c中具有常量参数的结构数组

Question

我定义了以下结构

typedef const struct _txmlAttribute
{
 const char * const ns;
} txmlAttribute;

在下面的函数中，我想初始化我的结构的动态数组：

int func(txmlAttribute* attrs){
    attrs = (txmlAttribute*) { {"as"}, {"bs"}};
    return 0;
}

int main(){
   txmlAttribute* attrs;
   func(attrs);
   return 0; 
}

但我收到以下警告：

main.c: In function ‘func’:
main.c:13:5: warning: braces around scalar initializer [enabled by default]
     attrs = (txmlAttribute*) { {"as"}, {"bs"}};
 ^
main.c:13:5: warning: (near initialization for ‘(anonymous)’) [enabled by default]
main.c:13:5: warning: initialization from incompatible pointer type [enabled by default]
main.c:13:5: warning: (near initialization for ‘(anonymous)’) [enabled by default]
main.c:13:5: warning: braces around scalar initializer [enabled by default]
main.c:13:5: warning: (near initialization for ‘(anonymous)’) [enabled by default]
main.c:13:5: warning: initialization from incompatible pointer type [enabled by default]
main.c:13:5: warning: (near initialization for ‘(anonymous)’) [enabled by default]
main.c:13:5: warning: excess elements in scalar initializer [enabled by default]
main.c:13:5: warning: (near initialization for ‘(anonymous)’) [enabled by default]

那为什么要这样做？以及如何清除它？

Answer 1

我想你想做这样的事情：

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

typedef struct
{
  char* ns;
} txmlAttribute;

int func(txmlAttribute* attrs){
  attrs[0].ns = malloc(3);
  if (attrs[0].ns == NULL)
    return -1;
  strcpy(attrs[0].ns, "as");
  attrs[1].ns = malloc(3);
  if (attrs[1].ns == NULL)
    return -1;
  strcpy(attrs[1].ns, "bs");
  return 0;
}

int main(){
  txmlAttribute attrs[2];
  func(attrs);
  //do something

  free(attrs[0].ns);
  free(attrs[1].ns);
  return 0; 
}

Answer 2

您的问题是{{"as"}, {"bs"}} 将在堆栈上分配，并且在您离开函数范围后将变为无效*。

我会通过编写一个“构造函数”函数来实现，该函数将字段作为参数，为结构及其成员分配（malloc）所需的空间，初始化它们并返回指向该结构的指针。然后填充数组的代码将类似于：

txmlAttribute attrs[2];
attrs[0] = make_txml_attr("as");
attrs[1] = make_txml_attr("bs");

txmlAttribute * make_txml_attr(const char * val) {
  txmlAttribute * res = malloc(sizeof(txmlAttribute));   
  res->ns = strdup(val);
  return res;
}


void free_txml_attr(const char ** p_attr) {
   free(*p_attr->ns);
   free(*p_attr);
   *p_attr = NULL;
}

（我省略了错误检查）

--

无效 - 因为访问它是未定义的行为

Answer 3

更改为attrs = (txmlAttribute[]) { {"as"}, {"bs"}}; 有效。
但(txmlAttribute[]) { {"as"}, {"bs"}}; 是本地范围值。
当您更改指针参数时，Original 也不会更改。
我的建议如下。

int func(txmlAttribute **attrs){
    static txmlAttribute att[] =  { {"as"}, {"bs"}};

    *attrs = att;
    return 0;
}