Function pointer cast

[Bartc]

I have a variable like this:

int *p;

Sometimes this points to an int location containing the address of a
void(void) function.

How can I cast it so that I can call that function?

I've tried casting to void **(void). The compiler keeps saying such a cast
is illegal.

(It should compile to something like this in x86 code:
mov esi,[p]
call [esi]
)

Thanks,

-- Bartc

[Dann Corbit]

Re: Function pointer cast

"Bartc" <bc@freeuk.comw rote in message
news:nhONj.1011 3$yD2.6254@text .news.virginmed ia.com...

>I have a variable like this:
>
int *p;
>
Sometimes this points to an int location containing the address of a
void(void) function.
>
How can I cast it so that I can call that function?
>
I've tried casting to void **(void). The compiler keeps saying such a cast
is illegal.
>
(It should compile to something like this in x86 code:
mov esi,[p]
call [esi]
)

According to the C99 standard, it may be possible as a common extension:
"J.5.7 Function pointer casts

1 A pointer to an object or to void may be cast to a pointer to a function,
allowing data to

be invoked as a function (6.5.4).

2 A pointer to a function may be cast to a pointer to an object or to void,
allowing a function to be inspected or modified (for example, by a debugger)
(6.5.4)."

Show the actual code you are trying to use.




** Posted from http://www.teranews.com **

[Eric Sosman]

Re: Function pointer cast

Bartc wrote:

I have a variable like this:
>
int *p;
>
Sometimes this points to an int location containing the address of a
void(void) function.
>
How can I cast it so that I can call that function?
>
I've tried casting to void **(void). The compiler keeps saying such a cast
is illegal.
>
(It should compile to something like this in x86 code:
mov esi,[p]
call [esi]
)

The call would look like

((void(*)(void) )*p)();

.... which might be more readable with a typedef (I dislike
typedefs that name object pointer types, but for function
pointers they are often helpful):

typedef void (*VoidOfVoid)(v oid);
((VoidOfVoid)*p )();

Be aware that treating an int value as a function pointer
is not something the Standard says must work. It is quite
likely to fail spectacularly in a 64-bit setting ...

--
Eric.Sosman@sun .com

[Bartc]

Re: Function pointer cast


"Eric Sosman" <Eric.Sosman@su n.comwrote in message
news:1208464501 .259090@news1nw k...

Bartc wrote:

>I have a variable like this:
>>
>int *p;
>>
>Sometimes this points to an int location containing the address of a
>void(void) function.
>>
>How can I cast it so that I can call that function?
>>
>I've tried casting to void **(void). The compiler keeps saying such a
>cast is illegal.

The call would look like
>
((void(*)(void) )*p)();
>
... which might be more readable with a typedef (I dislike
typedefs that name object pointer types, but for function
pointers they are often helpful):

That works perfectly, thanks!

I don't care what it looks like, it's only used in one place; in context it
looks like this (part of byte-code opcode dispatcher):

stopped=0;
do {
((void(*)(void) )*pcptr)();
} while (!stopped);

-- Bartc