Instead of using the <string.h> which will make the output way bigger you could just use this piece of code:

char *temp=a;while(*temp  =*b  );

• char *temp=a; - create a temporary pointer to the initial string
• (*temp =*b ); - make the first element of temp the first element of b and shift them both by one
• while - take the returned value (which is the current element of b) and if it's not '\0' continue

#include <stdio.h>
int main() {
    char a[6], *b = "test\n\0";
    char *temp=a;while(*temp  =*b  );
    printf("%s", a);
    return 0;
}

test