magical void * Type pointer (universal pointer | generic pointer)

Why is it called a universal pointer (generic pointer)?

Because the void pointer can hold any type of pointer, in Libev, Watcher->data is the void * type that holds the data for the asynchronous operation.

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Look at the following example:

I first assign any type of pointer to the void pointer, when used, the first force into the original pointer type, and then the solution, you can get the data we originally saved.

#include <stdio.h>

int main () {
	void *p = NULL;

	int a = ten;
	p = &a;
	printf ("A:%d\n", * (int *) p);

	float B = 11.22;
	p = &b;
	printf ("B:%f\n", * (float *) p);

	char *c = "abc";
	p = C;
	printf ("C:%s\n", p);

	return 0;
}