几个 str 库函数源代码

#include<stdio.h>

#include<assert.h>

char *my_strcat(char *dst,const char *src)

{

char *cp = dst;

assert(dst);

assert(src);

while(*cp)

{

cp++;

}

while(*cp++ = *src++)

{

;

}

return dst;

}

char strncat(char *front,const char *back,int  count)

{

char *start = front;

assert(front);

assert(back);

while(*front)

{

front++;

}

while(count--)

{

if(!(*front++ = *back++))//判断是否到back的末尾！

{                        //当遇到0就返回的就在条件前面加"!"

return start;

}

}

*front = '\0';

return start;

}

int strcmp(const char *src,const char *dst)

{

int ret = 0;

assert(src);

assert(dst);

while(!(ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)

{

src++;

dst++;

}

return ret;

}

int strncmp(const char *first,const char *last,int count)//compare first n characters of two strings

{

assert(first);

assert(last);

if(!count)

{

return 0;

}

while(--count && *first && *first == *last)

{

first++;

last++;

}

return (*(unsigned char *)first - *(unsigned char *)last);

}

int strlen(const char *str)

{

const char *eos = str;

assert(str);

while(*eos++)

{

;

}

return ((int)(eos - str - 1));//指针减指针为它们之间的元素个数

}

char * strncpy(char *dst,const char *src,int count)

{

char *start = dst;

assert(dst);

assert(src);

while(count-- && (*dst = *src))

{

;

}

if(count)

{

while(--count)

{

*dest++ = '\0';

}

}

return start;

}

char *strcpy(char *dst,const char *src)

{

char *start = dst;

assert(dst);

assert(src);

while(*dst++ = *src++)

{

;

}

return start;

}