C的误差：前pression必须有一个恒定的值 [英] C error: expression must have a constant value

问题描述

我写一些嵌入式code拥有超过SPI外部设备接口。该器件具有不同长度，并让事情简单一点我已经定义的结构如下几个寄存器

  typedef结构
{
    uint16_t签名：1; //注册带符号
    uint16_t CommLengthBytes：3; //寄存器的字节宽度
    uint16_t地址：12; //注册地址
} ts_register;
 

我已经定义，然后在我的消息来源的各个寄存器如下：

 静态常量ts_register SAGCYC = {0，1，量0x000};
静态常量ts_register DISNOLOAD = {0，1，0×001};
静态常量ts_register LCYCMODE = {0，1，量0x004};
静态常量ts_register IRMSA = {0,4} 0x31A;
静态常量ts_register IRMSB = {0,4} 0x31B;
静态常量ts_register VRMS = {0,4} 0x31C;
 

等。

我将采取一个指向ts_registers数组，排队读取所有寄存器阵列中，并调用回调函数来处理回复所需的SPI传输功能

我的问题是当我试图让我想读ts_registers的排列如下：

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

这会产生错误：恩pression必须有一个恒定的值3次（每个数组元素一次）。

由于他们被定义为常数，你有什么我忽略了？

解决方案

  

由于他们被定义为常数，你有什么我忽略了？

在与常量修改宣布C对象不是真正的常量。为常量一个更好的名字很可能是只读 - 它真正的意思是，编译器不会让的您的改变。而你需要真正的常数初始化与静态存储（我怀疑 regs_to_read 是全球性的）。

对象

您可以尝试在其他任何使用该阵列之前被调用函数赋值 regs_to_read 。

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?

解决方案

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.

这篇关于C的误差：前pression必须有一个恒定的值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！