C++ pointers and variables

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Carlos 2017-4-3

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https://ww2.mathworks.cn/matlabcentral/answers/333337-c-pointers-and-variables

评论： Guillaume 2017-4-4

采纳的回答： Jan

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Dear experts,

I am trying to convert the following C++ code to a matlab code:

double buffer = 72.0;
double *pBuffer = &buffer;
t = sizeof(buffer));

Does anyone know to solve this question?

Thank you in advance.

Carlos

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Answer 1

Jan 2017-4-3

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buffer = 72;

You do neither need a pointer in Matlab nor does the the size of the type matter.

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Carlos 2017-4-3

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Hello Jan! Thank you for your quick reply. Certainly, my problem is a bit complex. I have to convert C code y matlab code because I want to include this code in a GUI. On the one hand, I have the C functions within a file .h. Concretely, I would like to convert this C code lines in matlab commands:

double buffer = 72.0;
double *pBuffer = &buffer;
SetSystemParameter(MAXIMUM_RANGE,pBuffer,sizeof(buffer));

where SetSystemParameter is a function implemented in the library h and MAXIMUM_RANGE is a constant. The function SetSystemParameter sets the value provided in buffer but an error is generated if the buffer is incorrectly sized.

On the other hand, I use the following commands in matlab in order to obtain the same results as in C:

libstring = 'ATC3DG64'; %library h
buffer = double(72);
pRecord31 = libpointer('doublePtr',buffer);
calllib(libstring,'SetSystemParameter',MAXIMUM_RANGE,pRecord31,sizeof(buffer));

Sizeof is a .m file downloaded from matlab central. The author says this function does the same as the sizeof function of C code. The problem is my program alway returns an error related to the buffer size and I don't know why since it perfectly works in C.

Can you help me?

Thank you Jan!

Carlos 2017-4-4

Thank you Jan. I made it.

Guillaume 2017-4-4

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If you're using this filexchange sizeof you're using it incorrectly. The correct code should have been:

calllib(libstring,'SetSystemParameter',MAXIMUM_RANGE,pRecord31,sizeof('double'));

Or you could just not have bothered and pass 8 directly, since scalar double are always 8 bytes.

Note that matlab may have been clever enough to automatically make the conversion from double to void pointer, so possibly this may have been enough:

value = 72;
calllib(libstring,'SetSystemParameter',MAXIMUM_RANGE, value, 8)

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C++ pointers and variables

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