指针默认值。？ [英] pointer default value .?

问题描述

看，

  typedef struct jig 
   { 
         int *a;
         int *b;
   }temp;

现在阶段1

temp *b;
b= (temp*)malloc(sizeof(temp));

if(b->a != NULL)
    printf("a is not null\n");
else
    printf("a is null\n");
if(b->b != NULL)
    printf("b is not null\n");
else
    printf("b is null\n");

输出是：

a is null
b is null

现在阶段2

temp b;

if(b.a != NULL)
    printf("a is not null\n");
else
    printf("a is null\n");
if(b.b != NULL)
    printf("b is not null\n");
else
    printf("b is null\n");

输出是：

a is not null
b is not null

为什么发生这种情况？

why this is happening?

推荐答案

指针没有默认值。它们的价值就是无论垃圾是他们现在正在使用的内存。有时，一个特定的编译器将清零内存，但是这不是标准的，所以不要指望它。）

Pointers have no default value. The value they have is just whatever junk was in the memory they're using now. Sometimes a specific compiler will zero out memory, but that's not standard so don't count on it.)

这是的malloc空的记忆是一个巧合;它可能很容易被任何其他值。你需要和应始终手动所有指针设置为NULL。

The memory from malloc being NULL was a coincidence; it could have been any other value just as easily. You need to and should always manually set all your pointers to NULL.

另一种方法是，你还可以使用释放calloc，它做同样的事情，但malloc的所有设置位0在记忆它给你块。这不会堆栈变量帮助的，所以你还是自己设置这些为NULL。

Another alternative is you can also use calloc, which does the same thing as malloc but sets all bits to 0 in the block of memory it gives you. This doesn't help with stack variables though, so you still have to set those to NULL by yourself.

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