模拟實作strcpy,strcat,strstr,strchr,strcmp,memcpy,memmove

一、模拟strcpy

//模拟strcpy
#include<stdio.h>
#include<stdlib.h>
#include <assert.h>
char *my_strcpy(char *dest, const char *src) {
	assert(dest != NULL);
	assert(src != NULL);
	char *ret = dest;
	while (*dest&&*src){
		*dest = *src;
		dest++;
		src++;
	}
	return ret;
}

int main() {
	char a[] = "abcd";
	char b[] = "123";
	char *ret = my_strcpy(a, b);
	puts(ret);
	system("pause");
	return 0;
}
           

二、模拟strcat

//模拟strcat
#include<stdio.h>
#include<stdlib.h>
#include <assert.h>
char *my_strcat(char *dest, const char*src) {
	assert(dest != NULL);
	assert(src != NULL);
	char *ret = dest;
	while (*dest){
		dest++;
	}
	while (*src){
		*dest = *src;
		dest++;
		src++;
	}
	*dest = '\0';
	return ret;
}

int main() {
	char a[] = "abcd";
	char b[] = "123456";
	char *ret = my_strcat(a, b);
	puts(ret);
	system("pause");
	return 0;
}
           

三、模拟strstr

//模拟strstr
#include<stdio.h>
#include<stdlib.h>
#include <assert.h>
char *my_strstr(const char *str1, const char *str2) {
	assert(str1 != NULL);
	assert(str2 != NULL);
	char *tstr1 = str1;
	char *tstr2 = str2;
	while (*tstr1){
		char *flag = tstr1;
		while (*tstr1 == *tstr2 && *tstr1){
			tstr1++;
			tstr2++;
		}
		if (!*tstr2){
			return flag;
		}
		if (!*tstr1){
			return NULL;
		}
		tstr1++;
	}
	return NULL;
}

int main() {
	char a[] = "abcd123";
	char b[] = "bc";
	char *ret = my_strstr(a, b);
	puts(ret);
	system("pause");
	return 0;
}
           

四、模拟strchr

//模拟strchr
#include<stdio.h>
#include<stdlib.h>
char *my_strchr(const char *str, char c) {
	if (*str==NULL){
		return NULL;
	}
	char *p = (char *)str;
	while (*p){
	if (*p==c){
		return p;
	}
	p++;
	}
	return NULL;
}

int main() {
	char str[] = "1234567";
	char c = '3';
	char *ret = my_strchr(str, c);
	printf("%p\n", str);
	printf("%p\n", ret);
	system("pause");
	return 0;
}
           

五、模拟strcmp

//模拟strcmp
#include<stdio.h>
#include<stdlib.h>
#include <assert.h>
char *my_strcmp(const char *dest, const char *src) {
	assert(dest != NULL);
	assert(src != NULL);
	while (*dest == *src) {
		if (*dest=='\0'){
			return 0;
		}
		dest++;
		src++;
	}
	return *dest - *src;
}

int main() {
	char str1[] = "abcdef";
	char str2[] = "abcadf";
	char ret = my_strcmp(str1, str2);
	printf("%d\n", ret);
	system("pause");
	return 0;
}
           

六、模拟memcpy

//模拟memcpy
#include<stdio.h>
#include<stdlib.h>
void *my_memcpy(void *dest, const void *src, size_t n) {
	char *tmp = (char *)dest;
	char *s_src = (char *)src;
	while (n--){
		*tmp++ = *s_src++;
	}
	return dest;
}


int main() {
	char str1[] = "123456789";
	my_memcpy(str1, str1+2, 4);  //能正确顯示
	//my_memcpy(str1 + 2, str1, 4);  //記憶體重疊
	printf("%s\n", str1);
	system("pause");
	return 0;
}
           

七、模拟memmove

//模拟memmove
#include<stdio.h>
#include<stdlib.h>
void *my_memmove(void *dest, const void *src, size_t n) {
	char *s_dest = (char *)dest;
	char *s_src = (char *)src;
 if (s_dest>s_src&&s_src+n>s_dest){
	 s_dest = s_dest + n - 1;
	 s_src = s_src + n - 1;
	 while (n--){
		 *s_dest-- = *s_src--;
	 }
 }
 else
 {
	 while (n--){
		 *s_dest++ = *s_src++;
	 }
 }
 return dest;
}

int main() {
	char str1[] = "123456789";
	//my_memmove(str1, str1 + 2, 4);  //能正确顯示
	my_memmove(str1 + 2, str1, 4);  //memcpy記憶體重疊的情況也能正确顯示
	printf("%s\n", str1);
	system("pause");
	return 0;
}