【C语言】 实现strncpy,strncat,strncmp

//strncpy

与strcpy无异，只是拷贝的块大小不一样，strncpy只是向后拷贝n个字节的内容

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

char *my_strncpy(char *dest,const char *src,size_t n)
{

char *ret = dest;
assert(dest);
assert(src);
while(n--)   // 用字节数n控制循环的次数
{
*dest++ = *src++;
}
return dest;
}

int main()
{
char str1[] = "abcdef";
char str2[10] = {0};
my_strncpy(str2,str1,3);
printf("%s\n",str2);
return 0;
}

//strncat

与strcat无异，只是追加的块大小不一样，strncat只是向后追加n个字节的内容

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

char *my_strncat(char *dest,const char *src,size_t n)
{
char *ret = dest;
assert(dest);
assert(src);
while(*dest)
{
dest++;
}
while(n--)
{
*dest++ = *src++;
}
*dest = '\0';
return dest;
}

int main()
{
char str1[] = "abcd";
char str2[] = "efgh";
my_strncat(str1,str2,3);
printf("%s\n",str1);
return 0;
}

//strncmp

与strcmp无异，只是比较的块大小不一样，strncmp只是向后比较n个字节的内容

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

int *my_strncmp(const char *dest,const char *src,size_t n)
{
int ret = 0;
assert(dest);
assert(src);
while(n--)
{
if(!(ret = *(unsigned char *)dest - *(unsigned char *)src))
{
dest++;
src++;
}
}
if(ret < 0)
ret = -1;
else if(ret > 0)
ret = 1;

return ret;
}

int main()
{
char str1[] = "abcdef";
char str2[] = "abcd";
printf("%d\n",my_strncmp(str1,str2,3));
return 0;
}