在Matlab的mex函数中使用预分配的数组 [英] Working with preallocated arrays in Matlab's mex function

问题描述

我编写了一个简单的mex函数，该函数会更新Matlab数组已分配的内容:

I wrote a simple mex function which updates already allocated by Matlab array:

mex_test_array.c

mex_test_array.c

#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    double *x = mxGetPr(prhs[0]);
    x[0] = 3.1416;
}

测试1:

>> y = zeros(2, 2);
>> mex_test_array(y);
>> y

y =

    3.1416         0
         0         0

测试2:

>> y = zeros(2, 2);
>> mex_test_array(y(:, 1));
>> y

y =

     0     0
     0     0

为什么在子矩阵上不起作用(测试2)?有可能使其工作吗?

Why it doesn't work on sub-matrix (Test 2) ? Is it possible to make it work?

请告知.

备注: 我了解，更新输入数组并不是应如何编写mex文件，而且我确实知道如何从mex返回数组. 我尝试此技术的原因是避免两次分配数组的内存.

Remark: I understand, that updating input arrays is not how mex files are expected to be writted, and I do know how to return arrays from mex. The reason I tried this technique is to avoid allocation of the arrays' memory twice.

推荐答案

由于 MATLAB不想更改右侧参数"，因此不应该起作用.参见例如在线帮助:

prhs [是]输入数据的指针数组.输入数据是只读的， 不应通过您的mexFunction进行更改.

prhs [is an] Array of pointers to input data. The input data is read-only and should not be altered by your mexFunction .

这就是为什么您的函数标头

That is why your function header

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])

表示const mxArray *prhs[]，而不是mxArray *prhs[].如果要返回一个值，则应该 通过mxArray *plhs[]进行操作.我认为更改右侧参数"时的行为只是不确定的，建议阅读完整的

says const mxArray *prhs[] and not mxArray *prhs[]. If you want to return a value, you are supposed to do that through mxArray *plhs[]. I think the behaviour for when you change a "right-hand-side parameter" is just undefined and recommend reading the full MEX files guide for further details.

更新

要回答您的实际问题，我假设当您将y交给函数时，MATLAB会为您提供实际的指针，并且(错误地)相信您不会弄乱它；当您移交函数y(:,1)时，MATLAB会复制数组的该部分，并为您提供指向该副本的指针，该指针在函数调用后将被丢弃.

To answer your actual question, I assume that when you hand y to your function, MATLAB hands you the actual pointer and (falsely) trusts you not to mess with it; when you hand your function y(:,1) MATLAB makes a copy of that part of the array and hands you a pointer to that copy, which is discarded after your function call.

如果您坚持这样做，请至少阅读 reve_etrange 的注释中指出，nofollow noreferrer>在未公开记录的Matlab上进行Matlab mex就地编辑一个>！本质上，您必须运行

If you do insist on doing this, at least read Matlab mex in-place editing on Undocumented Matlab, as pointed out in the comments by reve_etrange! Essentially, you must run

mxUnshareArray(const_cast<mxarray *>(prhs[0]), true); 

在修改该数组之前.否则运行

before you modify that array. Otherwise running

>> y = zeros(2, 2);
>> y_backup=y;
>> mex_test_array(y);

将导致

>> y    
y =    
    3.1416         0
             0         0
>> y_backup
y =    
    3.1416         0
             0         0

...这导致了一些无法维护的代码！

... which makes for some hell-of-a-unmaintainable code!

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