问C中的字符串连接问题

EN

    #include<stdio.h>
    #include<math.h>
    #include<stdlib.h>

    int my_strlen(char *str)
    {
        int i;

        for (i = 0; str[i] != '\0'; i++);
        return (i);
    }

    int main()
    {
        int x,y;
        char s1[30],s2[30];
        printf("Enter string 1:");
        scanf("%s",s1);
        printf("Enter string 2:");
        scanf("%s",s2);
        x=my_strlen(s1),y=my_strlen(s2);
        char cs1[x+y+1];
        char cs2[x+y+1];
        for (int i = 0; s1[i] != '\0'; i++)
        {
            cs1[i]=s1[i];
        }
        for (int i = 0; s2[i] != '\0'; i++)
        {
            cs1[i + x] = s2[i];
        }
        for (int i = 0; s2[i] != '\0'; i++)
        {
            cs2[i] = s2[i];
        }
        for(int i = 0; s1[i] != '\0'; i++)
        {
            cs2[i + y] = s1[i];
        }
        cs1[x+y]='\0';
        cs2[x+y]='\0';
        printf("string 1+string 2 is:%s",cs1);
        printf("\nstring 2+ string 1 :%s",cs2);
        return (0);
    }

 x=sizeof(s1),y=sizeof(s2);

#include <string.h>

//...

size_t x,y;
char s1[30],s2[30];
printf("Enter string 1:");
scanf("%s",s1);
printf("Enter string 2:");
scanf("%s",s2);
x = strlen(s1), y = strlen(s2);
//...

for (int i =x; i < (x+y); i++)
{
    cs1[i]=s2[i];
}

//...

for(int i=y;i<(x+y);i++)
{
    cs2[i]=s1[i];
}

for ( size_t i = 0; i < y; i++)
{
    cs1[i + x] = s2[i];
}

//...

for( size_t i = 0; i < x; i++)
{
    cs2[i + y ] = s1[i];
}

    for (size_t i = 0; i < x; i++)
    {
        cs1[i]=s1[i];
    }
    for (int i = x; i < (x+y); i++)
    {
        cs1[i]=s2[i - x];
    }
    for (int i = 0; i < (y); i++)
    {
        cs2[i]=s2[i];
    }
    for(int i=y;i<(x+y);i++)
    {
        cs2[i]=s1[i - y];
    }