Incorrect DCB definition (winbase.h)

[andi.martin]

Hi Pelle,

IMHO there is a bug. Following code shows the problem:
#include <stdio.h>
#include <stddef.h>
#include <windows.h>

int main(int argc, char *argv[])
{
  DCB dcb={sizeof(DCB)};
  HANDLE h = CreateFileA("COM1:", GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);
  GetCommState(h, &dcb);
  printf("ByteSize is at offset %d\n", offsetof(DCB, ByteSize)); // 22 (and should be 18)
  printf("ByteSize is %d\n", dcb.ByteSize); // 0
  dcb.XonLim=20;
  if(!SetCommState(h, &dcb))
  {
    printf("Error where none should be...\n");
  }
  CloseHandle(h);
  return 0;
}


Kindly regards,
Andreas Martin

[andi.martin]

Workaround:
You have to change line 821 (winbase.h) from
DWORD fDummy2:17;
to
DWORD fDummy2:16;

to make it work.

Regards,
Andreas

[andi.martin]

Searched backwards to find PellesC version 4.50 which hasn't that bug yet.

Regards,
Andreas

[andi.martin]

Hi Pelle,

I think it's ANSIC anyway and not a bug, since alignment of bit-field members and non-bit-field members is in any point of view implementation defined or unspecified:
Ref: ISO/IEC 9899:1999 (E), 6.7.2.1 Structure and union specifiers (10-12)

Following example shows that in "struct s2" 4 bytes are "wasted", because no bitfield exceeds 4 bytes (32 bits).
Well that's still not a bug.
But poorly windows headers expect a compiler to leave only unused (bit) space appropriate to units type. That previous DCB structure sample is a good example, others might be found...

#include <stdio.h>
#include <stddef.h>

typedef struct s1 {
  int a:15;
  int b:16;
  char c;
};

typedef struct s2 {
  int a:16;
  int b:16;
  char c;
};


int main(int argc, char *args[])
{
  printf("%d\n", offsetof(struct s1, c));  // "4"
  printf("%d\n", offsetof(struct s2, c));  // "8"
  return 0;
}

Kindly regards,
Andreas Martin

[Pelle]

This sounds like the same bug I found, a few days ago, in a different context - I will check the fix with your code too...