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

char *join1(char *, char*);
void join2(char *, char *);
char *join3(char *, char*);

int main(void) {
    char a[4] = "abc"; // char *a = "abc"
    char b[4] = "def"; // char *b = "def"

    char *c = join3(a, b);
    printf("Concatenated String is %s\n", c);

    free(c);
    c = NULL;

    return 0;
}

/*方法一，不改变字符串a,b, 通过malloc，生成第三个字符串c, 返回局部指针变量*/
char *join1(char *a, char *b) {
    char *c = (char *) malloc(strlen(a) + strlen(b) + 1); //局部变量，用malloc申请内存
    if (c == NULL) exit (1);
    char *tempc = c; //把首地址存下来
    while (*a != \'\0\') {
        *c++ = *a++;
    }
    while ((*c++ = *b++) != \'\0\') {
        ;
    }
    //注意，此时指针c已经指向拼接之后的字符串的结尾\'\0\' !
    return tempc;//返回值是局部malloc申请的指针变量，需在函数调用结束后free之
}


/*方法二，直接改掉字符串a,*/
void join2(char *a, char *b) {
    //注意，如果在main函数里a,b定义的是字符串常量（如下）：
    //char *a = "abc";
    //char *b = "def";
    //那么join2是行不通的。
    //必须这样定义：
    //char a[4] = "abc";
    //char b[4] = "def";
    while (*a != \'\0\') {
        a++;
    }
    while ((*a++ = *b++) != \'\0\') {
        ;
    }
}

/*方法三，调用C库函数,*/
char* join3(char *s1, char *s2)
{
    char *result = malloc(strlen(s1)+strlen(s2)+1);//+1 for the zero-terminator
    //in real code you would check for errors in malloc here
    if (result == NULL) exit (1);

    strcpy(result, s1);
    strcat(result, s2);

    return result;
}