C error: expression must have a constant value

Question

I am writing some embedded code to interface with an external device over SPI. The device has several registers of varying length and to help keep things straight I have defined the following structure

typedef struct
{
    uint16_t    Signed          :1;  // Register is signed or unsigned
    uint16_t    CommLengthBytes :3;  // The width of the register in bytes 
    uint16_t    Address         :12; // Register address
}ts_register;

I have then defined each register in my sources as follows

static const ts_register    SAGCYC      = {0, 1, 0x000};
static const ts_register    DISNOLOAD   = {0, 1, 0x001};
static const ts_register    LCYCMODE    = {0, 1, 0x004};
static const ts_register    IRMSA       = {0, 4, 0x31A};
static const ts_register    IRMSB       = {0, 4, 0x31B};
static const ts_register    VRMS        = {0, 4, 0x31C};

etc.

I have a function that will take a pointer to an array of ts_registers and queue up the SPI transfers required to read all of the registers in the array and call a callback function to handle the reply

My issue comes when I try to make the array of ts_registers that I want to read as follows:

ts_register regs_to_read[3] = {VRMS, IRMSA, IRMSB};

This generates the error: "expression must have a constant value" 3 times (once per array element).

Since they are defined as constants, what have I overlooked?

Answer 1

Since they are defined as constants, what have I overlooked?

In C objects declared with the const modifier aren't true constants. A better name for const would probably be readonly - what it really means is that the compiler won't let you change it. And you need true constants to initialize objects with static storage (I suspect regs_to_read is global).

You could try assigning regs_to_read in a function called before anything else uses that array.

Answer 2

const doesn't make them constants at compile time. Make them #defines and the compiler will be happy.

Answer 3

I think this may be a compiler issue, and it would be helpful to know your platform and how you are building this code. I just took most of your code, doctored it up to compile it, and compiled on Linux using gcc. There were no warnings.

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

typedef struct
{
    uint16_t    Signed          :1;  // Register is signed or unsigned
    uint16_t    CommLengthBytes :3;  // The width of the register in bytes 
    uint16_t    Address         :12; // Register address
}ts_register;

int main(int argc, char **argv) {

    static const ts_register    SAGCYC      = {0, 1, 0x000};
    static const ts_register    DISNOLOAD   = {0, 1, 0x001};
    static const ts_register    LCYCMODE    = {0, 1, 0x004};
    static const ts_register    IRMSA       = {0, 4, 0x31A};
    static const ts_register    IRMSB       = {0, 4, 0x31B};
    static const ts_register    VRMS        = {0, 4, 0x31C};

    ts_register regs_to_read[3] = {VRMS, IRMSA, IRMSB};

    return(0);
}

Have you tried casting the values? It's not always the best thing to do, but will get you around the error.

Have you considered creating #define entries?

Also, please be aware consts take a bit of getting used to in C. They do not always behave the way you might expect.