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For library functions that take structure arguments, you need to pass structures that have field names that are the same as those in the structure definitions in the library. To determine the names and also the data types of structure fields, you can do one of the following:

You can also determine the field names of an externally defined structure from within MATLAB using the following procedure. When you create and initialize the structure, you don't necessarily have to match the data types of numeric fields. MATLAB converts to the correct numeric type for you when you make the call using the calllib function. To get the structure field names:

  1. Use libfunctionsview to display the signatures for all functions in the library you are using. libfunctionsview shows the names of the structures used by each function. For example, when you type
  1. MATLAB opens a new window displaying function signatures for the shrlibsample library. The line showing the addStructFields function reads:

  1. If the function you are interested in takes a structure argument, get the structure type from the libfunctionsview display, and invoke the libstruct function on that type. libstruct returns an object that is modeled on the structure as defined in the library:
  2. Get the names of the structure fields from the object returned by libstruct:
  3. Initialize the fields to the values you want to pass to the library function and make the call using calllib:

Specifying Structure Field Names

Here are a few general guidelines that apply to structures passed to external library functions:

Passing a MATLAB Structure

As with other data types, when an external function takes a structure argument (such as a C structure), you can pass a MATLAB structure to the function in its place. Structure field names must match field names defined in the library, but data types for numeric fields do not have to match. MATLAB converts each numeric field of the MATLAB structure to the correct data type.

Example of Passing a MATLAB Structure.   The sample shared library, shrlibsample, defines the following C structure and function:

The following code passes a MATLAB structure, sm, with three double fields to addStructFields. MATLAB converts the fields to the double, short, and long data types defined in the C structure, c_struct.

Passing a libstruct Object

When you pass a structure to an external function, MATLAB makes sure that the structure being passed matches the library's definition for that structure type. It must contain all the necessary fields defined for that type and each field must be of the expected data type. For any fields that are missing in the structure being passed, MATLAB creates an empty field of that name and initializes its value to zero. For any fields that have a data type that doesn't match the structure definition, MATLAB converts the field to the expected type.

When working with small structures, it is efficient enough to have MATLAB do this work for you. You can pass the original MATLAB structure with calllib and let MATLAB handle the conversions automatically. However, when working with repeated calls that pass one or more large structures, it may be to your advantage to convert the structure manually before making any calls to external functions. In this way, you save processing time by converting the structure data only once at the start rather than at each function call. You can also save memory if the fields of the converted structure take up less space than the original MATLAB structure.

Using the libstruct Function.   You can convert a MATLAB structure to a C-like structure derived from a specific type definition in the library in one step. Call the libstruct function, passing in the name of the structure type from the library, and the original structure from MATLAB. The syntax for libstruct is

The value s returned by this function is called a libstruct object. Although it is truly a MATLAB object, it handles much like a MATLAB structure. The fields of this new "structure" are derived from the external structure type specified by structtype in the syntax above.

For example, to convert a MATLAB structure, sm, to a libstruct object, sc, that is derived from the c_struct structure type, use

The original structure, sm, has fields that are all of type double. The object, sc, returned from the libstruct call has fields that match the c_struct structure type. These fields are double, short, and long.

Creating an Empty libstruct Object.   You can also create an empty libstruct object by calling libstruct with only the structtype argument. This constructs an object with all the required fields and with each field initialized to zero.

libstruct Requirements for Structures.   when converting a MATLAB structure to a libstruct object, the structure to be converted must adhere to the same guidelines that were documented for MATLAB structures passed directly to external functions. See Specifying Structure Field Names.

Using the Structure as an Object

The value returned by libstruct is not a true MATLAB structure. It is actually an instance of a class called lib.c_struct, as seen by examining the output of whos:

The fields of this structure are implemented as properties of the lib.c_struct class. You can read and modify any of these fields using the MATLAB object-oriented functions, set and get:

You can also read and modify the fields by simply treating them like any other MATLAB structure fields:

Example of Passing a libstruct Object

Repeat the addStructFields example, this time converting the structure to one of type c_struct before calling the function:

Now call the function, passing the structure sc:

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