I did a function f which takes as input a (void*), convert it to a (int*) and print the value.
#include <stdlib.h>
#include <stdio.h>
void f(void* p)
{
int *pi = (int*)p;
printf("%d\n", *pi);
}
int main()
{
int x = 1;
int *px = &x;
void *pv = (void*)px;
f(pv);
return 0;
}
Is it possible to implement a function:
void f2(void** pp);
such that it performs the "same" operations of the function f? My goal is to learn how to convert a (int*) to a (void**) and viceversa.
EDIT: error and warning of @tadman code (I did a mistake)
fvv.c: In function ‘f2’:
fvv.c:10:12: warning: initialization of ‘int *’ from incompatible pointer type ‘int **’ [-Wincompatible-pointer-types]
10 | int *pi = (int**)p;
| ^
fvv.c:12:17: error: invalid type argument of unary ‘*’ (have ‘int’)
12 | printf("%d\n", **pi);
|
EDIT2
fvv.c: In function ‘main’:
fvv.c:19:5: warning: passing argument 1 of ‘f2’ from incompatible pointer type [-Wincompatible-pointer-types]
19 | f2(&px);
| ^~~
| |
| int **
fvv.c:7:16: note: expected ‘void **’ but argument is of type ‘int **’
7 | void f2(void** p)
You can take any level of indirection you want, up to the blatantly, utterly absurd (void*******
), but I'm not sure why this would be useful:
void f2(void** p)
{
// Note you must maintain the same level of indirection
int **pi = (int**)p;
// Since this is a ** pointer, it requires ** to fully de-reference
printf("%d\n", **pi);
}
To call this you need a pointer to a pointer:
int x = 1;
int *px = &x;
f2((void**) &px);
In C terms a pointer to a pointer is often interpreted to mean one of the two following things:
Neither of those apply here.
That being said, in C there's not a lot of rules as to what you can and can't convert. If you want to do it, C isn't going to get in your way, even if the resulting code makes no sense or will crash immediately when executed.
You can convert int*
to void**
and back again, C won't care, but you should have a really good reason for doing such a thing. Normally arbitrary pointers are almost always specified as void*
, and as this can be recast into whatever you want, it's sufficient.
For example, you can specify void*
as an argument when that pointer is actually int**
, something you'll see quite often, as in thread_create
taking a void* arg
argument. That's not limited to mere pointers, you can cast to your heart's content.