库函数的模拟实现

模拟实现strlen

原型

size_t strlen ( const char * str );

size_t: unsigned int 无符号整型

模拟实现

//计数器方式
size_t my_strlen1(const char* str)
{size_t count = 0;while (*str){count++;str++;}return count;
}//不创建临时变量的计数器方式
size_t my_strlen2(const char* str)
{if (*str == '\0')return 0;elsereturn (size_t)(1 + my_strlen2(str + 1));
}//利用指针-指针方式获取长度
size_t my_strlen3(const char* str)
{const char *p = str;while (*p){p++;}return (size_t)(p - str);
}int main()
{//strlen(const char * str)-->my_strlen(const char *str)const char* str1 = "Hello world!";printf("模拟实现strlen函数实例#\n");size_t len1 = my_strlen1(str1);printf("The size of this string is %d,from my_strlen1\n",len1);size_t len2 = my_strlen2(str1);printf("The size of this string is %d,from my_strlen2\n", len2);size_t len3 = my_strlen3(str1);printf("The size of this string is %d,from my_strlen3\n", len3);printf("\n");system("pause");return 0;
}

模拟实现strcpy

原型

char * strcpy ( char * destination, const char * source );

返回值类型为char*，操作过程是将source所指向的的内容拷贝到destination指向的存储单元

模拟实现

//将src的内容拷贝至dest
char * my_strcpy(char *dest, const char *src)
{assert(src);assert(dest);char * begin = dest;//只有一个等号，赋值语句，要拷贝就要将所有内容都拷贝进去，包括'\0'while (*dest++ = *src++);return begin;
}int main()
{//strcpy(char *dest,const char* src)-->my_strcpy(char *dest,const char* src)printf("模拟实现strcpy函数实例#\n");const char *str2 = "Enjoyment!";char dest[] = " test Enjoyments ";printf("before；%s\n",dest);char * ret1 = my_strcpy(dest, str2);printf("after:%s\n",ret1);printf("\n");system("pause");return 0;}

模拟实现strcat

原型

char * strcat ( char * destination, const char * source );

模拟实现

char * my_strcat(char *dest, const char *src)
{assert(dest);assert(src);char *ret = dest;while (*dest){dest++;}while (*dest++ = *src++);return ret;
}
int main()
{
//char * strcat ( char * destination, const char * source )-->my_strcat(char*dest,const char *src);printf("模拟实现strcat函数实例#\n");char dest2[] = "Hello bit!";const char *str3 = "Enjoyment";char *ret2 = my_strcat(dest2, str3);printf("%s\n",ret2);printf("\n");system("pause");return 0;}

来看运行结果;

模拟实现strstr

原型

const char * strstr ( const char * str1, const char * str2 );

功能：在str1中找到第一次与str2所指内容匹配的位置，然后从这个位置向后输出str1所指向的内容
例如：
char *str1 = “abcdeabcda1234”;
char *str2 = “abcda”
调用strstr()函数之后，返回的就是“abcda1234”

模拟实现

char * my_strstr(const char *stra,const char *strb)
{assert(stra);assert(strb);char *p = (char*)stra;char *move_p = p;char * sp = (char*)strb;if (*strb == '\0')return NULL;while (*p){move_p = p;sp = (char*)strb;while (*sp && *sp == *move_p){sp++;move_p++;}if (*sp == '\0')return p;p++;}
}int main()
{//char * my_strstr(const char *str1, const char *str2)printf("模拟实现strstr函数实例#\n");const char *str4 = "hellobitHelloBit1234";const char *str5 = "Hello";char *ret3 = my_strstr(str4, str5);printf("%s\n",ret3);printf("\n");system("pause");return 0;}

模拟实现strcmp

原型

int strcmp ( const char * str1, const char * str2 );

此函数用于比较两个字符串是否相等，若相等返回0，若str1>str2,返回1，若str1<str2，返回值为-1

模拟实现

int my_strcmp(const char * str1, const char * str2)
{int ret = 0;assert(str1);assert(str2);while (!(ret = (unsigned int)*str1 - (unsigned int)*str2) && *str1){str1++;str2++;}if (ret < 0)return - 1;else if (ret>0)return 1;else;return ret;}int main()
{// int my_strcmp(const char * str1, const char * str2)printf("模拟实现strcmp函数实例#\n");const char *str6 = "my_strcmp实现";const char *str7 = "my_strcmp实现";int ret4 = my_strcmp(str6, str7);if (ret4 == 0)printf("str6=str7\n");else if (ret4 == -1)printf("str6<str7\n");elseprintf("str6>str7\n");printf("\n");system("pause");return 0;}

模拟实现memcpy

原型

void * memcpy ( void * destination, const void * source, size_t num );

将source所指的内容拷贝到destination所指处，拷贝的大小为num个字节。注意，此函数是按照内存的最小访存单位字节来进行操作的。

模拟实现

void * my_memcpy(void * dest, const void * src, size_t num)
{assert(dest);assert(src);char *_dest = (char *)dest;char*_src = (char *)src;if (_dest > _src&&_dest < _src + num){_dest = _dest + num - 1;_src = _src + num - 1;while (num--){*_dest = *_src;_dest--;_src--;}}else{while (num--){*_dest = *_src;_dest++;_src++;}}return dest;
}int main()
{//	void * memcpy(void * dest, const void * src, size_t num)printf("模拟实现memcmp函数实例#\n");char str8[64] = "hello bit!";const char * str9 = "my_memcpy 函数执行了！";my_memcpy(str8, str9, strlen(str9));printf("%s\n",str8);system("pause");return 0;}

模拟实现memmove

原型

void * memmove ( void * destination, const void * source, size_t num );

memmove()和memcpy()目前在功能上没有差异。他们两的实现代码一样，在这里就不再展示模拟实现。具体可以参考memcpy的实现代码

以上就是基本的库函数模拟，以后会不定期更新~~
如果对你有所帮助，三连支持走一波儿~