c++ - strncat _emulations_ multiple calls ignore changed n?

Question

为了自学，这里有我的 strncat 的两个版本(一个带有指针+偏移符号和一个数组版本):

// 08_38.cpp                                                                    
#include <iostream>
#include <cstring>

char * strncatPtr(char * a, char * b, size_t n);
char * strncatArr(char * a, char * b, size_t n);

int main (void) {

   char string1[20] = "foobarqwerty";
   char string2[20] = "asd";

   // strncat                                                                   
   std::cout << "-----------------------" << std::endl;
   std::cout << "--------STRNCAT--------" << std::endl;
   std::cout << "-----------------------" << std::endl;
   std::cout << strncat(string2, string1, 6) << std::endl;
   std::cout << strcpy(string2, "asd") << std::endl;

   std::cout << strncatPtr(string2, string1, 4) << std::endl;
   std::cout << strcpy(string2, "asd") << std::endl;

   std::cout << strncatArr(string2, string1, 3) << std::endl;
   std::cout << strcpy(string2, "asd") << std::endl;

   return 0;
}

// ------------------------------------                                         
char * strncatPtr(char * a, char * b, size_t n){
   unsigned int i = 0;
   // go to the end;                                                            
   for(; *(a+i) != '\0'; i++);
   // and start copying                                                         
   for(unsigned int j = 0;
       ((*(a+i+j) = *(b+j)) != '\0') && (j < n-1);
       j++);
   return a;
}

char * strncatArr(char * a, char * b, size_t n){
   unsigned int i = 0;
   // go to the end;                                                            
   for(; a[i] != '\0'; i++);
   // and start copying                                                         
   for(unsigned int j = 0;
       ((a[i+j] = b[j]) != '\0') && (j < n-1);
       j++);
   return a;
}

我不明白为什么当我测试它们时它认为每个函数调用的 size = 6

-----------------------
--------STRNCAT--------
-----------------------
asdfoobar
asd
asdfoobar
asd
asdfoobar
asd

但如果我分别测试它们，每次评论 2 个不同的调用，它们工作正常......你能告诉我吗？

Answer 1

如果复制的字符数小于要连接的字符串的长度，则您没有添加空终止符来指示字符串的结尾。