C —函数指针数组

Question

I am using C and looking to populate an array with pointers to various functions. I would like to use a function to pass the address of the function I want to put in the array. This is what I am trying to do with no success. The compiler is barking at my "task *ptr" deceleration in the prototype and registerTask function definition. I see a lot of examples of arrays with pointers to functions but none that first pass the address of the function through a function argument and then update the array. Any suggestions welcome.

typedef void (*task)(void);
task TaskArray[32];

// Prototypes
void taskA(void);
void taskB(void);
void registerTask(task *ptr);

// This is the problem function definition
void registerTask(task *ptr) {

    TaskArray[TaskIndex] = ptr;
    ++TaskIndex;

}

// Some example functions definitions
void taskA(void) { }   
void taskB(void) { }

int TaskIndex = 0;

main {

    registerTask(&taskA);       // place taskA pointer in array
    registerTask(&taskB);       // place taskB pointer in array

    ...later on...

    TaskIndex = value;          // Set the Task Array function Index
    (*TaskArray[TaskIndex]);    // call the function

}

Answer 1

task is a function pointer. Therefore, registerTask doesn't need to take a task *ptr parameter (which would be a function pointer pointer), just a task ptr .

Also, your syntax for calling the function is incorrect; you need the open and close parenthesis, just as if you were calling a regular ol' function.

Finally, you don't need the ampersand in front of the function names whose address you're taking - the function name is sufficient. And, you don't need to dereference the function pointer when calling it.

typedef void (*task)(void);
task TaskArray[32];

// Prototypes
void taskA(void);
void taskB(void);
void registerTask(task taskPtr);

void registerTask(task taskPtr) {

    TaskArray[TaskIndex] = taskPtr;
    ++TaskIndex;

}

// Some example functions definitions
void taskA(void) { }   
void taskB(void) { }

int TaskIndex = 0;

main {

    registerTask(taskA);       // place taskA pointer in array
    registerTask(taskB);       // place taskB pointer in array

    //...later on...

    TaskIndex = value;          // Set the Task Array function Index
    TaskArray[TaskIndex]();     // call the function

}

Answer 2

I have made a few changes so the code should compile and mostly work. The changes are:

1) move the scope of TaskIndex so it can be seen by registerTask().
2) remove a lot of extra indirection
3) fixed the function pointer call

typedef void (*task)(void);
task TaskArray[32];

// Prototypes
void taskA(void);
void taskB(void);

int TaskIndex = 0;

// This is the problem function definition
void registerTask(task ptr) {
    TaskArray[TaskIndex++] = ptr;
}

// Some example functions definitions
void taskA(void) { }   
void taskB(void) { }

int
main (void)
{

    registerTask(taskA);       // place taskA pointer in array
    registerTask(taskB);       // place taskB pointer in array

    ...later on...

    TaskIndex = value;          // Set the Task Array function Index
    TaskArray[TaskIndex]();    // call the function

}

Answer 3

IMHO things are a lot simpler if you don't use pointer typedefs. And if you use some sort of naming convention so that types are different to instances of that type. For example:

typedef void TASK(void);

// Prototypes
TASK taskA, taskB;

TASK *TaskArray[32];
int NumTasks = 0;

void registerTask(TASK *ptr)
{
    TaskArray[NumTasks++] = ptr;
}

// Some example functions definitions
void taskA(void) { }   
void taskB(void) { }

main {
    registerTask(&taskA);       // place taskA pointer in array
    registerTask(&taskB);       // place taskB pointer in array

// call the function
    TaskArray[0]();
}