"Writing systcik today_ When using the handler() interrupt function, an error is always reported. The global variable extern U16 ntime (ntime -- written in stm32f10x) defined at the beginning_ In the systick interrupt function of it. C), but errors are always reported during compilation. Baidu has a solution after that,
I have noticed the extern variable before, but I haven't thought about its use carefully. When I encounter it today, I must understand it and the general function of this keyword (just the general function, the breadth and depth of C dare not talk about it);
When you build a header file library, you often encounter a problem, that is, you define a temperature variable in file a and want to pass its value to file B for use, but the function in file a operation cannot take return parameters, such as interrupt service function. What should you do? If you define temperature in a, then include A.H and B.H in includes. H, and then put includes. H in a.c and B.C, there is no problem with single compilation, but there will be problems when linking,
“Symbol temple mulTIply defined（by A.o and B.o）”
This variable has been defined many times!!!
The solution is:
After defining the temp variable in a, declare it in B with extern, for example:
1. Define temp in a and assign value: U16 temp2 = 0;
2. Declare extern U16 temp2 in B;
This is just a declaration, no assignment, or an error will be reported!
Symbol temp2 mulTIply defined （by catch_ pwm.o and app.o）
Note the difference between variable definition and variable declaration:
The variable definition uses the form of "data type + variable name", and the compiler needs to allocate memory units to it;
The variable declaration uses the form of "external variable type + variable name" to tell the compiler that this variable will be defined in other external C files. I only use it externally here. The compiler will not allocate memory space to it, but will allocate memory space to it when it really encounters the variable definition.
//////////////////The following is the "Juhai" de post///////////////////////////////////
1. Common variables are defined as global variables
If it is a common type, it can be defined directly in the *. C file without the *. H file, and declared with extern at the calling file, because the compiler can recognize common types. For example, in a my. C file, I defined char name [10]; In other files, just use extern char name [] (because it is a declaration, the size of a one bit array can be omitted, but it is not recommended to use pointers. Comparing pointers and arrays is two different things). Tell the compiler that I have defined this variable. Take your time to find out. This makes sense because char is a type that the compiler can recognize autonomously.
2. A custom structure type is defined as a global variable
Unlike ordinary types, the compiler will not recognize your custom type without notifying the compiler in advance. At this time, the *. H file appears. Isn't the definition of structure type not occupying memory? Well, I put the definition of the large structure in the *. H file, so that no matter how many times you incude, the memory will not be occupied. In addition, there is another advantage. You can include this *. H file in other files, so that the compiler can recognize your custom type in this file. Isn't the purpose achieved? If I defined it in global. H
typedef struct _ POSITION
{
int x;
int y;
}POSITION;
Then I can implement the definition of global variables in a global. C file, but I need to include the *. H file, such as
include “global.h”
POSITION current，;
Thus, the variable cunrrent is defined. When this variable is referenced in other files, as long as external position current; Declare it, and then you can use it. However, this file must also include "global. H", because if it is not included, the position type is not recognized in this file.
1. How to reference a defined global variable?
Answer: extern can refer to the header file or the extern keyword. If you refer to a global variant declared in the header file by referring to the header file, you will report an error during compilation if you write it wrong. If you use extern, you will not report an error during compilation if you make the same error, An error is reported during connection.
2. Can global variables be defined in multiple. C files? Why?
A: Yes, you can declare a global variable with the same name in the form of static in different C files. Global variables with the same name can be declared in different C files, provided that only one C file can assign an initial value to this variable, and the connection will not make an error.
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Working principle of single chip microcomputer memory and concept analysis of chip selection and bus
Definition and analysis of extern global variable in STM32 Single Chip Microcomputer Design
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