C language improvement (3)

实现字符串拷贝函数：myStrcpy

void myStrcpy(char* dst, const char* src) {
	int len = strlen(src);
	for (int i = 0; i < len; ++i) {
		dst[i] = src[i];
	}
	dst[len] = '\0';
}

void test() {
	const char* src = "hello world!";
	char buffer[1024] = { 0 };
	myStrcpy(buffer, src);
	printf("%s\n", buffer);
}

1、It is best to use it when initializingbuffer置零,即：const char* src = "hello world!";

2、Remember to add one at the end after copying the strings one by one'\0'

方法2：

//Final judgment if*dst的值是0的话,程序就退出了

void myStrcpy03(char* dst, const char* src) {
	while (*dst++ = *src++);
}

while (*dst++ = *src++);
00C51D65  mov         eax,dword ptr [dst]  
00C51D68  mov         ecx,dword ptr [src]  
00C51D6B  mov         dl,byte ptr [ecx]  
00C51D6D  mov         byte ptr [eax],dl  
00C51D6F  mov         eax,dword ptr [dst]  
00C51D72  mov         cl,byte ptr [eax]  
00C51D74  mov         byte ptr [ebp-0C1h],cl  
00C51D7A  mov         edx,dword ptr [dst]  
00C51D7D  add         edx,1  
00C51D80  mov         dword ptr [dst],edx  
00C51D83  mov         eax,dword ptr [src]  
00C51D86  add         eax,1  
00C51D89  mov         dword ptr [src],eax  
00C51D8C  movsx       ecx,byte ptr [ebp-0C1h]  
00C51D93  test        ecx,ecx  
00C51D95  je          std::_Narrow_char_traits<char,int>::eof+19h (0C51D99h)  
00C51D97  jmp         __$EncStackInitStart+19h (0C51D65h)  

TestThe command logically ANDs two operands,并根据运算结果设置相关的标志位.但是,TestThe two operands of the command不会被改变.The result of the operation will be discarded after the relevant flag bit is set.

TEST AX,BX 与AND AX,BXcommand has the same effect,只是TestInstructions do not changeAX和BX的内容,而ANDThe command will save the result to AX中.

1、*dst=*src

2、dst++,src++

3、判断++之前的*dst是否为0,如果为0就跳出循环,Jump to if not zero1去执行

前置++和后置++的区别：

y=x++;
00DE1BF1  mov         eax,dword ptr [x]  
00DE1BF7  mov         dword ptr [y],eax  
00DE1BFD  mov         ecx,dword ptr [x]  
00DE1C03  add         ecx,1  
00DE1C06  mov         dword ptr [x],ecx  
		y = ++x;
00DE1C0C  mov         eax,dword ptr [x]  
00DE1C12  add         eax,1  
00DE1C15  mov         dword ptr [x],eax  
00DE1C1B  mov         ecx,dword ptr [x]  
00DE1C21  mov         dword ptr [y],ecx  

但是对于i++/++iThis addition does not affect the result

for循环的执行流程：

for (int i = 0; i < 5; ++i) {
00DE1BCD  mov         dword ptr [ebp-438h],0  
00DE1BD7  jmp         allocSpace+8h (0DE1BE8h)  
00DE1BD9  mov         eax,dword ptr [ebp-438h]  
00DE1BDF  add         eax,1  
00DE1BE2  mov         dword ptr [ebp-438h],eax  
00DE1BE8  cmp         dword ptr [ebp-438h],5  
00DE1BEF  jge         __vfprintf_l+9h (0DE1C29h)  

 字符串反转：

写入访问权限冲突：If it is a lower address it is possible to use a null pointer,If it is a different address,It is possible that wild pointers are used or access rights are not available(For example, the constant area) 

注意：形如char*str=(char*)helloSuch string constants are stored in the constant area,Him and the worldconst一样,Once initialized, it cannot be modified.(It also cannot be modified through pointers)

实现字符串反转——指针型

void myStrReverse(char* src) {
    if(NULL==src){
        return;
    }
	int len = strlen(src);
	char tmp = 0;
	char* pHead = src;
	char* pTail = src + (len - 1);
	while (pHead < pTail) {
		tmp = *pHead;
		*pHead= *pTail;
		*pTail = tmp;
		--pTail;
		++pHead;
	}
}

实现字符串反转——subscript

void myStrReverse2(char* src) {
     if(NULL==src){
        return;
    }
	int len = strlen(src);
	char tmp = 0;
	int start = 0;
	int end = len - 1;
	while (start < end) {
		tmp = src[start];
		src[start] = src[end];
		src[end] = tmp;
		++start;
		--end;
	}
}

When a function uses a formal parameter pointer, you must first judge！！！

格式化字符串——sprintf

 The buffer we often refer to is a blockbuffer,也就是一块内存

printfIt is to put the formatted string into the display,而sprintfis to put the formatted string inbuffer里面了

sprintf的三大作用：

1、格式化字符串

2、拼接字符串

3、将数字转化成字符串

void test() {

	//1.格式化字符串
	char buffer[1024] = { 0 };
	sprintf(buffer, "hello %s", "world");
	printf("%s\n", buffer);

	//2.拼接字符串
	const char* str1 = "hello";
	const char* str2 = "Obama";
	memset(buffer, 0, 1024);
	sprintf(buffer, "%s %s", str1, str2);
	printf("%s\n", buffer);

	//3.Convert numbers to string format
	int num = 666;
	memset(buffer, 0, 1024);
	sprintf(buffer, "老铁双击%d\n", num);
	printf("%s\n", buffer);

	
}

void test() {

	char** p = (char**)malloc(sizeof(char*) * 5);
	if (NULL == p) {
		return;
	}
	memset(p, 0, sizeof(char*) * 5);
	for (int i = 0; i < 5; ++i) {
		p[i] = (char*)malloc(64);
		memset(p[i], 0, 64);
		sprintf(p[i], "string%d", i + 1);
	}
	for (int i = 0; i < 5; ++i) {
		printf("%s\n", p[i]);
	}
	for (int i = 0; i < 5; ++i) {
		if (p[i] != NULL) {
			free(p[i]);
			p[i] = NULL;
		}
	}
	if (p != NULL) {
		free(p);
		p = NULL;
	}
}

Do some judgment before freeing heap memory！！！

calloc和realloc：（仅供了解）

 

 骚操作：按住alt键,Drag the mouse to select down,可以打印多行

Find a string：

const char* myStrStr(const char* str, const char* subStr) {
	const char* myStr = str;
	const char* mySub = subStr;
	while (*myStr!='\0')
	{
		if (*myStr != *subStr) {
			++myStr;
			continue;
		}
		const char* tmp = myStr;
		while (*mySub != '\0') {
			if (*myStr != *mySub) {
				mySub = subStr;
				break;
			}
			++mySub;
			++myStr;
		}
		if (*mySub == '\0') {
			return tmp;
		}
		++myStr;
	}
	return NULL;
}


void test() {

	const char* str = "abcdefg";
	const char* subStr = "gh";
	const char*sub=myStrStr(str, subStr);
	printf("%s\n", sub);
}

 Pointers are prone to errors：

Move the value of a pointer to heap space,使得无法freeremove this memory

void test() {

	char* p = (char*)malloc(100);
	p++;
	free(p);
	p = NULL;
}

 

 发现调用freefunction crashes,This is the case because we moved the pointermalloc地址的指针！

When you operate a certain piece of memory, make sure that it is legal(自己申请的,Also not released)

So it must not return the address of a local variable！！！！！！

 所以freeBe sure to set the pointer after it is droppedNULL