关于一些在c与c++中的特殊关键字的详解与用法

悠悠 2022-11-27 00:58 179阅读 0赞

#### 关于一些在c与c++中的特殊关键字的详解与用法，这些都是个人目前见过并记得的，不能说这个很全面，请见谅 ####

###### 1、sizeof：计算某个类型或者一些变量所占的字节数 ######

用法：

printf("%d\n",sizeof(int));
    	//计算int类型所占的字节数,结果为：4
    	
    	int num[] = { 1,2,3};	
    	printf("%d\n",sizeof(num));
    	//计算num数组所占的字节数，结果为：4*3 = 12

###### 2、LINE:常用于调试代码，找出错的位置 ######

用法：

printf("%d\n",__LINE__);
    			//打印当前行数

###### 3、func：指当前所在的函数,用于调试 ######

用法：

printf("%s\n",__func__);
    			//打印当前所在函数

###### 4、BUFSIZ:系统缓冲区的大小（注：系统缓冲区与文件缓冲区不是同一个） ######

原名：Buffer size
    
    	This macro constant expands to an integral expression with the size of the 
    	buffer used by the setbuf function.

用法：

printf("%d\n",BUFSIZ);
    		// 64位系统下结果为：8192

###### 5、EOF:到达文件末尾或者遇到CTRL+c时将让程序结束运行，指示其他一些故障情况 ######

原名：end of file		
    
    	It is a macro definition of type int that expands into a negative integral constant expression (generally, -1).
    
    	It is used as the value returned by several functions in header <cstdio> to indicate that the End-of-File has been reached or to signal some other failure conditions.
    
    	It is also used as the value to represent an invalid character.

用法：

int num;
    			while(scanf("%d",&num)!=EOF);
    			//代表只要没有按下CTRL+c就一直循环

###### 6、FOPEN\_MAX:指使用fopen等文件io函数打开文件时，可以打开的文件最大个数，所有系统的FOPEN\_MAX应大于7。 ######

原名：Potential limit of simultaneous open streams
    	
    	This macro constant expands to an integral expression that represents the maximum number of files that can be opened simultaneously.
    
    	Particular library implementations may count files opened by tmpfile towards this limit. Implementations may also allow more files to be opened beyond this limit.
    
    	FOPEN_MAX shall be greater than 7 on all systems.

用法：

printf("%d\n",FOPEN_MAX);
    		// 64位系统下结果为：16

###### 7、L\_tmpnam：代表临时文件名字的最小长度 ######

原名：Minimum length for temporary file name
    
    	This macro constant expands to an integral expression corresponding to 
    	
    	the size needed for an array of char elements to hold the longest file name 
    
    	string possibly generated by tmpnam.

用法：

printf("%d\n",L_tmpnam);
    		// 64位系统下结果为：20

###### 8、TMP\_MAX:临时文件数，不能小于25。 ######

原名：Number of temporary files
    	
    	This macro expands to the minimum number of unique temporary file 
    	
    	names that are guaranteed to be possible to generate using tmpnam.
    
    	This value cannot be lower than 25.
    
    	Particular library implementations may count file names used by files created with tmpfile towards this limit.

用法：

printf("%d\n",TMP_MAX);
    		// 64位系统下结果为：238328

###### 9、NULL:空指针，一般用于定义指针时先存NULL的地址,防止它指向内存一些其他地方 ######

原名：Null pointer		
    
    	This macro expands to a null pointer constant.
    
    	CC++98C++11：
    	A null-pointer constant is an integral constant expression that evaluates to 
    	zero (like 0 or 0L), or the cast of such value to type void* (like (void*)0).
    
    	A null pointer constant can be converted to any pointer type (or pointer-to-
    	member type), which acquires a null pointer value. This is a special value 
    	that indicates that the pointer is not pointing to any object.

用法：

int *p = NULL;
    		//把p指针初始化为空指针

###### 10、FILENAME\_MAX:系统所支持的文件名最大长度 ######

原名：Maximum length of file names
    
    	This macro constant expands to an integral expression corresponding to the size needed for an array of char elements
    	 
    	to hold the longest file name string allowed by the library. Or, if the library imposes no such restriction, 
    	
    	it is set to the recommended size for character arrays intended to hold a file name.

用法：

printf("%d\n",FILENAME_MAX);
    		// 64位系统下结果为：4096

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##### 这些都是关于我个人的一些了解；如果有什么地方写错或者画错了麻烦告知一下，真的非常感谢 #####