string.c

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

typedef struct {
    int len;
    char *s;
} string_t;

typedef string_t string;

void set(string* s1, char *s);
void concat(string* s1, string* s2);
void stringFree(string* s1);

int main(void) {
    string str1 = {0};
    string str2 = {0};

    set(&str1,"hello ");
    set(&str2,"world!");
    concat(&str1,&str2);

    printf("%p , %s\n",str1.s,str1.s);

    stringFree(&str1);
    stringFree(&str2);

}

void set(string* s1, char *s){
    free(s1->s);
    s1->s = strdup(s);
    s1->len = strlen(s);
}

void concat (string* s1, string* s2){
    int totalLen = s1->len + s2->len;
    char rslt[totalLen+1];
    int i=0;
    for(i=0;i<s1->len;i++){
        rslt[i] = s1->s[i];
    }
    for(i=s1->len;i<totalLen;i++){
        int j=i-s1->len;
        rslt[i] = s2->s[j];
    }
    rslt[totalLen] = '\0';
    set(s1,rslt);
}

void stringFree(string* s1) {
    free(s1->s);
}