|  | Pointer assigned by a function problem... | | |
| | | jamaj |  |
|  Posted: Wed Sep 03, 2008 12:53 am Post subject: 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 |
| |
| | | Ian Collins | |
| | Posted: Wed Sep 03, 2008 12:53 am Post subject: Re: Pointer assigned by a function problem... | |
jamaj wrote:
| Quote: | #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?
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. |
| |
| | | Ian Collins | |
| | Posted: Wed Sep 03, 2008 12:53 am Post subject: Re: Pointer assigned by a function problem... | |
jamaj wrote:
[please don't top-post]
| Quote: | I have altered for this version, and it worked. But is this the unique
and right way?
Yes. |
| Quote: | 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);
}
|
--
Ian Collins. |
| |
| | | jamaj | |
| | Posted: Wed Sep 03, 2008 1:02 am Post subject: 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.com> wrote:
| Quote: | #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 |
|
| |
| | | Guest | |
| | Posted: Wed Sep 03, 2008 1:11 am Post subject: Re: Pointer assigned by a function problem... | |
| | | |
On Sep 3, 4:02 am, jamaj <jama...@gmail.com> wrote:
| Quote: | 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 is of type (double *). `p' expects (void *).
Change to:
printf("foo ptr=%p\n", (void *)*ptr);
| Quote: | *ptr = malloc(sizeof(**ptr));
|
Don't forget that it's possible for malloc to return NULL.
| Quote: | printf("foo ptr=%p\n",*ptr);
|
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;
| Quote: | printf("foo value=%f\n",**ptr);}
int main(int argc, char **argv)
{
double *ptr=NULL;
printf("main ptr=%p\n",ptr);
|
And here
| Quote: | foo(&ptr);
printf("main ptr=%p\n",ptr);
|
and here.
| Quote: | 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
|
Please don't top post.
LINK
LINK
LINK
LINK
etc. |
| |
| | | Guest | |
| | Posted: Wed Sep 03, 2008 1:19 am Post subject: Re: Pointer assigned by a function problem... | |
On Sep 3, 4:11 am, vipps...@gmail.com wrote:
| Quote: | On Sep 3, 4:02 am, jamaj <jama...@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; |
| |
| | | jamaj | |
| | Posted: Wed Sep 03, 2008 1:20 am Post subject: Re: Pointer assigned by a function problem... | |
| Quote: | Please don't top post.http://www.catb.org/jargon/html/T/top-post.htmlhttp://www.cs.tut.fi/~jkorpela/usenet/brox.htmlhttp://www.caliburn.nl/topposting.htmlhttp://www.html-faq.com/etiquette/?toppost
etc.
|
Ok. I won't do it anymore.
But the problem was that showed by Ian Collins. Many thanks.
Regards,
jamaj |
| |
| | | jamaj | |
| | Posted: Wed Sep 03, 2008 1:26 am Post subject: Re: Pointer assigned by a function problem... | |
On 2 set, 22:19, vipps...@gmail.com wrote:
| Quote: | On Sep 3, 4:11 am, vipps...@gmail.com wrote:
On Sep 3, 4:02 am, jamaj <jama...@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;
|
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 |
| |
| | | Guest | |
| | Posted: Wed Sep 03, 2008 1:31 am Post subject: Re: Pointer assigned by a function problem... | |
On Sep 3, 4:26 am, jamaj <jama...@gmail.com> wrote:
| Quote: | On 2 set, 22:19, vipps...@gmail.com wrote:
On Sep 3, 4:11 am, vipps...@gmail.com wrote:
On Sep 3, 4:02 am, jamaj <jama...@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;
I think that you already know that these little snippets of code have
only an illustrative purpose, and they did the work
|
No they did not.
| Quote: | - with or without
the memory leaks that you correctly showed. So you can rest, cause my
doubt was kindly removed by Ian Collins. |
|
| |
| | | Guest | |
| | Posted: Wed Sep 03, 2008 1:32 am Post subject: Re: Pointer assigned by a function problem... | |
On Sep 3, 4:20 am, jamaj <jama...@gmail.com> wrote:
| Quote: | Please don't top post.
Ok. I won't do it anymore.
|
Thanks, now please leave the attributes untouched.
(original message was by me) |
| |
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