How to resolve the error 'declaration is incompatible with "<variable declaration>"' when generating code with Embedded Coder R2025b?
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I am generating code from my Simulink model using Embedded Coder in R2025b. I have included a manual variable declaration in the generated code using custom code settings. When the generated code is compiled, I encounter the following error:
error: declaration is incompatible with "<variable declaration>"
How can I resolve this?
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MathWorks Support Team
2026-4-6，0:00
The error occurs because a user-written variable declaration conflicts with one automatically generated by Embedded Coder R2025b.
Note: User-written code can be included in generated code by using a "Model Header" block or by specifying custom header files, source files, or libraries to be included, as described in the documentation on Specifying Custom Code Settings for Code Generation.
For example, suppose the following declaration for a variable in your Simulink model is manually added to the generated code:
extern unsigned int myVariable;
However, Embedded Coder already generates its own declaration for this variable in the "model.h" file:
extern uint16_T myVariable;
The type "uint16_T" is a target-specific, 16-bit integer type defined by Embedded Coder in the "rtwtypes.h" file. For example,
typedef unsigned short uint16_T;
Because the size of built‑in C/C++ types (such as "unsigned int") can vary across target hardware, the manually declared type may not match the generated type. This type mismatch results in a conflicting declaration and causes the error when generating code.
For this reason, it is not recommended to manually add type declarations for variables referenced in a Simulink model. If you need to introduce user‑defined types or override the types generated by Embedded Coder, refer to this MATLAB Answers post on Including User‑Defined Types in Generated Code.
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