#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;      }