what is typecasting a pointer to the type (void *)p mean?

**Michael Rasmussen** · Jan 25 '06, 09:05 PM

Re: what is typecasting a pointer to the type (void *)p mean?

On Wed, 25 Jan 2006 12:17:23 -0800, Abhishek wrote:
[color=blue]
> int func(int i,(void *)p) where p is a pointer or an address which is
> passed from the place where it is called. what do you mean by pointing to
> a void and why is it done?[/color]
A pointer to a void is a pointer to an unqualified place in memory. If you
want to work with threads this is unavoidable. One of the reasons is a
way of transfering data from one process to another. You cannot do this
and at the same time preserve the type information. This also means that
the receiving process must know how to dereference the pointer - cast it
to a known type.

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

void test_p(void *t)
{
int i;
char *text = (char *) t;
for (i = 0; i < 10; ++i)
printf("%s\n", text);
pthread_exit(NU LL);
}

int main()
{
pthread_t t1 = 0;
char *text = "test";

printf("Howdy world!\n");
pthread_create( &t1, NULL, (void *)&test_p, (void *) text);
pthread_join(t1 , NULL);
return 0;
}

[color=blue]
> Aso, what happens when we typecast a pointer to another type. say[/color]
for[color=blue]
> example int *i=(char *)p;[/color]
This is dangerous since you could loose information an depending on your
compiler could course either a warning or an error.
--
Hilsen/Regards
Michael Rasmussen

http://keyserver.veridis.com:11371/pks/lookup?op=get&search=0xE3E80917

**Scorpio** · Jan 25 '06, 09:05 PM

Re: what is typecasting a pointer to the type (void *)p mean?

Abhishek wrote:[color=blue]
> why do I see that in most C programs, pointers in functions are
> accepted as:
> int func(int i,(void *)p) where p is a pointer or an address which is
> passed from the place where it is called. what do you mean by pointing
> to a void and why is it done?[/color]

void pointer is a generic pointer, meaning that it can be used to store
address of any data type.

by (void*)p you are casting the address of p to void* type.
This is actually redundant. Typecasting is not necessary
while assigning value to a void* pointer.
[color=blue]
> Aso, what happens when we typecast a pointer to another type. say for
> example int *i=(char *)p;[/color]

This should not even compile because you're trying to assign a wrong
type
of address to int *i.
[color=blue]
> under different situations? I am kind of confused..can anybody clear
> this confusion by clearly explaining to me what actually is happening
> out here?
> Bye[/color]

Sharath A.V

**Abhishek** · Jan 25 '06, 09:15 PM

Re: what is typecasting a pointer to the type (void *)p mean?

Does this alsomean that I get the following flexibility?

like I can use the test_p function to accept different types of
pointers by typecasting them to type void * and later perform an action
depending on a particular flag variable I pass to the function?
like void test_p(void *t, int i)
{
int k;
if(i==1)
{
char *text = (char *) t;
for (k = 0; k < 10; ++k)
printf("%s\n", text);
}
else
{
int *j=(int *)t;
for(k=0;k<10;k+ +)
printf("%d\n",* j);
}

}

here...dependin g upon the value of 'i' I pass, I can take appropriate
action. However, I know that if I pass i=1, then I pass a pointer to a
string by typecasting to void * and if I pass i !=2 then I pass a
pointer to an integer by type casting the integer pointer to type void
*???
Pls clarify

Michael Rasmussen wrote:[color=blue]
> On Wed, 25 Jan 2006 12:17:23 -0800, Abhishek wrote:
>[color=green]
> > int func(int i,(void *)p) where p is a pointer or an address which is
> > passed from the place where it is called. what do you mean by pointing to
> > a void and why is it done?[/color]
> A pointer to a void is a pointer to an unqualified place in memory. If you
> want to work with threads this is unavoidable. One of the reasons is a
> way of transfering data from one process to another. You cannot do this
> and at the same time preserve the type information. This also means that
> the receiving process must know how to dereference the pointer - cast it
> to a known type.
>
> #include <stdio.h>
> #include <pthread.h>
> #include <unistd.h>
>
> void test_p(void *t)
> {
> int i;
> char *text = (char *) t;
> for (i = 0; i < 10; ++i)
> printf("%s\n", text);
> pthread_exit(NU LL);
> }
>
> int main()
> {
> pthread_t t1 = 0;
> char *text = "test";
>
> printf("Howdy world!\n");
> pthread_create( &t1, NULL, (void *)&test_p, (void *) text);
> pthread_join(t1 , NULL);
> return 0;
> }
>
>
>[color=green]
> > Aso, what happens when we typecast a pointer to another type. say[/color]
> for[color=green]
> > example int *i=(char *)p;[/color]
> This is dangerous since you could loose information an depending on your
> compiler could course either a warning or an error.
> --
> Hilsen/Regards
> Michael Rasmussen
> http://keyserver.veridis.com:11371/p...rch=0xE3E80917[/color]

**Emmanuel Delahaye** · Jan 25 '06, 10:05 PM

Re: what is typecasting a pointer to the type (void *)p mean?

Abhishek a écrit :[color=blue]
> Does this alsomean that I get the following flexibility?[/color]

'flexibility' and 'void*' are often associated. Your intuition is correct.
[color=blue]
> like I can use the test_p function to accept different types of
> pointers by typecasting them to type void * and later perform an action
> depending on a particular flag variable I pass to the function?[/color]

Somehow, yes. Note that the conversion from/to void* doesn't need an
explicit typecast.
[color=blue]
> like void test_p(void *t, int i)
> {
> int k;
> if(i==1)
> {
> char *text = (char *) t;[/color]

char *text = t;
if (text != NULL)
{[color=blue]
> for (k = 0; k < 10; ++k)
> printf("%s\n", text);[/color]
}[color=blue]
> }
> else
> {
> int *j=(int *)t;[/color]

int *j = t;
if (j != NULL)
{
[color=blue]
> for(k=0;k<10;k+ +)
> printf("%d\n",* j);[/color]
}[color=blue]
> }
>
> }
>
> here...dependin g upon the value of 'i' I pass, I can take appropriate
> action.[/color]

Correct.

--
A+

Emmanuel Delahaye

**Michael Rasmussen** · Jan 25 '06, 10:15 PM

Re: what is typecasting a pointer to the type (void *)p mean?

On Wed, 25 Jan 2006 13:01:56 -0800, Abhishek wrote:
[color=blue]
> Does this alsomean that I get the following flexibility?
>
> like I can use the test_p function to accept different types of pointers
> by typecasting them to type void * and later perform an action depending
> on a particular flag variable I pass to the function? like void
> test_p(void *t, int i)[/color]
For this kind of problem I would recommend using a struct.

#include <stdio.h>

enum Kind {integer, string};

struct container {
enum Kind kind;
void *ptr;
};

void test_p(void *data)
{
struct container *c = (struct container *) data;
switch (c->kind) {
case integer:
printf("Receive d integer:\t%d\n" , (int) c->ptr);
break;
case string:
printf("Receive d string:\t%s\n", (char *) c->ptr);
break;
default:
printf("Undefin ed data type\n");
}
}

int main()
{
struct container c;
char *text = "test";
int i = 1;

c.kind = string;
c.ptr = (void *)text;
test_p((void *) &c);
c.kind = integer;
c.ptr = (void *)i;
test_p((void *) &c);
return 0;
}

--
Hilsen/Regards
Michael Rasmussen

http://keyserver.veridis.com:11371/pks/lookup?op=get&search=0xE3E80917

**Michael Rasmussen** · Jan 25 '06, 10:15 PM

Re: what is typecasting a pointer to the type (void *)p mean?

On Wed, 25 Jan 2006 22:56:57 +0100, Emmanuel Delahaye wrote:
[color=blue]
>
> Somehow, yes. Note that the conversion from/to void* doesn't need an
> explicit typecast.
>[/color]
Well, not always but in this situation it is mandatory:
void foo(void *t) {
printf("%s", (char *) t);
}

int main() {
char *t = "test";
foo(t);
return 0;
}

So might as well use explicit casting since it is a lot more readable for
an outsider which later has to maintain the code. IMHO

--
Hilsen/Regards
Michael Rasmussen

http://keyserver.veridis.com:11371/pks/lookup?op=get&search=0xE3E80917

**Al Balmer** · Jan 25 '06, 10:25 PM

Re: what is typecasting a pointer to the type (void *)p mean?

On Wed, 25 Jan 2006 23:07:54 +0100, Michael Rasmussen <mir@miras.or g>
wrote:
[color=blue]
>On Wed, 25 Jan 2006 22:56:57 +0100, Emmanuel Delahaye wrote:
>[color=green]
>>
>> Somehow, yes. Note that the conversion from/to void* doesn't need an
>> explicit typecast.
>>[/color]
>Well, not always but in this situation it is mandatory:
>void foo(void *t) {
> printf("%s", (char *) t);
>}
>
>int main() {
> char *t = "test";
> foo(t);
> return 0;
>}
>
>So might as well use explicit casting since it is a lot more readable for
>an outsider which later has to maintain the code. IMHO[/color]

void foo(void *t) {
char *rt = t;
printf("%s", rt);
}

--
Al Balmer
Sun City, AZ

**Abhishek** · Jan 25 '06, 10:25 PM

Re: what is typecasting a pointer to the type (void *)p mean?

Thank you all for that wonderful explanation.
So, whats the difference between a null pointer and a pointer which is
pointing to void?
Are these same?

Michael Rasmussen wrote:[color=blue]
> On Wed, 25 Jan 2006 22:56:57 +0100, Emmanuel Delahaye wrote:
>[color=green]
> >
> > Somehow, yes. Note that the conversion from/to void* doesn't need an
> > explicit typecast.
> >[/color]
> Well, not always but in this situation it is mandatory:
> void foo(void *t) {
> printf("%s", (char *) t);
> }
>
> int main() {
> char *t = "test";
> foo(t);
> return 0;
> }
>
> So might as well use explicit casting since it is a lot more readable for
> an outsider which later has to maintain the code. IMHO
>
> --
> Hilsen/Regards
> Michael Rasmussen
> http://keyserver.veridis.com:11371/p...rch=0xE3E80917[/color]

**Michael Rasmussen** · Jan 25 '06, 10:55 PM

Re: what is typecasting a pointer to the type (void *)p mean?

On Wed, 25 Jan 2006 14:18:42 -0800, Abhishek wrote:
[color=blue]
> Thank you all for that wonderful explanation. So, whats the difference
> between a null pointer and a pointer which is pointing to void?
> Are these same?
>[/color]
A null pointer points to nothing - eg. you have a pointer of some type
which has not jet been assigned any value. A pointer to void is a pointer
of some unknown type assigned an address in memory.

--
Hilsen/Regards
Michael Rasmussen

http://keyserver.veridis.com:11371/pks/lookup?op=get&search=0xE3E80917

**Keith Thompson** · Jan 26 '06, 12:35 AM

Re: what is typecasting a pointer to the type (void *)p mean?

"Abhishek" <abhisheksgumad i@gmail.com> writes:[color=blue]
> Thank you all for that wonderful explanation.
> So, whats the difference between a null pointer and a pointer which is
> pointing to void?
> Are these same?[/color]

Please don't top-post. Your response belongs below, or mixed with,
the article to which you're replying, and you should trim anything
that's not relevant to your response. This makes it much easier to
read each individual article from top to bottom.

No, they're very different things.

A null pointer is a particular pointer *value*. There's a null
pointer value for each pointer type (typically with the same
representation) . A null pointer doesn't point to anything. (It's
typically, but not necessarily, represented as all-bits-zero.)

A void pointer is a pointer *type*, declared as "void*". It's a
generic pointer type. An object of type void* can point to any
object, or it can have a null pointer value.

You should read sections 4 and 5 of the comp.lang.c FAQ, available at
<http://www.c-faq.com/>. In fact, you should read the whole thing.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

**Flash Gordon** · Jan 26 '06, 02:15 AM

Re: what is typecasting a pointer to the type (void *)p mean?

Abhishek wrote:

Please don't top post. Your reply belongs under the text you are
replying to after snipping anything not relevant to your reply. I've
corrected it this time.
[color=blue]
> Michael Rasmussen wrote:[color=green]
>> On Wed, 25 Jan 2006 22:56:57 +0100, Emmanuel Delahaye wrote:
>>[color=darkred]
>>> Somehow, yes. Note that the conversion from/to void* doesn't need an
>>> explicit typecast.
>>>[/color]
>> Well, not always but in this situation it is mandatory:
>> void foo(void *t) {
>> printf("%s", (char *) t);
>> }
>>
>> int main() {
>> char *t = "test";
>> foo(t);
>> return 0;
>> }
>>
>> So might as well use explicit casting since it is a lot more readable for
>> an outsider which later has to maintain the code. IMHO[/color][/color]

In general casts are a bad thing. They tell the compiler you know what
you are doing even if you don't. So those who do not have a thorough
understanding should not use them until they know *why* they are needed
in a given situation. My experience is that I have seen more casts to
shut the compiler up without solving the underlying problem (and leaving
in significant errors without the compiler complaining as a result) then
I have seen casts that were required. They also add visual clutter which
IMHO makes it harder to read.
[color=blue][color=green]
>> --
>> Hilsen/Regards
>> Michael Rasmussen
>> http://keyserver.veridis.com:11371/p...rch=0xE3E80917[/color][/color]

Please snip peoples signatures (the bit after the -- normally) unless
you are commenting on them.
[color=blue]
> Thank you all for that wonderful explanation.
> So, whats the difference between a null pointer and a pointer which is
> pointing to void?
> Are these same?[/color]

What is the difference between an integer variable and the value 0?

The null pointer is a specific pointer value that does not point
anywhere. You can have null pointers of any pointer type and they always
compare equal to each other when converted to a common pointer type.
Null pointers are also returned by various library functions to indicate
failure.

A pointer to void is a specific type of pointer (just as long is a
specific type of integer). Any pointer to object type (not a pointer to
a function) can be converted to void* and back safely. Converting
to/from void* to other object pointer types does not require a cast. So
pointers to void are a generic pointer type.
--
Flash Gordon
Living in interesting times.
Although my email address says spam, it is real and I read it.

**Flash Gordon** · Jan 26 '06, 02:15 AM

Re: what is typecasting a pointer to the type (void *)p mean?

Michael Rasmussen wrote:[color=blue]
> On Wed, 25 Jan 2006 14:18:42 -0800, Abhishek wrote:
>[color=green]
>> Thank you all for that wonderful explanation. So, whats the difference
>> between a null pointer and a pointer which is pointing to void?
>> Are these same?
>>[/color]
> A null pointer points to nothing - eg. you have a pointer of some type
> which has not jet been assigned any value.[/color]

No, a pointer that has not been assigned any value is *not* a null
pointer in the general case. If it has static duration that is true, but
if it has automatic duration (i.e. a pointer variable declared in a
function that is not specified as static) it would be a pure fluke if it
happened to be a null pointer.

It is true that a null pointer does not point anywhere. It is also the
only pointer value that you are guaranteed to be able to convert to any
other pointer value. Null pointers are normally either generated by you
explicitly using them in the source or on a library routine that returns
a pointer failing.
[color=blue]
> A pointer to void is a pointer
> of some unknown type assigned an address in memory.[/color]

A variable of type void* might point to something, or on the other hand
it could be uninitialised or a null pointer. The pointer itself has no
type information so it could point at any type of object (not a
function) and it is up to the programmer to keep track of what type it
points to.
--
Flash Gordon
Living in interesting times.
Although my email address says spam, it is real and I read it.

**Lucien Kennedy-Lamb** · Jan 26 '06, 03:25 AM

CALL FOR COMMENT ON NULL: Re: what is typecasting a pointer to the type (void *)p mean?

Abhishek wrote:[color=blue]
> So, whats the difference between a null pointer and a pointer which is
> pointing to void?
> Are these same?[/color]

You require a proper explanation of what the terms 'null' and 'void'
actually mean in C. Their meanings differ from the standard English
sense (just to confuse everyone).

Null...

In the English language "null" means insignificant, missing or
amounting to nothing. It can also mean zero. In fact the German
language for number zero is "null".

http://german.about.com/library/anfang/blanfang07.htm

Don't let that confuse you.

In C terms a null pointer is a pointer that doesn't point to any
variable, structure or function *anywhere*. It's like having a finger
that is designed to point to a spot on a map. Point to anywhere on the
map and it will have some well defined coordinate (address), but if you
haven't decided where to point that finger quite yet... point that
finger upwards into space (null).

The null value for pointers is only useful for setting pointers that
haven't been initialised to a *real* address.

Null pointers *must* be assigned using the NULL macro (defined by
<stdio.h>). The reason is that, while most of the time NULL is defined
as 0, it may not be on every platform.

char *string = NULL; /* I'm not pointing to anything useful yet */

CALL FOR COMMENT HERE: I've never seen a compiler *not* use 0 as NULL,
so who has? Who can list an example where this is not the case? I've
seen so much code that relies on the NULL == 0 assumption.

Use of a null pointer?

Any pointer can be assigned the NULL value. I can be a char, int, long
or any structure or union pointer.

void PrintMyString(c har *string)
{
if (string != NULL) /* Check if this is a real address of a
possible string */
printf("This is the string: %s", string);
}

and my favourite is:

void ProcessTaxRetur n(struct TAX_RETURN *p_tax_return)
{
if (p_tax_return != NULL) /* Has a valid tax return structure been
received? */
{
SendLargeTaxBil l(p_tax_return) ;
}
}

I've been trying for years to send a null tax return, but it's never
worked. :-)

Void...

In the English language "void" means empty, useless or a vacuum.

In C it has quite a different meaning. My best explanation is a type
of zero length data.

Consider:

void DoSomthing(void );

The compiler knows what there is no return and no arguments to this
function.

void*

What does this mean? It's a pointer like any other pointer. It holds
the address of a memory location.

It simply means that "I've got the address of *something*, but I don't
know what it is".

void *pointers are generally used between software components that
expose an interface where the type of data is unknown.

Consider

void *malloc(size_t size);

This standard C library function returns a void * as it has no idea how
this memory block is to be used for (something addressed in C++ with
the new operator).

void* pointers is a convenient way of passing around data pointers when
types con't be conveyed. Use them sparingly because real types (and
typedefs) should be used whenever possible.

I hope this goes some way to explaining how void and null should be
used.

Lucien Kennedy-Lamb

**pete** · Jan 26 '06, 04:45 AM

Re: CALL FOR COMMENT ON NULL: Re: what is typecasting a pointer to the type (void *)p mean?

Lucien Kennedy-Lamb wrote:
[color=blue]
> Null pointers *must* be assigned using the NULL macro (defined by
> <stdio.h>). The reason is that,
> while most of the time NULL is defined
> as 0, it may not be on every platform.
>
> char *string = NULL; /* I'm not pointing to anything useful yet */[/color]

There's nothing wrong with:
char *string = 0;
It means exactly the same thing thing as
char *string = NULL;
no matter how NULL is defined.

--
pete

what is typecasting a pointer to the type (void *)p mean?

what is typecasting a pointer to the type (void *)p mean?

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