使用mxGetPr和mxGetData [英] Using mxGetPr vs mxGetData

问题描述

我正在尝试编写一个简单的mex函数.我有一个整数输入，它是我的对象数. 当我编译myMEX_1.cpp并由MATLAB用任何输入值调用它时，我总是得到:

I am trying to write a simple mex function. I have an integer input which is the number of my objects. When I compile myMEX_1.cpp and call it by MATLAB with any input value, I always get:

请求的对象数:= 0

Number of Requested Objects := 0

但是myMEX_2.cpp可以正常工作，并显示从MATLAB命令窗口输入的数字. 我在myMEX_1.cpp中的错误在哪里?

But the myMEX_2.cpp works fine and show the number inputted from MATLAB command window. Where is the my mistake in myMEX_1.cpp?

我的环境:MATLAB R2013a和Microsoft SDK 7.1编译器.

// myMEX_1.cpp
#include "mex.h" 
void mexFunction(int nlhs,       mxArray *plhs[], 
                 int nrhs, const mxArray *prhs[]) 
{

    char str11[100];
    unsigned short frameCount;
    //unsigned short *frameCountPtr;
    frameCount = (*((unsigned short*)mxGetData(prhs[0])));
    sprintf(str11, "Number of Requested Objects := %d:\n", frameCount);
    mexPrintf(str11);
}





// myMEX_2.cpp
#include "mex.h" 
void mexFunction(int nlhs,       mxArray *plhs[], 
                 int nrhs, const mxArray *prhs[]) 
{
   char str11[100];
   unsigned short frameCount;
   double* dblPointer; 
   dblPointer = mxGetPr(prhs[0]);
   frameCount = (unsigned short)(*dblPointer);
   sprintf(str11, "Number of Requested Objects := %d:\n", frameCount);
   mexPrintf(str11);
}

推荐答案

mxGetData 返回一个void指针，该指针必须强制转换为 correct 数据类型的指针.

mxGetData returns a void pointer which must be cast to a pointer of the correct datatype.

在C中，mxGetData返回void指针(void *).由于void指针指向没有类型的值，因此将返回值转换为与pm

In C, mxGetData returns a void pointer (void *). Since void pointers point to a value that has no type, cast the return value to the pointer type that matches the type specified by pm

在您的情况下，我假设虽然看起来像您传入了整数，但实际上它是double，因为这是MATLAB的默认数据类型，因此您遇到的问题是由于您尝试将其转换指向unsigned short指针.

In your case, I'm assuming that although it looks like you've passed in an integer, it's actually a double since that is MATLAB's default datatype so your issue is due to the fact that you try to convert it to an unsigned short pointer.

myMEX_1(1)          % Passes a double
myMEX_1(uint16(1))  % Passes an integer

要解决此问题，我们需要将mxGetData的输出强制转换为double指针，然后取消引用，强制转换并分配

To fix this, we would need to cast the output of mxGetData as a double pointer instead, and then dereference it, cast it and assign it

frameCount = (unsigned short)*(double*)mxGetData(prhs[0]);

mxGetPr 与mxGetData 除外，它会自动将mxGetData的输出强制转换为double指针.因此，它可以节省您一步，但仅适用于double输入(您拥有).

mxGetPr is the same as mxGetData except that it automatically casts the output of mxGetData as a double pointer. Thus, it saves you a step but is only appropriate for double inputs (which you have).

如果要适当地处理多种类型的输入，则需要使用 mxIsClass .

If you want to handle inputs of multiple types appropriately, you'll need to check the type of the input using either mxIsDouble or mxIsClass.

if ( mxIsDouble(prhs[0]) ) {
    frameCount = (unsigned short)*mxGetPr(prhs[0]);
} else if ( mxIsClass(prhs[0], "uint16") {
    frameCount = *(unsigned short*)mxGetData(prhs[0]);
} else {
    mexPrintf("Unknown datatype provided!");
    return;
}

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