First output of calllib is always double.

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Zachary Smoot
Zachary Smoot 2024-4-24
回答： Paras Gupta 2024-4-30
If I have a function in a C library declared in a C header like so:
long long some_function();
And call the function like so:
loadlibrary('the_library', 'the_header.h');
out = calllib('the_library', 'some_function');
The output "out" is of type double, instead of int64.
When generating a prototype file for the library, the value of "LHS" is detected as "int64", but this does not seem to effect what the actuall output type is when calling the function.

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采纳的回答

Paras Gupta
Paras Gupta 2024-4-30
Hey Zachary,
In my understanding, you want the output type of the 'calllib()' function to be 'int64' instead of 'double' for a C function with return type "long long".
The type for all scalar values returned by MATLAB is 'double' by default. Please refer to the note given in the following documentation:
https://www.mathworks.com/help/matlab/matlab_external/passing-arguments-to-shared-library-functions.html
To convert the output to 'int64', you can try the following two workarounds:
  1. Using the 'int64()' function on the output - https://www.mathworks.com/help/matlab/ref/int64.html
  2. Modifying the function definition to return an array pointer with type "long long" instead of a scalar output. The 'out' variable in MATLAB will then be a 'libpointer' object - https://www.mathworks.com/help/matlab/ref/libpointer.html
Hope this helps.

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