How to read and write to global variables of shared library(using loadlibrary)?

A C dll contains functions fn1(),fn2() etc which take some input arguments and give some outputs.

The processing and output of the fn1() depends on global varaibles (exported in dll header).

When the dll is loaded using

>> loadlibrary('dllname','dllname.h') 
>> m = libfunctions('dllname', '-full')

Output is:

>> m = 

    'fn1'
    'fn2'
    'lib.pointer globalVar1'
    'lib.pointer globalVar2'
    .
    .

Question is how to modify value of this globalVar1 in matlab?

Ex:

prev_value = globalVar1;
globalVar1 = newValue;
output = calllib('dllname','fn1',arg1);

Similarly, how to modify if the global variables are structures?

Thanks in Advance.

Answer 1

[Mohammad Sami]

You need to get the pointer first. Try the following to see if it works.

Also you did not specify what data type the pointer is pointing to in the question.

globalpointer1 = calllib('dllname','globalVar1');
prev_value = globalpointer1.Value;
globalpointer1.Value = int8(2); % assuming a data type and value.
output = calllib('dllname','fn1',arg1);

Structures should be returned as libstruct objects. See the libstruct documentations for details.

[Nilendra Gadre]

Thanks for your quick reply.

The global variable that is accessed is of int32 type.

Based on your suggestion i have tried:

>> globalpointer1 = calllib('dllname','globalVar1');
>> prev_value = globalpointer1.Value;

The assignment gives me the following error:

"The datatype and size of the value must be defined before the value
can be retrieved."

Contents of "globalpointer1" after calllib:

>> globalpointer1
 globalpointer1 =
 libpointer

Then i tried creating a libpointer with datatype specified:

>> globalpointer2 = libpointer('int');
>> globalpointer2 = calllib('dllname','globalVar1');
>> prev_value2 = globalpointer2.Value;

Still getting the same error:

"The datatype and size of the value must be defined before the value
can be retrieved."

[Mohammad Sami]

try this way.

globalpointer1 = calllib('dllname','globalVar1');
setdatatype(globalpointer1,'int32Ptr',1,1);
prev_value = globalpointer1.Value;
globalpointer1.Value = int32(2); % assuming a data type and value.
% arg1 ....
output = calllib('dllname','fn1',arg1);

[Nilendra Gadre]

Thank You so much.

This worked.