Jump to address syntax

[Guillaume Dargaud]

Hello all,
I have an example of working code under my eyes that goes as follow:

unsigned long address=0x40000 0;
(void (*)(void)addres s)();

It's supposed to jump start a kernel loaded at that address from a small
bootloader.

But my cross compiler chokes on the second line (89) and I must say I've
tried variations of the syntax without success:

TestGpio/TestGpio.c: In function 'main':
TestGpio/TestGpio.c:89: error: expected ')' before numeric constant
TestGpio/TestGpio.c:89: error: expected expression before ')' token

It would make more sense to me as:
((void *)address)();
TestGpio/TestGpio.c: In function 'main':
TestGpio/TestGpio.c:90: error: called object '4194304u' is not a function

Maybe this one ?:
((void *)(void)address )();
TestGpio/TestGpio.c: In function 'main':
TestGpio/TestGpio.c:90: error: void value not ignored as it ought to be

--
Guillaume Dargaud

http://www.gdargaud.net/

[Robert Gamble]

Re: Jump to address syntax

On Jul 16, 6:44 am, "Guillaume Dargaud"
<use_the_form_o n_my_contact_p. ..@www.gdargaud .netwrote:

Hello all,
I have an example of working code under my eyes that goes as follow:
>
unsigned long address=0x40000 0;
(void (*)(void)addres s)();
>
It's supposed to jump start a kernel loaded at that address from a small
bootloader.
>
But my cross compiler chokes on the second line (89) and I must say I've
tried variations of the syntax without success:
>
TestGpio/TestGpio.c: In function 'main':
TestGpio/TestGpio.c:89: error: expected ')' before numeric constant
TestGpio/TestGpio.c:89: error: expected expression before ')' token
>
It would make more sense to me as:
((void *)address)();
TestGpio/TestGpio.c: In function 'main':
TestGpio/TestGpio.c:90: error: called object '4194304u' is not a function
>
Maybe this one ?:
((void *)(void)address )();
TestGpio/TestGpio.c: In function 'main':
TestGpio/TestGpio.c:90: error: void value not ignored as it ought to be

The syntax to cast address to a function pointer and then call it
would look like this:

((void (*)(void))addre ss)();

It might be clearer to do something like this though:

void (*fptr)(void) = (void (*)(void))addre ss;
fptr();

--
Robert Gamble

[Eric Sosman]

Re: Jump to address syntax

Guillaume Dargaud wrote:

Hello all,
I have an example of working code under my eyes that goes as follow:
>
unsigned long address=0x40000 0;
(void (*)(void)addres s)();
[...]

ITYM `((void (*)(void))addre ss)();'.

Usually I disapprove of smuggling pointer-ness into a
typedef'ed type, but I make an exception for function pointers
because a pointerized typedef seems to help readability:

typedef void (*VFptr)(void);
((VFptr)address )();

--
Eric Sosman
esosman@ieee-dot-org.invalid

[Richard Tobin]

Re: Jump to address syntax

In article <g5kjem$ajj$1@c cpntc8.in2p3.fr >,

>(void (*)(void)addres s)();

You're trying to cast address to type "void (*)(void)". So you put
that type in parentheses before the variable:

(void (*)(void))addre ss

and you want to call it; you'll need more parentheses to get the
grouping right:

((void (*)(void))addre ss)();

-- Richard
--
Please remember to mention me / in tapes you leave behind.

[Ben Bacarisse]

Re: Jump to address syntax

Eric Sosman <esosman@ieee-dot-org.invalidwrit es:

Guillaume Dargaud wrote:

>Hello all,
>I have an example of working code under my eyes that goes as follow:
>>
>unsigned long address=0x40000 0;
>(void (*)(void)addres s)();
>[...]

>
ITYM `((void (*)(void))addre ss)();'.
>
Usually I disapprove of smuggling pointer-ness into a
typedef'ed type, but I make an exception for function pointers
because a pointerized typedef seems to help readability:
>
typedef void (*VFptr)(void);
((VFptr)address )();

I used to make this same exception, but is

typedef void VFptr(void);
((VFptr *)address)();

really any less readable?

(I think I got in to the habit because some pre-standard C compilers
refused to typedef a function type. I can't find any evidence for now
except a sense memory of surprise when I discovered that it is
standardised. Am I misremembering? )

--
Ben.

[Guillaume Dargaud]

Re: Jump to address syntax

>typedef void (*VFptr)(void);

>((VFptr)addres s)();

or

typedef void VFptr(void);
((VFptr *)address)();

Thanks, yes those two forms are a lot clearer.
--
Guillaume Dargaud

http://www.gdargaud.net/

[Peter Nilsson]

Re: Jump to address syntax

Guillaume Dargaud wrote:

typedef void (*VFptr)(void);
((VFptr)address )();

>
or
>

typedef void VFptr(void);
((VFptr *)address)();

This reads as a pointer to a function pointer. I'd prefer...

typedef void VFunc(void);
((VFunc *)address)();

>
Thanks, yes those two forms are a lot clearer.

In cases where you're calling the function often, it's clearer still
to transfer the casts to the declaration, or hide it in a function
macro...

#define frobnisticate() ((VFunc *) 0x400000)
or
VFunc *frobnisticate = (VFunc *) 0x400000;

frobnisticate() ;

--
Peter

[Ben Bacarisse]

Re: Jump to address syntax

Peter Nilsson <airia@acay.com .auwrites:

Guillaume Dargaud wrote:

typedef void (*VFptr)(void);
((VFptr)address )();

>>
>or
>>

typedef void VFptr(void);
((VFptr *)address)();

>
This reads as a pointer to a function pointer. I'd prefer...
>
typedef void VFunc(void);
((VFunc *)address)();

Yes so would I :-) That was not Guillaume Dargaud's fault -- when I
flipped the * from the typedef to the cast I kept the name to show the
symmetry but it would have been wiser to change the name at the same
time.

--
Ben.