gcc使用malloc进行内存对齐 [英] gcc memory alignment using malloc

问题描述

我有以下结构：

 ＃定义M 3 
 
 #pragma pack（push）
 #pragma pack（1）
 struct my_btree_node {
 struct my_btree_node * pointers [M]; 
 unsigned char * keys [M  -  1]; 
 int data [M  -  1]; 
 unsigned char number_of_keys; 
}; 
 #pragma pack（pop）
  

sizeof（struct my_btree_node ）函数为这个结构返回一个49字节的值。是否使用 malloc 为该结构分配内存会返回一个64字节的数据块，因为在64位系统中指针是16字节对齐的，还是确实是49字节？

是否有一种方法可以将内存与小于2的幂比对，并且可以在应用程序内获得分配内存的真实大小？

我想减少填充字节的数量以节省内存。我的应用程序分配了数百万个结构，我不想浪费内存。

code>使用内部堆结构。它依赖于实现，但人们可能会期望内存由全部（内部）块分配。所以通常不可能通过一个 malloc 调用分配恰好49个字节。你可以在 malloc 之上建立一个你自己的子系统来做到这一点，但我没有理由为什么你可能需要它。

PS为了减少内存浪费，你可以预先分配一个由100个结构组成的数组，当你只需要一个时，就返回& a [i]，直到所有的空闲索引被浪费。由于数组永远不会被填充，所以内存浪费将会减少大约100倍。

I've the following struct:

#define M 3

#pragma pack(push)
#pragma pack(1)
struct my_btree_node {
    struct my_btree_node *pointers[M];
    unsigned char *keys[M - 1];
    int data[M - 1];
    unsigned char number_of_keys;
};
#pragma pack(pop)

The sizeof(struct my_btree_node) function returns a value of 49 byte for this struct. Does allocating memory for this struct using malloc return a 64 byte block because on 64 bit systems pointers are 16-byte-aligned or will it indeed be 49 bytes?

Is there a way to align memory with a smaller power of two than 16 and is it possible to get the true size of the allocated memory inside the application?

I want to reduce the number of padding bytes in order to save memory. My applications allocates millions of those structs and I do not want to waste memory.

解决方案

malloc uses internal heap-structure. It is implementation-dependent yet one may expect that the memory is allocated by a whole number of (internal) blocks. So usually it's not possible to allocate exactly 49 bytes by a single malloc call. You can build some subsystem of your own on top of malloc to do this, yet I see no reason why you may want it.

P.S. To reduce memory wasting, you can pre-allocate an array consisting of, say, 100 structs, when you need just one more, and return &a[i] until all free indexes are wasted. As arrays are never padded, the memory wasting would be reduced in about 100 times.

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