Pointer assigned by a function problem...

#include <stdlib.h>
#include <stdio.h>

void foo(double *ptr)
{
printf("foo ptr=%p\n",ptr);
ptr = malloc(sizeof(*ptr));
printf("foo ptr=%p\n",ptr);
ptr[0] = 12;
printf("foo value=%f\n",*ptr);
}


int main(int argc, char **argv)
{
double *ptr=NULL;
printf("main ptr=%p\n",ptr);
foo(ptr);
printf("main ptr=%p\n",ptr);
}


Returns:

main ptr=(nil)
foo ptr=(nil)
foo ptr=0x804a008
foo value=12.000000
main ptr=(nil)

Why the pointer in the main function is not updated?

Thnaks in advance.

jamaj

re: Pointer assigned by a function problem...

jamaj wrote:Because you are updating the local copy in foo. ptr in foo is copy of
the one in main.

You have to write

void foo(double **ptr)
{
*ptr = malloc(sizeof(*ptr));
}

--
Ian Collins.

re: Pointer assigned by a function problem...

I have altered for this version, and it worked. But is this the unique
and right way?

void foo(double **ptr)
{
printf("foo ptr=%p\n",*ptr);
*ptr = malloc(sizeof(**ptr));
printf("foo ptr=%p\n",*ptr);
*ptr[0] = 12;
printf("foo value=%f\n",**ptr);
}
int main(int argc, char **argv)
{
double *ptr=NULL;
printf("main ptr=%p\n",ptr);
foo(&ptr);
printf("main ptr=%p\n",ptr);
printf("main value=%f\n",*ptr);
}

Now the results are:

main ptr=(nil)
foo ptr=(nil)
foo ptr=0x804a008
foo value=12.000000
main ptr=0x804a008
main value=12.000000


On 2 set, 21:53, jamaj <jama...@gmail.comwrote:

re: Pointer assigned by a function problem...

jamaj wrote:

[please don't top-post]
Yes.
--
Ian Collins.

re: Pointer assigned by a function problem...

On Sep 3, 4:02 am, jamaj <jama...@gmail.comwrote:*ptr is of type (double *). `p' expects (void *).
Change to:

printf("foo ptr=%p\n", (void *)*ptr);
Don't forget that it's possible for malloc to return NULL.
Same here; *ptr is (double *), `p' expects (void *).
So here you have a problem, since you don't check if *p is indeed non-
null, and you just dereference it.
Change this to:

if(*ptr == NULL) return;
*ptr[0] = 12;
And here
and here.
Please don't top post.
http://www.catb.org/jargon/html/T/top-post.html
http://www.cs.tut.fi/~jkorpela/usenet/brox.html
http://www.caliburn.nl/topposting.html
http://www.html-faq.com/etiquette/?toppost
etc.

re: Pointer assigned by a function problem...

On Sep 3, 4:11 am, vipps...@gmail.com wrote:<snip>

Ah yes, also there's a memory leak, since you never free *p.
So add a free(*p); after the last printf, and then add a return 0;

re: Pointer assigned by a function problem...

Ok. I won't do it anymore.

But the problem was that showed by Ian Collins. Many thanks.

Regards,

jamaj

re: Pointer assigned by a function problem...

On 2 set, 22:19, vipps...@gmail.com wrote:I think that you already know that these little snippets of code have
only an illustrative purpose, and they did the work - with or without
the memory leaks that you correctly showed. So you can rest, cause my
doubt was kindly removed by Ian Collins.

Thanks again.

jamaj

re: Pointer assigned by a function problem...

On Sep 3, 4:26 am, jamaj <jama...@gmail.comwrote:No they did not.

re: Pointer assigned by a function problem...

On Sep 3, 4:20 am, jamaj <jama...@gmail.comwrote:Thanks, now please leave the attributes untouched.

(original message was by me)

re: Pointer assigned by a function problem...

On 2 set, 22:31, vipps...@gmail.com wrote:But they are working in my machine!!! Maybe a bug of gcc... I'm
wasting my time with you. Thanks a lot...

The real program i'm building is now working, and what is important
for me.

Thanks Ian Collins...
Regards,

jamaj

re: Pointer assigned by a function problem...

On Tue, 2 Sep 2008 18:19:06 -0700 (PDT), vippstar@gmail.com wrote:
In main, *p is a double. You meant
free(p);

--
Remove del for email

re: Pointer assigned by a function problem...

On Tue, 2 Sep 2008 18:49:09 -0700 (PDT), jamaj <jamajbr@gmail.com>
wrote:
It is possible you are wasting your time but if so it is because you
are learning bad habits and rejecting efforts to correct them.
Refusing to correct undefined behavior just because the program
appears to work is a recipe for disaster.

--
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re: Pointer assigned by a function problem...

On Sep 3, 6:35 am, Barry Schwarz <schwa...@dqel.comwrote:Right, thanks for that correction.