Go deep into the usage of global variables in uCOS

Global variables are often encountered in C programming. The following method is generally used to define global variables:

. C file Definition

Unsigned int gVariable;

. H file Declaration

Extern unsigned int gVariable;

This aims to prevent repeated declarations of variables and improve compilation efficiency. However, if such a plan is not properly arranged, various problems may occur. Repeated declarations during compilation, or even errors.

After reading the book "uC/OS-II" Translated by Shao Beibei, I found that a very clever global variable definition method was used in it. I will record my understanding below, your own notes. Also, write it out to learn with everyone.

The uC/OS-II defines a header file that is globally used. This file is referenced in any. C file.

This macro is defined in each. h file. \

Copy codeThe Code is as follows: # ifdef XXX_GLOBALS

# Define XXX_EXT

# Else

# Define XXX_EXT extern

# Endif

In the. H file, each global variable is prefixed with xxx_EXT. Xxx indicates the name of the module. The. C file of this module has the following definitions:

# Define XXX_GLOBALS

# Include "des. h"

When the compiler processes the. C file, it forces xxx_EXT (which can be found in the corresponding. h file) to be null (because xxx_GLOBALS has been defined ). Therefore, the compiler allocates memory space for each global variable. When the compiler processes other. C files, xxx_GLOBAL is not defined, and xxx_EXT is defined as extern, so that you can call external global variables. To illustrate this concept, see uC/OS _II.H, which includes the following definitions:

Copy codeThe Code is as follows: # ifdef OS _GLOBALS

# Define OS _EXT

# Else

# Define OS _EXT extern

# Endif

OS _EXT INT32U OSIdleCtr;

OS _EXT INT32U OSIdleRun;

OS _EXT INT32U OSIdleMax;

At the same time, uCOS_II.H has the following definitions:

# Define OS _GLOBALS

# Include "des. h"

When the compiler processes uCOS_II.C, it turns the header file into the following, because OS _EXT is set to null.

INT32U OSIdleCtr;

INT32U OSIdleRun;

INT32U OSIdleMax;

In this way, the compiler will allocate these global variables to the memory. When the compiler processes other. C files, the header file becomes as follows. Because OS _GLOBAL is not defined, OS _EXT is defined as extern.

Extern INT32U OSIdleCtr;

Extern INT32U OSIdleRun;

Extern INT32U OSIdleMax;

In this case, no memory allocation is generated, and these variables can be used in any. C file. In this way, you only need to define it once in the. H file.