Date: Tue, 12 Aug 2008 18:22:21 +0200 From: Jordi Moles Blanco <jordi@cdmon.com> To: Giorgos Keramidas <keramida@ceid.upatras.gr> Cc: freebsd-questions@freebsd.org Subject: Re: error allocating memory with realloc(). how can i increase max_allowed in the system? [solved] Message-ID: <48A1B8BD.3080306@cdmon.com> In-Reply-To: <87fxpab4gy.fsf@kobe.laptop> References: <48A1A613.5020407@cdmon.com> <87fxpab4gy.fsf@kobe.laptop>
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Hello,
thank you very much for your time and help, i had completely
misunderstood how realloc() works.
i though i was able to write some C code but now i feel a complete
newbie, hehehe.
anyway... that made everything clear to me and now my script is working
like a charm.
thanks for everything
En/na Giorgos Keramidas ha escrit:
> On Tue, 12 Aug 2008 17:02:43 +0200, Jordi Moles Blanco <jordi@cdmon.com> wrote:
>
>> Hi,
>>
>> i'm running a FreeBSD 7.0 amd64 machine and struggling with some C
>> code i'm writing.
>>
>> I've had some trouble with this home-made script as it keeps crashing
>> while launching a "realloc()" call.
>>
>> I narrowed down the problem and here i'm sending you a short example of
>> code that crashes:
>>
>> *************
>> #include <stdio.h>
>> #include <stdlib.h>
>>
>> int main()
>> {
>>
>> int midataula;
>>
>> midataula = 3000;
>>
>> char *missatge = (char *)malloc(midataula * sizeof(char));
>>
>> missatge[0]='h';
>> missatge[1]='o';
>> missatge[2]='l';
>> missatge[3]='a';
>>
>> printf("\n\ntaula1: %s",missatge);
>>
>> int voltes;
>> voltes = 0;
>>
>> while(voltes<4)
>> {
>> midataula = midataula+500;
>> realloc(missatge, midataula * sizeof(char));
>> voltes++;
>> }
>>
>
> There's your problem. realloc() works fine, but it *returns* the new
> pointer; it does _not_ modify missatge "in place".
>
> The program should work fine if you use size_t for midataula (it is the
> 'size' of an array, which may not necessarily fit in an 'int'), and if
> you use realloc() correctly, as in:
>
> #include <stdlib.h>
> #include <err.h>
>
> size_t midataula;
> char *missatge;
>
> /*
> * DON'T cast the result of malloc(). It may 'hide' the bug of
> * a missing <stdlib.h> include, and cause troubles when
> * malloc() is implicitly defined by the compiler as:
> *
> * int malloc(...);
> *
> * On a 64-bit machine converting a 64-bit pointer to `int' will
> * lose the high-order 32 bits of the address, and you will try
> * to access unexpected memory areas.
> */
> midataula = 3000;
> missatge = malloc(midataula * sizeof(*missatge));
> if (missatge == NULL)
> err(1, "malloc");
>
> Then when you use realloc() keep both midataula and missatge in
> temporary copies until you are sure that realloc() worked:
>
> while (voltes < 4) {
> char *tmp;
> size_t newsize;
>
> newsize = midataula + 500;
> tmp = realloc(missatge, newsize * sizeof(*missatge));
> if (tmp == NULL)
> err(1, "realloc");
>
> /*
> * Now that you know the resize has succeeded, update
> * midataula and missatge. realloc() is allowed to
> * relocate missatge. See the following note in its
> * manpage:
> *
> * Note that realloc() and reallocf() may move the
> * memory allocation, resulting in a different return
> * value than ptr.
> */
> midataula = newsize;
> missatge = tmp;
> }
>
> Right now you are calling realloc() as:
>
> realloc(missatge, newsize * sizeof(*missatge));
>
> and throwing away the resulting pointer. The first time that realloc()
> discovers that the `resized' vector cannot fit in its original location,
> it relocates the array, and returns the new location. You throw away
> that location and your next iteration through the loop tries to access
> an invalid (already freed) memory region.
>
> That's what causes your segmentation fault.
>
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