编译预警无效**和无效* [英] Compilation warning for void ** and void *

问题描述

我有一个关于一个问题无效* 和无效** ，我知道这是有点老问题和计算器之前一直问（有点）。所以现在的问题是：

当我编译这个code用gcc 4.4.3 ubuntu下10.10，我得到以下警告：

  zz.c：在函数'主'：
zz.c：21：警告：从兼容的指针类型过客酒吧的参数1
zz.c：9：注：应为无效**，但参数的类型为浮**
 

那为什么其确定传递变量x作为的foo（）的变量，但它不是确定传递变量y，酒吧的参数（）。我可以通过两个变量明确铸造成解决这个问题无效* 和无效** 预期。

 无效美孚（无效*一）{
}空巴（无效**一）{
     * A =（浮点*）malloc的（100 * sizeof的（浮动））;
}诠释主（）{    浮动* X =（浮点*）malloc的（100 * sizeof的（浮动））;
    富（X）;
    免费（X）;    浮动* Y;
    酒吧（安培; Y）;
    免费（Y）;    返回0;
}
 

解决方案

无效* A 意味着 A 点未知类型的对象。然而， A 本身并不是一个未知的类型，因为 A 是已知有键入无效* 。只有 A 点对象有一个未知类型。

无效** A 意味着 A 指向类型的对象无效* 。该 *一个点，有一个未知的类型，但对象 *一个本身是有类型的指针<对象code>无效* 。

＆安培; Y 是一个指针类型的对象浮法* 。 ＆安培; Y 不是一个指针类型的对象无效*

。

I have a question regarding void* and void** and I know this is sort of an old question and has been asked (somewhat) before in stackoverflow. So the question is the following:

When I compile this code with gcc 4.4.3 under ubuntu 10.10, I get the following warning:

zz.c: In function ‘main’:
zz.c:21: warning: passing argument 1 of ‘bar’ from incompatible pointer type
zz.c:9: note: expected ‘void **’ but argument is of type ‘float **’

why is it that its ok to pass variable x as an argument of foo() but its not ok to pass variable y as an argument of bar(). I can fix this by explicitly casting both variables into void* and void** as expected.

void foo (void* a){
}

void bar(void **a){
     *a = (float *) malloc(100*sizeof(float));
}

int main (){

    float *x = (float*) malloc(100*sizeof(float));
    foo(x);
    free(x);

    float *y;
    bar(&y);
    free(y);

    return 0;
}

解决方案

void *a means that a points to an object of unknown type. However, a itself is not an unknown type because a is known to have type void*. Only the object that a points to has an unknown type.

void **a means that a points to an object of type void*. The object that *a points to has an unknown type, but the object *a itself is a pointer with type void*.

&y is a pointer to an object of type float*. &y is not a pointer to an object of type void*.

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