[C++知识库]C语言自我实现函数atoi、itoa 、memcpy

atoi、itoa

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <assert.h>

int myatoi(const char * str)
{
	int i = 0;
	int tmp = 0;
	int flag = 1;
	
	assert(str != NULL);

	//如果是前面有空格先消去
	while (isspace(*str)){	//用函数isspace判断是否为空白字符
		str++;
	}
	if(*str == '-'){
		flag = -1;
		str++;
	}
	while(*str != '\0'){
		if(*str > 57 || *str < 48) return -1;
		str++;
		i++;
	}
	if(i != 0){
		str -= i;
	}
	while(*str != '\0'){
		tmp = tmp*10+(*str -'0');//0-48
		str++;	
		i++;
	}
	return tmp*flag;
}
void  myitoa(int n,char *str) 
{
	char *head = str;
	char ch;
	int i=0,j=0;
	while(n != 0){
		*str = n%10 +'0';
		n /= 10;
		j++;
		str++;
	}
	str--;
	while(head < str){
		ch = *head;
		*head++ = *str;
		*str-- = ch;
		i++;
	}
	//printf("head  %s\n",head);
	//printf("str  %s\n",str);
	str += i+1;
	*str = '\0';

}

int main()
{
	int i;
	char *c = "   -123456";
	char *k = NULL;
	char *m = "dfdfg";
	i = myatoi(m);
	printf("%d\n",i);
	int b = 123456789;
	char str[32];
	myitoa(b,str);
	printf("%s\n",str);

	return 0;
}

memcpy

void *mymemcpy(void *dest, const void *src, int count)
{
void *ptr = dest;
if (NULL == dest || NULL == src || count <= 0)
return NULL;
 
while (count--)
*(char *)dest++ = *(char *)src++;
 
return ptr;
}

由于拷贝是从低地址开始，按字节进行拷贝的，需要考虑内存重叠：

void *memcpy(void *dest, const void *src,size_t n)
{
if(dest==NULL||src==NULL)
    return NULL;
void *res = dest;
if(dest<=src||(char*)dest>=(char*)src+n){
        while(n--){
//从低地址开始复制
        *(char*)dest =*(char*)src;
        dest=(char*)dest+1;
        src=(char*)src+1;
        }
}
else{
//从高地址开始复制
      src = (char*)src + n -1;
      dst = (char*)dst + n -1;
      while(n--){
            *(char*)dst = *(char*)src;
            dst = (char*)dst -1;
            src = (char*)src -1;
          }
    }
return res;
}