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

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?

CodePudding user response：

Very much in contrast to the best practice you mention, I follow what I consider a rule:

"Include what you need. Do not include what you do not need."

I am very much with M. Oehm:

If a header file requires definitions from other header files, it should include that header. It sure isn't any worse practice than trying to work around it by using void *.

The concept of forward declaring structs allows to use pointer-to-that before including or typedefining the struct in detail. (As mentioned in the comment be some programmer dude.)

CodePudding user response：

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 *.

CodePudding user response：

If TaskHandle_t is a pointer to struct then you can forward declare the struct and add an alias for a pointer to it.

typedef struct TaskHandle_s *TaskHandle_t;
void my_function(TaskHandle_t _taskHandle);