strcat用于动态char指针 [英] strcat for dynamic char pointers

问题描述

我需要用C语言编写一个strcat.我尝试了以下操作:

I need to write a strcat in C. I tried below things:

 char * c_strcat( char * str1, const char * str2)
 {

      char * ret = (char *) malloc(1 + strlen(str1)+ strlen(str2) );

     if(ret!=NULL)
     {
         strcpy(ret, str1);
         strcat(ret, str2);

     if(str1 !=NULL) free str1; // If I comment this everything will be fine 
       return ret;
     }

     return NULL;
 }     


 int main()
 {

 char * first = "Testone";
 char *second = "appendedfromhere";

 first = c_strcat(first,second);
 if(second !=NULL) free(second);

// I want to store the result in the same variable 

 }

每当我在c_strcat函数中释放str1内存时，它就会崩溃.如果我不释放空间，那么分配给第一个的内存将被泄漏(如果我理解正确的话).

It crashed whenever I free the str1 memory in the c_strcat function. If I don't free then the memory allocated for the first will be leaked (if I understood properly).

如何将返回值存储到相同的变量中(第一个)?

How to store returned value to same variable (first)?

推荐答案

来自 7.20.3.2 C99标准的免费功能:

free函数导致ptr指向的空间被释放，即 可用于进一步分配.如果ptr是空指针，则不执行任何操作. 否则，如果 该参数与calloc，malloc或 realloc函数，或者如果通过调用free或realloc释放了空间， 行为是不确定的.

The free function causes the space pointed to by ptr to be deallocated, that is, made available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if the argument does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined.

和str1(又名first)不是由三个动态内存分配功能之一分配的，从而导致崩溃. second也是如此.

And str1 (aka first) was not allocated by one of the three dynamic memory allocating functions, causing the crash. Same for second also.

如果不释放空间，则分配给first的内存将泄漏(如果理解正确).

if don't free then the memory allocated for the first will be leaked(if am understood properly).

如果使用malloc()，realloc()或calloc()，并且以后没有free() d内存，则内存将泄漏.由于字符串文字"Testone"不会动态分配，因此不能释放.

Memory will be leaked if malloc(), realloc() or calloc() is used and the memory is not free()d at a later point. As the string literal "Testone" is not dynamically allocated it must not be freed.

请注意，调用free()之前的指针NULL检查不需要为free()，如上所述，如果指针参数为NULL，则不执行任何操作.

Note that the pointer NULL check prior to invoking free() is unrequired as free(), as described above, does nothing if the pointer argument is NULL.

我是否强制转换malloc的结果?

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