[avr-gcc-list] Bit-wise structure and unions

[Keith Gudger]

Below is a program which illustrates the following issue:

When using AVR ports, I would like to define them with bit-wise
structures.  This way I can easily access individual bits (or groups of
bits).  e.g.:

typedef struct port_reg {
        unsigned bit0:1   ;
        unsigned bit1:1   ;
        unsigned unused:6 ;
} PORT_REG;

However, I cannot assign the 8 bit port value directly to a structure
defined this way.  Here is the code and the gcc error:

    PORT_REG *pLED_DIR = (PORT_REG *)0x37 ; ///< LED Port DIR REG PORT B
    *pLED_DIR = (PORT_REG) 0x03 ;

main.c:27: error: conversion to non-scalar type requested

I can fix this problem by introducing a union:

typedef union {
    PORT_REG port_bits ;
    uint8_t  byte ;
} PORT_UNION ;

   PORT_UNION *pLED_DIR = (PORT_UNION *)0x37 ; ///< LED Port DIR REG PORT B
   PORT_UNION *pLED_OUT = (PORT_UNION *)0x38 ; ///< LED Port Output PORT B
   pLED_DIR->byte = 0x3 ;

But I would rather not have the union.  Is there any other way around
this?  Thanks.

Keith

(full code follows)

typedef struct port_reg {
        unsigned bit0:1   ;
        unsigned bit1:1   ;
        unsigned unused:6 ;
} PORT_REG;

typedef union {
    PORT_REG port_bits ;
    uint8_t  byte ;
} PORT_UNION ;

int main(void)
{
#ifdef UNION
  PORT_UNION *pLED_DIR = (PORT_UNION *)0x37 ; ///< LED Port DIR REG PORTB
  PORT_UNION *pLED_OUT = (PORT_UNION *)0x38 ; ///< LED Port Output PORTB
  pLED_DIR->byte = 0x3 ;
#else
  PORT_REG *pLED_DIR = (PORT_REG *)0x37 ; ///< LED Port DIR REG PORT B
  PORT_REG *pLED_OUT = (PORT_REG *)0x38 ; ///< LED Port Output PORT B
  *pLED_DIR = 0x03 ;
#endif
     while (1)
    {
#ifdef UNION
        pLED_OUT->port_bits.bit1 ^= 1 ;
#else
        pLED_OUT->bit1 ^= 1 ;
#endif
     }
     return 0;
}