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cfunctionheader-filesextern

How to declare a function with an unknown parameter datatype in a header file?

I am pretty new to C and have the following issue with a framework specific datatype that is used within an extern declared function for a parameter inside a header file.

//FILE: example.h

extern void my_function(void *);

//FILE: example.c

#include "example.h"
#include "freertos/task.h"

void my_function(TaskHandle_t _taskHandle){
 //function body
}

The datatype TaskHandle_t is of course not known in the header as I would have to include the freertos header here as well. I heard including other header files inside a header file is bad practise.

If I try to compile the example above the following error raises:

error: conflicting types for 'my_function'
 void my_function(TaskHandle_t _taskHandle)

Whats the best approach to solve this problem?

Solution

  • Declaring an otherwise unknown datatype is about the only good reason why you should include a header in a header. However there is also the so-called 'forward declaration', which you can use when you need to pass a pointer to a struct. (Or class in C++)

    A forward declaration simply looks like this:

    //.h file
struct YourType;
void yourFunction(struct YourType *variable);

//.c file
#include "HeaderContainingYourType.h"

void yourFunction(struct YourType *variable)
{
}

    As for your specific problem, your prototype and your declaration must have the exact same signature, so if you declare the prototype with void *, you must use void * in the declaration too.

    Ways to solve your problem:

    • Include the header file you need.
    • Pull out the typedef from the header file and redeclare it verbatim in your new header file.
    • Pass void * and cast it to the appropriate type in the C file.

    Please note, that the third approach negates the aspect of 'typesafety', since you can pass anything to a void *.