C String Function strcmp\strcpy\strcat\memcpy

1.strcmp

int strcmp (const char* str1, const CHAR*STR2) {
    assert (str1! = null&&str2! = NULL);
    while (*str1&&*str1 = = *str2) {
        str1++;
        str2++;
    }

    if (* (unsigned char*) str1 < * (unsigned char*) str2) {
        return-1;
    }
    else if (* (unsigned char*) str1 > * (unsigned char*) str2) {
        return 1;
    }
    else{
        return 0;
    }

}

Attention:
1. The parameter is const
2. Exception input processing assert (str1! = null&&str2! = NULL);
3. You must first convert the char* pointer to unsigned when comparing the size of the characters to the char*
Because the range of signed character values is -128~127, the range of unsigned character values is 0~255, and the ASCII of the string has no negative value.
For example *STR1, the value of 1,*STR2 is 255.
Originally *STR1<*STR2, but the signed number of 255 is-1. 2.strcat

Char *strcat (char* strdest, const CHAR*STRSRC) {
    assert (strdest! = null&&strsrc! = NULL);
    char* address = strdest;
    while (*strdest! = ') ') strdest++;

    while (*strsrc! = ') ' {
        *strdest = *strsrc;
        strdest++;
        strsrc++;
    }
    *strdest = ' + ';//
    return address;
}

1. Note the parameters and return values
2. To overwrite the original string ' + ', add ' 3.strcpy ' at the end

Char *strcpy (char* strdest, const CHAR*STRSRC) {
    assert (strdest! = null&&strsrc! = NULL);
    char* address = strdest;

    while (*strsrc! = ') ' {
        *strdest = *strsrc;
        strdest++;
        strsrc++;
    }
    *strdest = ' + ';
    return address;
}

Address overlap issues should also be considered if necessary 4.memcpy

First look at the explanation of the standard memcpy ():

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

When you implement memcpy () yourself, you need to consider the case of overlapping addresses .

#include <iostream>
#include <cstdio>
using namespace std;


void* mymemcpy (void* DST, const void* SRC, size_t n) {
    if (DST = = NULL | | src = = NULL) return null;
    char* PDST;
    char* psrc;

    if (src >= DST | | (char*) DST >= (char*) src + n-1) {
        pdst = (char*) DST;
        PSRC = (char*) src;

        while (n--) {
            *pdst++ = *psrc++;
        }
    }
    else{
        pdst = (char*) dst+ n-1;
        PSRC = (char*) src+ n-1;

        while (n--) {
            *pdst--= *psrc--;
        }
    }

    return DST;
}



int main () {

    char buf[100] = "Abcdefghijk";
    memcpy (Buf+2, buf, 5);
    mymemcpy (buf + 2, buf, 5);
    printf ("%s\n", buf + 2);

    return 0;
}

Results
Abcdehijk