为什么的malloc分配不同的字节数比请求？ [英] Why does malloc allocate a different number of bytes than requested?

问题描述

我有这块code的

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

int main(){
void *a, *b;

a=malloc(16);
b=malloc(16);
printf("\n   block size (for a): %p-%p : %li",b,a,b-a);

a=malloc(1024);
b=malloc(1024);
printf("\n   block size (for a): %p-%p : %li",b,a,b-a);	
}

这不应该显示最后分配的块大小？ （16或1024）。
它打印2​​4安培; 1032，因此它具有8个额外的字节...

This shouldn't display the the last allocated block size? (16 or 1024). It prints 24 & 1032, so it has 8 extra bytes...

我的问题是（使这个测试用例之前），我做一个函数（1024字节）malloc和返回分配的结果。当检查对函数返回块的大小，我得到516块......，我不明白为什么。我想这可能是这样做的分配的缓冲区一些处理后，会出现内存curruption的原因：）

My problem is (before making this testcase) that i do malloc in a function (1024 bytes), and return the allocated result. When checking the block size on the function return i get 516 blocks... and i dont get it why. I guess this might be the reason for the memory curruption that occurs after doing some processing on the allocated buffers:)

编辑：我见过<一个href=\"http://stackoverflow.com/questions/232691/array-size-from-pointer-in-c\">http://stackoverflow.com/questions/232691/array-size-from-pointer-in-c似乎要问同样的事情，对不起，重新发布。

i've seen http://stackoverflow.com/questions/232691/array-size-from-pointer-in-c and seems to ask the same thing, sorry for reposting.

我重做我的例子我更具体的code：

I've redone my example to my more specific code:

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

short int * mallocStuff(long int number, short int base){
	short int *array;
	int size=1024;

	array=(short int*)calloc(1,size);
	//array=(short int*)malloc(size);

	return array;
}

int main(){
	short int **translatedArray;

	translatedArray=malloc(4*sizeof(short int));

	int i;
	for(i=0;i<4;i++){
		translatedArray[i]=mallocStuff(0,0);

		if(i>0)
			printf("\n   block size (for a): %p-%p : %i",translatedArray[i],translatedArray[i-1],translatedArray[i]-translatedArray[i-1]);
	}

	return 0;
}

和输出是
   块大小（对于一）：0x804a420-0x804a018：516
   块大小（对于一）：0x804a828-0x804a420：516
   块大小（一）：0x804ac30-0x804a828：516

And the output is block size (for a): 0x804a420-0x804a018 : 516 block size (for a): 0x804a828-0x804a420 : 516 block size (for a): 0x804ac30-0x804a828 : 516

根据以上的职位，比1024更大......我错了？

According to the above post that be bigger than 1024...am i wrong?

推荐答案

您有一个错误。相反的：

You have a bug. Instead of:

translatedArray=malloc(4*sizeof(short int));

您应该有

translatedArray=malloc(4*sizeof(short int*));

请注意在code缺少的指针。我怀疑这是你观察到的行为源于

Note the missing pointer in your code. I suspect this is where your observed behavior stems from.

---

另请注意， 0x804a420 - 0x804a018 = 1032 ，而不是 516 。其计算公式 translatedArray [I] - translatedArray [我 - 1] 给你的元素（短整型，或者更简单，短裤）的数量这两个地址之间，而不是数量的字节

Also notice that 0x804a420 - 0x804a018 = 1032, not 516. The formula translatedArray[i] - translatedArray[i - 1] gives you the number of elements (short ints, or more simply, shorts) in between the two addresses, not the number of bytes.

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