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As Objective-C is a superset of C, it allows developers to mix pure c source files with Object-C source files.

But to call a C function from Objective-C, you should not import the .c file in your Objective-C .m file, that won’t work.

To call a C function from the Objective-C, you need to create a .h header file to include the C function declaration, and, of cause, include the .h file in the .c source file where the C function resides. Then import the .h file in the .m source file where you would like to call the C function, and simply call the function.