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Joe Shmo
#1 / 10
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 simple question about malloc & free char *ptr;
ptr = malloc(sizeof(char) * 256);
ptr += 100;
free(ptr);
Since I've changed the address of ptr, is this going to cause problems
freeing the memory?
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| Sun, 28 May 2000 03:00:00 GMT |
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Danette & Murray Ro
#2 / 10
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simple question about malloc & free
in comp.lang.c:
=> char *ptr;
=> ptr = malloc(sizeof(char) * 256);
=> ptr += 100;
=> free(ptr);
=>
=> Since I've changed the address of ptr, is this going to cause problems
=> freeing the memory?
Yes.
Instead do:
char *ptr;
char *ptr2;
ptr = malloc(sizeof(char) * 256);
ptr2 = ptr + 100;
free(ptr);
But remember that ptr2 is also invalid after the free().
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Be Different -
THINK!
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http://mroot.home.mindspring.com
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| Tue, 30 May 2000 03:00:00 GMT |
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Miguel Carrasquer Vid
#3 / 10
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simple question about malloc & free On Wed, 10 Dec 1997 04:46:40 -0600, "Joe Shmoe"
Quote:
>char *ptr;
>ptr = malloc(sizeof(char) * 256);
sizeof(char) is 1, by definition. Quote: >ptr += 100; >free(ptr); >Since I've changed the address of ptr, is this going to cause problems
>freeing the memory?
You bet! ==
Miguel Carrasquer Vidal ~ ~
Amsterdam _____________ ~ ~
========================== Ce .sig n'est pas une .cig
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| Tue, 30 May 2000 03:00:00 GMT |
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Ralph Silverm
#4 / 10
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simple question about malloc & free
: On Wed, 10 Dec 1997 04:46:40 -0600, "Joe Shmoe"
: >char *ptr;
: >ptr = malloc(sizeof(char) * 256);
: sizeof(char) is 1, by definition.
: >ptr += 100;
: >free(ptr);
: >
: >Since I've changed the address of ptr, is this going to cause problems
: >freeing the memory?
: You bet!
: ==
: Miguel Carrasquer Vidal ~ ~
: Amsterdam _____________ ~ ~
: ========================== Ce .sig n'est pas une .cig
evident intent is achieved
by use of
realloc()
...
--
Ralph Silverman
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| Tue, 30 May 2000 03:00:00 GMT |
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Kurt Watz
#5 / 10
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simple question about malloc & free Quote:
>: On Wed, 10 Dec 1997 04:46:40 -0600, "Joe Shmoe"
>: >char *ptr;
>: >ptr = malloc(sizeof(char) * 256);
>: sizeof(char) is 1, by definition.
>: >ptr += 100;
>: >free(ptr);
>: >
>: >Since I've changed the address of ptr, is this going to cause problems
>: >freeing the memory?
> evident intent is achieved
> by use of
> realloc()
> ...
realloc() can be used to shorten an allocated chunk of memory, but I am not aware of a use of realloc that makes it possible to use just the "rest" of an allocated chunk. I doubt very strongly that there is a reasonable connection between realloc() and what happens in the above piece of code. Kurt
--
| Kurt Watzka Phone : +49-89-2178-2781
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| Tue, 30 May 2000 03:00:00 GMT |
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Ulric Erikss
#6 / 10
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simple question about malloc & free
Quote:
>> evident intent is achieved
>> by use of
>> realloc()
>> ...
>realloc() can be used to shorten an allocated chunk of memory, but I
>am not aware of a use of realloc that makes it possible to use just the
>"rest" of an allocated chunk. I doubt very strongly that there is a
>reasonable connection between realloc() and what happens in the
>above piece of code.
I think Silverman is right this time, at least if the intent is to keep the first 100 chars and free the rest. They may not end up in their original address, but that doesn't seem to matter here. Ulric
--
"You say to-mah-to, I say to-mah-to, and he says to-mah-to,
but ISO disagrees with all three of us and says to-mah-to."
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| Tue, 30 May 2000 03:00:00 GMT |
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Pete Becke
#7 / 10
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simple question about malloc & free Quote:
> char *ptr;
> ptr = malloc(sizeof(char) * 256);
> ptr += 100;
> free(ptr);
> Since I've changed the address of ptr, is this going to cause problems
> freeing the memory?
Yes, serious problems. When you call free you must pass a pointer that was returned to you by malloc, calloc, or realloc, or a NULL pointer. -- Pete
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| Tue, 30 May 2000 03:00:00 GMT |
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firewin
#8 / 10
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simple question about malloc & free
Quote:
> : On Wed, 10 Dec 1997 04:46:40 -0600, "Joe Shmoe"
> : >char *ptr;
> : >ptr = malloc(sizeof(char) * 256);
> : sizeof(char) is 1, by definition.
> : >ptr += 100;
> : >free(ptr);
> : >
> : >Since I've changed the address of ptr, is this going to cause problems
> : >freeing the memory?
> : You bet!
> evident intent is achieved
> by use of
> realloc()
> ...
How's that, Ralph? The intent seems to be, to allocate some memory, play around with the resulting pointer a bit, then free the original block of memory. How realloc() accomplishes this, I don't really know. With the exception that his free() was incorrect, the code the original poster had was right. He just needed to make a copy and modify -that- instead of the original, then go back and free the original. [- firewind -]
[- "You're just jealous because the voices talk to -me-." -]
[- Have a good day, and enjoy your C. -]
[- (on a crusade of grumpiness where grumpiness is due) -]
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| Tue, 30 May 2000 03:00:00 GMT |
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Alicia Carla Longstree
#9 / 10
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simple question about malloc & free Quote:
> char *ptr;
> ptr = malloc(sizeof(char) * 256);
> ptr += 100;
> free(ptr);
> Since I've changed the address of ptr, is this going to cause problems
> freeing the memory?
Absolutely. do this instead: char *ptr, *bptr;
bptr = malloc(256); /* sizeof( char ) is 1 by definition. */
ptr = bptr+100;
/* some code. */
free( bptr ); /* Free the memory, never change bptr. */
ptr = NULL; /* Nullify any pointers based on a free()ed pointer. */
Make sure that any pointers pointing into memory that you have freed are
assigned either a NULL pointer value or some other valid value. Any
effort to dereference or access a pointer into free()ed memory is
undefined behavior, its value is indeterminant and may cause you system
to crash.
e.g.
char *ptr, *bptr;
bptr = malloc(256);
ptr = bptr+100;
free( bptr );
if( ptr == NULL ) { /* this produces undefined behavior. */
The value of ptr is indeterminant. You can not use it to access memory,
you can't even compare its value with another (Really, how can you
compare a value whan that value is indeterminant - unable to be
determined)!
--
*****************************************************************
If you {*filter*}a smurf, what color does it turn?
What happens if you get scared half to death twice?
Energizer Bunny arrested, charged with battery.
I spilled Spot remover on my dog. Now he's gone.
I used to have an open mind but my brains kept falling out.
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| Thu, 01 Jun 2000 03:00:00 GMT |
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Lawrence Kir
#10 / 10
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simple question about malloc & free
Quote:
>> char *ptr;
>> ptr = malloc(sizeof(char) * 256);
>> ptr += 100;
>> free(ptr);
>> Since I've changed the address of ptr, is this going to cause problems
>> freeing the memory?
>Absolutely. do this instead:
>char *ptr, *bptr;
>bptr = malloc(256); /* sizeof( char ) is 1 by definition. */
>ptr = bptr+100;
>/* some code. */
>free( bptr ); /* Free the memory, never change bptr. */
>ptr = NULL; /* Nullify any pointers based on a free()ed pointer. */
>Make sure that any pointers pointing into memory that you have freed are
>assigned either a NULL pointer value or some other valid value. Any
>effort to dereference or access a pointer into free()ed memory is
>undefined behavior, its value is indeterminant and may cause you system
>to crash.
The same is true for trying to dereference a null pointer. If the code tries to dereference a pointer to freed memory then the program has a logic fault. The program logic needs to be fixed rather then propped up like this. Also it may not be possible to write to all pointers to a freed block since the section of code in question may not know where they all are or have access to them. Quote: >e.g.
>char *ptr, *bptr;
>bptr = malloc(256);
>ptr = bptr+100;
>free( bptr );
While I'm not advocating this as good style it is worth noting that free(ptr-100);
is valid too.
--
-----------------------------------------
-----------------------------------------
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| Thu, 01 Jun 2000 03:00:00 GMT |
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