常见字符串操作函数

转载自http://blog.youkuaiyun.com/h971379154/article/details/48846755

//常见字符串操作函数内部实现
#include <stdio.h>


size_t strlen(const char *s)//char型指针间距可表示长度
{
    const char *sc;
    for (sc = s; *sc != '\0'; ++sc);
    return sc - s;
}

char *strcpy(char *dest, const char *src)//将src字符串拷贝到dest字符串
{
    char *tmp = dest;
    while ((*dest++ = *src++) != '\0');
    return tmp;
}

char *memset(char *s, int c, size_t count)//将字符串的前count个字符设置成字符c
{
    char *xs = s;
    while (count--)
        *xs++ = c;
    return s;
}

void *memcpy(char *dest, const char *src, size_t count)
{
	char *tmp = dest;
	const char *s = src;
	while (count--)
		*tmp++ = *s++;
	return dest;
}

char * strcat(char * dest, const char * src)  //连接字符串
{  
    char *tmp = dest;   
    while (*dest)  
        dest++;  
    while ((*dest++ = *src++) != '\0');  
    return tmp;  
}  

/*
若str1==str2，则返回零；
若str1>str2，则返回正数；
若str1<str2，则返回负数。
*/
int strcmp(const char *str1,const char *str2)
{
    /*不可用while(*str1++==*str2++)来比较，当不相等时仍会执行一次++，
    return返回的比较值实际上是下一个字符。应将++放到循环体中进行。*/
    while(*str1 == *str2)
    {
        if(*str1 == '\0')
            return 0;        
        str1++;
        str2++;
    }
    return *str1 - *str2;
}
  
int main(void)
{
	printf("%d\n",strlen("abchello"));
	char *src = "blain";
	char dest[10];
    char *s="blain";
	char pdest[10] ="goon";
	printf("%s\n",strcat(pdest,s));

	char *s1 = "abc";
	char *s2 = "ab";
	printf("%d\n",strcmp(s1,s2));

	//printf("%s\n",strcpy(dest,src));
	//printf("%s\n",memset(dest,'a',3));
	printf("%s\n",memcpy(dest,src,6));
	return 0;
}