Simulation implementation of string function (2)

• memcpy

Function: Starting from the starting position of the memory address pointed to by src, copy count bytes of data to the starting position of the memory address pointed to by the target dest.

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

void* my_memcpy(void* dest,const void* src,size_t count){
    assert(dest);
    assert(src);
    void* ret = dest;
    while(count--){
        *(char*)dest = *(char*)src;
        dest = (char *) dest + 1;
        src = (char*)src+1;
    }
    return ret;
}

int main(){
    char buf[] = "abcdefghigk";
    my_memcpy(buf+2,buf,5);
    printf("%s\n",buf+2);
    return 0;
}

• memmove

Function: Copy count bytes from the memory area pointed to by src to the memory area pointed to by dest.

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

void* my_memmove(void* dest,const void* src,size_t count){
    assert(dest);
    assert(src);
    void* ret = dest;
    if(dest <= src || (char*)dest >= ((char*)src+count)){
        while(count--){
            *(char*)dest = *(char*)src;
            dest = (char *) dest + 1;
            src = (char*)src+1;
        }
    }
    else{
        dest = (char *) dest + count-1;
        src = (char*)src+count-1;
        while(count--){
            *(char*)dest = *(char*)src;
            dest = (char *) dest-1;
            src = (char*)src-1;
        }
    }
    return ret;
}

int main(){
    char s[] = "hello world";
    my_memmove(s,s+3,strlen(s)+1-3);
    printf("%s\n",s);
    return 0;
}

• memset

Function: The memory area of ​​count bytes starting from dest is filled with the integer c.

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

void* my_memset(void* dest,int c,size_t count){
    assert(dest);
    if(count < 0)
        return NULL;
    char * tmp = (char *) dest;
    while(count--)
        *tmp++ = c;
    return dest;
}

int main(){
    char buf[10];
    int i = 0;
    printf("sizeof(buf) = %d\n",sizeof(buf));
    for(i = 0;i < 10;i++)
        printf("buf[%d] = %d\n",i,buf[i]);
    printf("-----------------------------------\n");
    my_memset(buf,1,sizeof(buf));
    for(i = 0;i < 10;i++)
        printf("buf[%d] = %d\n",i,buf[i]);
    return 0;
}

• memcmp

Function: Compare the first num bytes of ptr1 and ptr2 in the memory area.

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

int my_memcmp(const void* ptr1,const void* ptr2,size_t num){
    assert(ptr1);
    assert(ptr2);
    while(num--){
        while(*(char*)ptr1 == *(char*)ptr2){
            if(*(char*)ptr1 == '\0')
                return 0;   
            ptr1++;
            ptr2++;
        }
    }
    if(*(char*)ptr1 > *(char*)ptr2)
        return 1;
    else if(*(char*)ptr1 < *(char*)ptr2)
        return -1;
}

int main(){
    char* p = "adcc";
    char* q = "axcc";
    printf("%d\n",my_memcmp(p,q,3));
    return 0;
}

• memchr

Function: Find the character c from the first count bytes of the memory area pointed to by buf

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

void* memchr(const void* buf,int c,size_t count){
    assert(buf);
    while(count--){
        if(*(char*)buf == (char)c)
            return (void*)buf;
        buf = (char*)buf+1;
    }
    return NULL;
}

int main(){
    char* pch;
    char str[] = "hello world";
    pch = (char*)memchr(str,'w',strlen(str));
    if(pch != NULL)
        printf("fpund at position %d\n",pch-str+1);
    else
        printf("not found\n");
    return 0;
}