Q68872: Ternary Operator Generates Incorrect Code for Bit Fields

Article: Q68872
Product(s): See article
Version(s): 6.00   | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | buglist6.00 fixlist6.00a | mspl13_c
Last Modified: 6-FEB-1991

In some situations, the ternary operator does not recognize that the
first bit in a structure has been set. For example, when the code
example below is compiled with

   cl test.c

it prints out

   NOT_SET  NOT_SET

when it should print the following:

   SET      NOT_SET

Code Example
------------

#include <stdio.h>

typedef struct tag
{
   unsigned int a: 1;
   unsigned int b: 1;
   unsigned int c: 14;
} BIT, *PBIT;

void foo(PBIT px)
{
   printf("%s\t%s", px->a ? "SET" : "NOT_SET", px->b ? "SET" : "NOT_SET");
}

void main(void)
{
   BIT  x;

   x.a = 1;
   x.b = 0;
   foo( &x );
}

The following are three workarounds:

1. Update to C version 6.00a.

2. Compile with /Od.

3. Simplify the printf() statement by moving the ternary condition to
   another line, or by using an else if statement.

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