1.实现strcpy 2.实现strcat 3.实现strstr
4.实现strchr 5.实现strcmp 6.实现memcpy
7.实现memmove 8.实现strncpy
9.实现strncat 10.实现strncmp
1.实现strcpy
Copies the C string pointed by source into the array pointed by destination, including the terminating null character (and stopping at that point).
源字符串必须以 ‘\0’ 结束。
会将源字符串中的 ‘\0’ 拷贝到目标空间。
目标空间必须足够大，以确保能存放源字符串。
目标空间必须可变
char* myStrcpy(char* dest,const char* src)
{
char* ret = dest;
assert(dest != NULL);
assert(src != NULL);
while ((*dest++=*src++))
{
;
}
return ret;
}
2.实现strcat
Appends a copy of the source string to the destination string. The terminating null character in destination is overwritten by the first character of source, and a null-character is included at the end of the new string formed by the concatenation of both in destination.
源字符串必须以 ‘\0’ 结束。
目标空间必须有足够的大，能容纳下源字符串的内容。
目标空间必须可修改
char* myStrcat(char* dest,const char* src)
{
char* ret = dest;
assert(dest != NULL);
assert(src != NULL);
while (*dest)
{
dest++;
}
while ((*dest++=*src++))
{
;
}
return ret;
}
3.实现strstr
这个函数用于判断两个字符串是否一个是另一个的字串
如果不是返回null
char* myStrstr(const char* str1,const char* str2)
{
assert(str1);
assert(str2);
char* cp = (char*)str1;
char* substr = (char*)str2;
char* s1 = NULL;
if (str2=='\0')
{
return NULL;
}
while (*cp)
{
s1 = cp;
substr = str2;
while (*s1&&*substr&&(*s1==*substr))
{
s1++;
substr++;
}
if (substr=='\0')
return cp;
cp++;
}
}
4.实现strchr
该函数用来寻找目标字符串中是否有我们所寻找的字符
char* myStrchr(const char* dest, char c)
{
assert(dest);
while (dest)
{
if (*dest==c)
{
return dest;
}
dest++;
}
return NULL;
}
5.实现strcmp
比较从ptr1和ptr2指针开始的num个字节
标准规定： 第一个字符串大于第二个字符串，则返回大于0的数字 第一个字符串等于第二个字符串，则返回0 第一个字符串小于第二个字符串，则返回小于0的数
int myStrcmp(const char* str1,const char* str2)
{
int ret = 0;
assert(str1 != NULL);
assert(str2 != NULL);
while (!(ret = *(unsigned char *)str1 - *(unsigned char *)str2) && *str2)
{
++str1, ++str2;
}
if (ret < 0)
{
ret = -1;
}
else if (ret > 0)
{
ret = 1;
}
return(ret);
}
6.实现memcpy
函数memcpy从source的位置开始
向后复制num个字节的数据到destination的内存位置。
这个函数在遇到 ‘\0’ 的时候并不会停下来。
如果source和destination有任何的重叠，复制的结果都是未定义的
void* myMemcpy(void* dst,const void* src,size_t num)
{
assert(dst&&src);
char* strSrc = (char*)src;
char* strDst = (char*)dst;
while (num)
{
*strDst= *strSrc;
strDst++;
strSrc++;
num--;
}
return dst;
}
7.实现memmove
和memcpy的差别就是memmove函数处理的源内存块和目标内存块是可以重叠的。
如果源空间和目标空间出现重叠，就得使用memmove函数处理
`void* myMemove(void* dst,const void* src,size_t num)
{
assert(dst&&src);
char* strDst = (char*)dst;
char* strSrc = (char*)src;
//分为前重叠和后重叠
if (strDst<strSrc&&(strDst+num)>strSrc)
{
while (num)
{
*strDst = *strSrc;
num--;
strSrc++;
strDst++;
}
}
else if(strSrc<strDst&&strSrc+num>strDst)
{
while (num)
{
*(strDst + num) = *(strSrc + num);
num--;
}
}
else
{
while (num)
{
*strDst = *strSrc;
num--;
strSrc++;
strDst++;
}
}
return dst;
}
8.实现strncpy
Copies the first num characters of source to destination. If the end of the source C string (which is signaled by a null-character) is found before num characters have been copied, destination is padded with zeros until a total of num characters have been written to it.
拷贝num个字符从源字符串到目标空间。
如果源字符串的长度小于num，
则拷贝完源字符串之后，在目标的后边追加0，直到num个
char* myStrncpy(char* dest, const char* src, size_t num)
{
assert(dest&&src);
char* ret = dest;
num--;
while ((*dest++ = *src++) && (num--))
{
;
}
return ret;
}
9.实现strncat
Appends the first num characters of source to destination, plus a terminating null-character. If the length of the C string in source is less than num, only the content up to the terminating nullcharacter is copied.
char* myStrncat(char*dest, const char*src, size_t num)
{
assert(dest&&src);
char* ret = dest;
while (*++dest);
while (src&&num)
{
*dest++ = *src++;
num--;
}
return ret;
}
10.实现strncmp
比较到出现另个字符不一样或者一个字符串结束或者num个字符全部比较完
int myStrncmp(const char* dest, const char* src, size_t num)
{
assert(dest&&src);
while (dest++&&src++&&num--)
{
if (*dest > *src)
{
return 1;
}
if (*dest < *src)
{
return -1;
}
}
return 0;
}