为什么在此 MPLAB C 示例中使用“extern”？
Why is `extern` used in this MPLAB C example?
在 MPLAB XC8 Compiler User Guide 中，第 162 页上的示例（转载如下）将 extern 关键字与 @ 说明符结合使用。鉴于我们自己指定地址，为什么需要这样做？它本身不会分配任何内存。
我能想到的唯一原因可能是外部变量在启动时没有归零。但是，C 变量通常包含垃圾，直到您显式分配给它们。所以...我不知道。
可能跟在头文件里有关系？要避免多个 #include 语句导致某种 "variable already declared" 错误？
If the pointer has to access objects in data memory, you need to define a different object to act as a dummy target. For example, if the checksum was to be calculated over 10 bytes starting at address 0x90 in data memory, the following code could be used.
const char * cp;
extern char inputData[10] @ 0x90;
cp = &inputData;
// cp is incremented over inputData and used to read values there
No memory is consumed by the extern declaration, and this can be mapped over the top of existing objects.
没有区别。主要是显式还是隐式的选择。
在 MPLAB XC8 Compiler User Guide 中，第 162 页上的示例（转载如下）将 extern 关键字与 @ 说明符结合使用。鉴于我们自己指定地址，为什么需要这样做？它本身不会分配任何内存。
我能想到的唯一原因可能是外部变量在启动时没有归零。但是，C 变量通常包含垃圾，直到您显式分配给它们。所以...我不知道。
可能跟在头文件里有关系？要避免多个 #include 语句导致某种 "variable already declared" 错误？
If the pointer has to access objects in data memory, you need to define a different object to act as a dummy target. For example, if the checksum was to be calculated over 10 bytes starting at address 0x90 in data memory, the following code could be used.
const char * cp; extern char inputData[10] @ 0x90; cp = &inputData; // cp is incremented over inputData and used to read values thereNo memory is consumed by the extern declaration, and this can be mapped over the top of existing objects.
没有区别。主要是显式还是隐式的选择。