[Bug c/11885] New: Problem with bitfields in packed structs

[stanrost at lcs dot mit dot edu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11885

           Summary: Problem with bitfields in packed structs
           Product: gcc
           Version: 3.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: stanrost at lcs dot mit dot edu
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu avr-unknown-unknown

Hi,

I have been hacking for Berkley Motes using nesC, which produces code later
compiled by AVR-GCC.  There is a problem with bitfields within packed
structures, which is also reproducible on the native platform (i686-pc-linux-gnu).

The bug is illustrated by the following simple program:
--------------------------------------------------------------

include <stdio.h>
#include <stdint.h>

typedef struct {
    
    uint8_t flag1:2;
    uint8_t flag2:1;
    uint8_t flag3:1;
   
    uint8_t flag4;

} __attribute__ ((packed)) MyType;

int main(void) {

    MyType a;
    MyType *b = &a;

    b->flag1 = 0;
    b->flag2 = 0;
    b->flag3 = 0;

    b->flag4 = 0;

    b->flag4++;
    
    if (b->flag1)
	printf("Set\n");
    else
	printf("Clear\n");

}

--------------------------------------------------------------

This is clearly an error because flag4 should be allocated its own byte
and not packed into the byte with other variables.

Please look into this, as the current methods of circumvention force me
to pad the structures with "reserved" bitfields.

Respectfully,

Stan Rost