当有足够的内存时，为什么malloc()失败? [英] Why does malloc() fail when there is enough memory?

问题描述

我正在使用具有128GB内存的服务器进行一些计算.我需要 malloc()大小为56120 * 56120的2D浮点数组.示例代码如下:

I'm using a server with 128GB memory to do some computation. I need to malloc() a 2D float array of size 56120 * 56120. An example code is as follows:

int main(int argc, char const *argv[])
{
    float *ls;
    int num = 56120,i,j;
    ls = (float *)malloc((num * num)*sizeof(float));
    if(ls == NULL){
        cout << "malloc failed !!!" << endl;
        while(1);
    }
    cout << "malloc succeeded ~~~" << endl;
    return 0;
}

代码可以成功编译，但是当我运行它时，它显示为"malloc failed !!!" .根据我的计算，整个阵列仅需要大约11GB的内存.在开始代码之前，我检查了服务器，发现有110GB的可用内存.为什么会发生错误?

The code compiles successfully but when I run it, it says "malloc failed !!!". As I calculated, it only takes about 11GB of memory to hold the whole array. Before I started the code, I checked the server and there was 110GB of free memory available. Why does the error happen?

我还发现，如果将 num 减少到40000，则malloc将成功.

I also found that if I reduce num to, say 40000, then the malloc will succeed.

这是否意味着 malloc()可以分配的最大内存有限制?

Does this mean that there is a limit on the maximum memory that can be allocated by malloc()?

此外，如果我更改分配方式，则直接声明具有这种大小的2D浮点数组，如下所示:

Moreover, if I change the way of allocation, directly declaring a 2D float array of such size, as follows:

int main(int argc, char const *argv[])
{
    int num = 56120,i,j;
    float ls[3149454400];
    if(ls == NULL){
        cout << "malloc failed !!!" << endl;
        while(1);
    }
    cout << "malloc succeeded ~~~" << endl;
    for(i = num - 10 ; i < num; i ++){
        for( j = num - 10; j < num ; j++){
            ls[i*num + j] = 1;
        }
    }
    for(i = num - 11 ; i < num; i ++){
        for( j = num - 11; j < num ; j++){
            cout << ls[i*num + j] << endl;
        }
    }
    return 0;
}

然后我编译并运行它.我收到细分错误" .

then I compile and run it. I get a "Segmentation fault".

我该如何解决?

推荐答案

问题是，您的计算

(num * num) * sizeof(float)

作为32位有符号整数计算完成，并且num = 56120的结果为

is done as 32-bit signed integer calculation and the result for num=56120 is

-4582051584

然后解释为size_t的值非常大

Which is then interpreted for size_t with a very huge value

18446744069127500032

您没有太多的内存;)这就是 malloc()失败的原因.

You do not have so much memory ;) This is the reason why malloc() fails.

将 num 转换为 size_t ，然后它将按预期工作.

Cast num to size_t in the calculation of malloc, then it should work as expected.

这篇关于当有足够的内存时，为什么malloc()失败?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！