如何使用__malloc_hook？ [英] How to use __malloc_hook?
问题描述
在 GNU C库参考手册中，有一个例子程序（第65页），但我不知道什么是三句话： __ malloc_hook = old_malloc_hook; old_malloc_hook = __malloc_hook; __ malloc_hook = my_malloc_hook;
意思。 Especailly第二个，谁可以给我解释一下吗？谢谢。
In GNU C Library Reference Manual, there is an example program(p.65), But I don't know what the three sentences:
__malloc_hook = old_malloc_hook;
old_malloc_hook = __malloc_hook;
__malloc_hook = my_malloc_hook;
mean. Especailly the second one, who can explain for me? thanks.
static void *
my_malloc_hook (size_t size, const void *caller)
{
void *result;
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
__free_hook = old_free_hook;
/* Call recursively */
result = malloc (size);
/* Save underlying hooks */
old_malloc_hook = __malloc_hook;
old_free_hook = __free_hook;
/* printf might call malloc, so protect it too. */
printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
__free_hook = my_free_hook;
return result;
}
我写了一个小程序来测试一下：
I write a little program to test it:
#include <stdio.h>
#include <malloc.h>
/* Prototypes for our hooks. */
static void my_init_hook(void);
static void *my_malloc_hook(size_t, const void *);
/* Variables to save original hooks. */
static void *(*old_malloc_hook) (size_t, const void *);
/* Override initializing hook from the C library. */
void (*__malloc_initialize_hook) (void) = my_init_hook;
static void my_init_hook(void)
{
old_malloc_hook = __malloc_hook;
__malloc_hook = my_malloc_hook;
}
static void *my_malloc_hook(size_t size, const void *caller)
{
void *result;
/* Restore all old hooks */
__malloc_hook = old_malloc_hook;
printf("1: __malloc_hook = %x old_malloc_hook = %x\n", __malloc_hook, old_malloc_hook);
/* Call recursively */
result = malloc(size);
printf("2: __malloc_hook = %x old_malloc_hook = %x\n", __malloc_hook, old_malloc_hook);
/* Save underlying hooks */
old_malloc_hook = __malloc_hook;
printf("3: __malloc_hook = %x old_malloc_hook = %x\n", __malloc_hook, old_malloc_hook);
/* printf() might call malloc(), so protect it too. */
printf("malloc(%u) called from %p returns %p\n",
(unsigned int)size, caller, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
printf("4: __malloc_hook = %x old_malloc_hook = %x\n", __malloc_hook, old_malloc_hook);
return result;
}
int main(void)
{
char *p;
p = malloc(10);
free(p);
return 0;
}
该方案的结果是：
the result of the program is :
1: __malloc_hook = 0 old_malloc_hook = 0
2: __malloc_hook = 0 old_malloc_hook = 0
3: __malloc_hook = 0 old_malloc_hook = 0
malloc(10) called from 0xb7797f38 returns 0x932c008
4: __malloc_hook = 804849d old_malloc_hook = 0
但现在我有更多的问题，为什么 old_malloc_hook 都是0，在1,2,3，为什么 __ malloc_hook 0？我真的很困惑。帮帮我吧。
but now I have more problems, why old_malloc_hook are all 0, in 1,2,3, why __malloc_hook are 0? I am really confused. Help me.
推荐答案
据我可以告诉，一切工作完全按照预期和输出是好的。
As far as I can tell, everything is working exactly as expected and the output is fine.
变量， __ malloc_hook 0（或空），可能是因为系统默认是的不的有一个malloc挂钩。
The variable, __malloc_hook, is 0 (or null) probably because the system's default is to not have a malloc hook.
当大卫·施瓦茨如上所述，保存原始 __ malloc_hook 很重要，这样就可以只原始的malloc之前恢复（）被调用。这只是下面的评论 / *行恢复所有旧的钩子* / 。我猜，在这种特殊情况下，这是不必要的，因为原来的malloc挂钩空，但为了安全起见，应该做的。
As David Schwartz mentioned above, saving the original __malloc_hook is important so that it can be restored just before the original malloc() is called. That's the line just below the comment /* Restore all old hooks */. I'm guessing that in this specific case, it's unnecessary, since the original malloc hook is null, but to be safe it should be done.
请放心，这code上，就像你想要它。现在，我只想让这个炖了一会儿，也许是一个电灯泡熄灭，有一天，你会完全理解它。 （抱歉，这是我今天能够做的最好的。）
Please rest assured that this code is running just like you want it to. For now, I would simply let this stew for a while and perhaps a light-bulb will go off and one day, you'll understand it completely. (Sorry, but that's the best I can do today.)
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