 1 or 2 bytes. This can be done with the #pragma pack(2)
just before the #include <stdio.h>. After the include file is
compiled, packing can be set back to 4 bytes with #pragma pack(4).
Using this method allows for normal use of the standard devices.

The following program will compile with /Zp4 /AL and run successfully:

#pragma pack(2)
#include <stdio.h>
#pragma pack(4)

#include <conio.h>

void main(void)
{
  fprintf(stderr,"U1001: Syntax Error\n");
  getch();
}

The reason STDIO.H must be compiled with packing set to 1 or 2 bytes
is quite simple. The structure FILE is declared inside STDIO.H, but
storage for the streams is not allocated. The stdin...stderr streams
are assigned to an external array of stream file handles that were
previously compiled inside the combined library with /Zp2 (the
default). It becomes a simple case of linking two objects with
different packing options.

Microsoft has confirmed this to be a problem with C versions 5.10,
6.00, and 6.00a; and QuickC versions 2.00, 2.01, 2.50, and 2.51
(buglist2.00, buglist2.01, buglist2.50, and buglist2.51). We are
researching this problem and will post new information here as it
becomes available.

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