如何在C中使用函数指针？

Question

I am writing pthread program which contain functions as given below and I want to call another function in between one function.

void *fun1(void *arg1)
{
fun2(NULL);
}
void *fun2(void *arg)
{
fun1(NULL);
}

When I am calling another function as shown above I am getting error as shown below

error: conflicting types for 'fun2'

note: previous implicit declaration of 'fun2' was here

How can I call fun2 in between fun1

Answer 1

Declare :

void *fun2(void *);

before fun1

The compiler assumes the default return type as int so you need to tell it the actual prototype before use

Answer 2

During compilation you have not given a prototype for fun2 like:

void * fun2(void *);

Since there is no forward declaration hence the compiler implicitly assumes it will return an int . (I mean this is how my compiler behaves) So it will have conflicting types.

The best practice is:

void * fun1(void *);
void * fun2(void *);

void * fun1(void *arg1)
{
    fun2(NULL);
    //return missing
}
void * fun2(void *arg)
{
    fun1(NULL);
    //return missing
}

Answer 3

/* forward declaration */
void *fun2(void *arg);

void *fun1(void *arg1)
{
  fun2(NULL);
}
void *fun2(void *arg)
{
  fun1(NULL);
}

Note that, as written, you have an infinite recursion. These two functions will just call each other until the program runs out of stack and crashes.

Answer 4

You must declare your function before calling it.Keep habit of declaring functions in header file or top of your .c file . You should

void *fun2(void *args);

Answer 5

Lets go wild and assume that you have you have used forward declarations.

fun1 (there is a misnomer) calls fun2 then that calls fun1 that calls fun2 ad infinitum - until the blessed machine runs of of stack (patience?) and gives up.