C 错误:表达式必须有一个常量值 [英] C error: expression must have a constant value

问题描述

我正在编写一些嵌入式代码以通过 SPI 与外部设备连接.该设备有几个不同长度的寄存器，为了帮助保持直线，我定义了以下结构

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;

然后我在我的源代码中定义了每个寄存器

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};

等

我有一个函数，它将获取一个指向 ts_registers 数组的指针，并将读取数组中所有寄存器所需的 SPI 传输排队，并调用回调函数来处理回复

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

当我尝试按如下方式制作我想要读取的 ts_registers 数组时，我的问题出现了:

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};

这会产生错误:表达式必须有一个常量值"3 次(每个数组元素一次).

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

既然它们被定义为常量，我忽略了什么?

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

推荐答案

既然它们被定义为常量，我忽略了什么?

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

在 C 中用 const 修饰符声明的对象不是真正的常量.const 的更好名称可能是 readonly - 它的真正含义是编译器不会让 你 更改它.并且您需要真正的常量来初始化具有静态存储的对象(我怀疑 regs_to_read 是全局的).

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).

您可以尝试在其他任何使用该数组之前调用的函数中分配 regs_to_read.

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

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