Q67086: va_arg() Fails on Odd-Size Structs Packed on 1-Byte Boundaries

Article: Q67086
Product(s): See article
Version(s): 6.00 6.00a | 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: 19-JAN-1991

Because the va_arg() macro does not handle odd-sized arguments
properly, odd-sized structures should not be passed by value to
variable argument functions if the structures are packed on 1-byte
boundaries. Because an even number of bytes is always put on the stack
for each argument passed, a packed structure with an odd size will not
be retrieved properly. Using an even packing size or passing the
structure by reference will eliminate this problem.

For more information on the va_arg() problem with odd-sized arguments,
query on the following words:

   va_arg and char

To demonstrate this problem, assume a program contains code resembling
the following:

#pragma pack(1)

struct foo {
             int  x;
             char y[5];
           } oddstruct;

If the structure "oddstruct" is passed to a function by value and that
function takes a variable number of arguments, then accessing the
structure from within the function with va_arg() will fail. The
sizeof(oddstruct) is actually 7 in this case, but 8 bytes will have
been pushed on the stack.

Microsoft has confirmed this to be a problem in 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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