C中的外部和静态指针

Question

嗨什么可能是静态和外部指针的用法？如果他们存在

Answer 1

要回答关于何时可以使用它们的问题,请举几个简单的例子:

静态指针可用于实现一个总是向程序返回相同缓冲区的函数,在第一次调用它时分配它:

char * GetBuffer() {
   static char * buff = 0;
   if ( buff == 0 ) {
       buff = malloc( BUFSIZE );
   }
   return buff;
}

可以使用extern(即全局)指针允许其他编译单元访问main的参数:

extern int ArgC = 0;
extern char ** ArgV = 0;

int main( int argc, char ** argv ) {
   ArgC = argc;
   ArgV = argv;
   ...
}

Answer 2

简短回答:它们不存在.C99 6.7.1说"最多可以在声明中的声明说明符中给出一个存储类说明符".extern并且static都是存储类说明符.

Answer 3

假设我有一个指针，我想让多个翻译单元全局可用。我想在一个地方（foo.c）定义它，但允许在其他翻译单元中对其进行多个声明。“extern”关键字告诉编译器这不是对象的定义声明；实际的定义将出现在其他地方。它只是使对象名称可供链接器使用。编译和链接代码时，所有不同的翻译单元都将通过该名称引用同一对象。

假设我还有一个指针，我确实希望使其对单个源文件中的函数全局可用，但不让它对其他翻译单元可见。我可以使用“static”关键字来指示对象的名称不导出到链接器。

假设我还有一个指针，我只想在单个函数中使用该指针，但在函数调用之间保留该指针的值。我可以再次使用“static”关键字来指示该对象具有静态范围；它的内存将在程序启动时分配，直到程序结束才释放，因此该对象的值将在函数调用之间保留。

/** 
 * foo.h
 */
#ifndef FOO_H
#define FOO_H

/**
 * Non-defining declaration of aGlobalPointer; makes the name available
 * to other translation units, but does not allocate the pointer object
 * itself.
 */
extern int *aGlobalPointer;

/**
 * A function that uses all three pointers (extern on a function 
 * declaration serves roughly the same purpose as on a variable declaration)
 */
extern void demoPointers(void);
...
#endif

/**
 * foo.c
 */
#include "foo.h"

/**
 * Defining declaration for aGlobalPointer.  Since the declaration 
 * appears at file scope (outside of any function) it will have static 
 * extent (memory for it will be allocated at program start and released 
 * at program end), and the name will be exported to the linker.
 */
int *aGlobalPointer;

/**
 * Defining declaration for aLocalPointer.  Like aGlobalPointer, it has 
 * static extent, but the presence of the "static" keyword prevents 
 * the name from being exported to the linker.  
 */
static int *aLocalPointer;

void demoPointers(void)
{
  /**
   * The static keyword indicates that aReallyLocalPointer has static extent, 
   * so the memory for it will not be released when the function exits,
   * even though it is not accessible outside of this function definition
   * (at least not by name)
   */
  static int *aReallyLocalPointer;
}