Pass anonymous struct as parameter in C

Question

I have the following line of c (carriage returns added for readability - they aren't in the code):

#define i2c_write(slave_addr, reg_addr, len, *data_ptr)
    twi_master_write(MPU_TWI, {
        .addr = reg_addr,
        .addr_length = 1,
        .buffer = *data_ptr,
        .length = len,
        .chip = slave_addr
    })

Where twi_master_write() is declared as:

uint32_t twi_master_write(Twi *p_twi, twi_packet_t *p_packet);

and twi_packet_t is declared as:

typedef struct twi_packet {
    uint8_t addr[3];
    uint32_t addr_length;
    void *buffer;
    uint32_t length;
    uint8_t chip;
} twi_packet_t;

The parameters of twi_write() are all required to be of type unsigned char.

When compiling, I receive the following error:

expected expression before '{' token

Is there a correct way to do what I'm trying to do here, or is it just not possible?

Answer 1

My take on it, in a compilable sample. This is a compilation stub and will not run correctly, so don't try to run it as-is !

//
// Cobbling up a compilation stub
//

#include <stdint.h>

struct Twi;
typedef struct Twi Twi;

#define MPU_TWI (Twi*)0

typedef struct twi_packet {
    uint8_t addr[3];
    uint32_t addr_length;
    void *buffer;
    uint32_t length;
    uint8_t chip;
} twi_packet_t;

uint32_t twi_master_write(Twi *p_twi, twi_packet_t *p_packet);


//
// Now for my answer :
//

#define i2c_write(slave_addr, reg_addr, len, data_ptr) \
    twi_master_write(MPU_TWI, &(twi_packet_t){         \
        .addr = reg_addr,                              \
        .addr_length = 1,                              \
        .buffer = *data_ptr,                           \
        .length = len,                                 \
        .chip = slave_addr                             \
    })

main()
{
    // Trigger that macro !
    i2c_write(0, 0, 0, (void**)0);
}

This instanciates a compound literal and passes its address to the function. The literal's lifetime does not exceed that of the full call expression.