BitfieldGotchas

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---
title: BitfieldGotchas
---

# Assignment from Fake Boolean

In the following example a boolean flag is passed across an interface.

*   In the source object, flags were implemented using an enum. 
*   In the destination object they were implemented as a bitfield. 
*   The bug was hidden by the use of a C89-style typedeffed BOOL (an int)

The resulting code fails to assign the boolean flag correctly. The bug is a non-issue if C99 stdbool is used instead.

## Source

#include <stdio.h> //printf
    
    #if \_\_STDC\_VERSION\\_\_ >= 199901
    #   include <stdbool.h>
        typedef _Bool BOOL;
    #   define FALSE false
    
    #else 
        /* C89 Fake bool */
        typedef int BOOL;
    #   define FALSE (0)
    
    #endif
    
    
    
    /* Source data */
    enum EFLAGS {
        FLAG_NONE = 0x00,
        FLAG_FOO = 0x01,
        FLAG_BAR = 0x02,
        FLAG_MAX
    } srcFlags = FLAG_NONE;
    
    struct SFLAGS
    {
        int isFoo   :1;
        int isBar   :1;
    } dstFlags = {, };
    
    
    int main(void)
    {
        BOOL tmpBool = FALSE;
    
        srcFlags |= FLAG_BAR; /* set BAR in src */
    
        tmpBool = (srcFlags & FLAG_BAR); /* pass in BOOL */
        /* fake BOOL: tmpBool has value 2
         * C99 _Bool: tmpBool has value true
         */
    
        dstFlags.isBar = tmpBool; /* set in dst */
        /* fake BOOL: isBar is assigned (2 & 0x01) = 0  !!!
         * C99 _Bool: isBar is assigned true
         */
    
        if (dstFlags.isBar)
        {
            printf("dstFlags.isBar is TRUE");
        }
        else
        {
            printf("dstFlags.isBar is FALSE");
        }
    
        return ;
    }

## Results

$ gcc -std=c89 -pedantic -Wall -o bitfield1 bitfield1.c && ./bitfield1
    dstFlags.isBar is FALSE
    
    $ gcc -std=c99 -pedantic -Wall -o bitfield1 bitfield1.c && ./bitfield1
    dstFlags.isBar is TRUE

## Lesson

*   Always sanitise fake BOOL with !!(x) or (x != 0) 
*   Or avoid the problem entirely by using C99 _Bool

[1]: BitfieldGotchas?action=sourceblock&num=1

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