Volatile in struct causing warning in macro

Question

I use the following type:

/* double_buffer.h */

typedef struct
{
    uint8_t * active_buffer_p;       //< Address of active buffer
    uint8_t current_writing_index;   //< Current writing index in active buffer
    uint8_t buffer_1[BUFFER_SIZE];   //< First buffer
    uint8_t buffer_2[BUFFER_SIZE];   //< Second buffer
} double_buffer_t;


#define DoubleBuffer_init(buffers) do {                 \
        (buffers).active_buffer_p = (buffers).buffer_1; \
        (buffers).current_writing_index = 0;            \
        } while(0)

In my code, I declare an array of double buffer, using the volatile keywoard (because the buffers can be updated/read asynchronously in interrupts and in functions):

static volatile double_buffer_t m_double_buffers[NB_DOUBLE_BUFFERS];

I then initialize those buffers individually:

DoubleBuffer_init(m_double_buffers[id]);

When I compile the software (gcc), I got the following warning:

 error: assignment discards 'volatile' qualifier from pointer target type [-Werror=discarded-qualifiers]
   28 |         (buffers).active_buffer_p = (buffers).buffer_1; \

The reason why I have this warning is quite unclear to me, and I am not sure how to fix it.

Any help would be appreciated (I can update the question if something is not clear).

Answer 1

You get this warning because you have a volatile object, and you create a non-volatile pointer to it.

This is bad as the compiler could access the volatile object without knowing that it is volatile. E.g. it could transform two reads into a single, it could change the order etc.

One way to fix it is to define active_buffer_p to uint8_t volatile * active_buffer_p.