一篇文章带你实现C语言中常用库函数的模拟

目录

• 前言
• 函数介绍

• strlen（求字符串长度）
• strcpy（字符串拷贝）
• strcat（字符串追加）
• strcmp（字符串比较）
• strstr（找子字符串）
• memcpy（内存拷贝）
• memmove（内存移动）

#include<stdio.h>
size_t my_strlen(char* arr) {
	int count = 0;
	while (*arr) {
		arr++;
		count++;
	}
	return count;
}
int main() {
	char arr[] = "abcdef";
	size_t ret=my_strlen(arr);
	printf("%u\n", ret);
	return 0;
}

#include<stdio.h>
#include<assert.h>
char * my_strcpy(char* arr,const char* arr1) {
	assert(arr && arr1);
		char* ret = arr;
	while (*arr++ = *arr1++) {
		;
	}
 return ret;
}
int main() {
	char arr[] = "xxxxxxxxxxxxxxx";
	char arr1[] = "abcd";
	my_strcpy(arr, arr1);
	printf("%s\n", arr);
	return 0;
}

#include<stdio.h>
#include<assert.h>
char* my_strcat(char* dest, const char* src) {
	assert(dest && src);     //断言-保护dest和src指针都不为空
	char* ret = dest;         //保存目标字符串的首地址
	//找到目标字符串的末尾\0
	while (*dest) {
		dest++;
	}
	//把源字符串追加到目标字符串直到\0为止
	while (*dest++ = *src++) {
		;
	}
	return ret;
}
int main() {
	char arr[20] = "abc";
	char arr1[] = { 'd','e','f','\0' };
	printf("%s\n", my_strcat(arr, arr1));//把arr1数组中的内容追加到arr数组中
	return 0;
}

#include<stdio.h>
#include<assert.h>
int my_strcmp(const char* a, const char* b) {
    assert(a && b);
	while (*a == *b) {
		if (*a == '\0')
			return 0;
		a++;
		b++;
	}
	return *a - *b;
}
int main() {
	char a[] = "abc";
	char b[] = "abcq";
	int ret=my_strcmp(a, b);
	if (ret > 0)
		printf(">\n");
	else if (ret == 0)
		printf("=\n");
	else
		printf("<\n");
	return 0;
}

#include<stdio.h>
#include<assert.h>
char* my_strstr(const char* dest,const char* src) {
	assert(dest && src);
	char* s1=dest;
	char* s2=src;
	if (*src == '\0')
		return dest;
	while (*s1) {
		dest = s1;
		src = s2;
		while (*dest!='\0'&&*src!='\0'&&*dest == *src) {
			dest++;
			src++;
		}
		if (*src == '\0')
			return s1;
		s1++;
	}
	return NULL;
}
int main() {
	char arr[] = "I am a students";
	char arr1[] = "am";
	char *ret=my_strstr(arr, arr1);
	if (ret == NULL)
		printf("找不到");
	else
		printf("%s\n", ret);
	return 0;
}

#include<stdio.h>
#include<assert.h>
void* my_memcpy(void* dest, const void* src, size_t count) {
	assert(dest && src);
	void * ret = dest;
	while (count--) {
		*(char*)dest = *(char*)src;
		dest=(char *)dest+1;
		src=(char *)src+1;
	}
	return ret;
}
int main() {
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
	int arr1[20] = { 0 };
	my_memcpy(arr1, arr, 10 * sizeof(int));
	for (int i = 0; i < 20; i++) {
		printf("%d ", arr1[i]);
	}
	printf("\n");
	return 0;
}

#include<stdio.h>
#include<assert.h>
void* my_memmove(void* dest, const void* src, size_t count) {
	assert(dest && src);
	void* ret = dest;
	//从前往后
	if (dest < src) {
		while (count--) {
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	//从后往前
	else {
		while (count--) {
			*((char*)dest + count) = *((char*)src + count);
		}
	}
	return ret;
}
int main() {
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };
	my_memmove(arr + 2, arr, 16);
	for (int i = 0; i < 10; i++) {
		printf("%d ", arr[i]);
	}
	printf("\n");
	return 0;
}