* Re: Segmentation fault from free()
2004-08-24 20:34 Segmentation fault from free() Lei Yang
2004-08-24 21:12 ` Robert Schiele
@ 2004-08-25 3:05 ` joy
2004-08-27 20:26 ` Mariano Moreyra
2 siblings, 0 replies; 6+ messages in thread
From: joy @ 2004-08-25 3:05 UTC (permalink / raw)
To: Lei Yang; +Cc: linux-c-programming, linux-gcc
As already said, not much to ay w/o the full source.
However, you have allocated a 4Kb buffer. possibly
you are reading beyond that limit into some memory space
not belonging to your program and when you try to free it,
you get a segfault. A wild guess, but is possible since you say this is
happening only for large files.
regards,
Joy.M.Monteiro
Lei Yang wrote:
> Hi friends,
>
> I am writing a c code and have been bugged by this segmentation fault
> for a while.
>
> What I did is simply like:
>
> -----------------------------------------------------------------------------------------------------
>
> .........
> unsigned long blocksize = 2048;
> char *in_buffer
> char *out_buffer;
> if(( in_buffer= malloc(blocksize)) == NULL)
> {
> fprintf(stderr, "*** Can't malloc(%ld) forbuffer.\n",blocksize);
> return NULL;
> }
>
> if(( out_buffer= malloc(2*blocksize)) == NULL)
> {
> fprintf(stderr, "*** Can't malloc(%ld) forbuffer.\n",blocksize);
> free(in_buffer);
> return NULL;
> }
>
> loop: until all the data are read from file
> {
> //read a block of data from a file to in_buffer
> // do some data processing with in_buffer
> //write the result to out_buffer
> //memcpy out_buffer to list
> }
>
> free(in_buffer);
> free(out_buffer);
>
> return list;
> ......
> -----------------------------------------------------------------------------------------------------------------
>
>
> I've debugged with gdb to see where the segmentation fault happens, it
> is at free(in_buffer).
> I've verified that the value for in_buffer after malloc() and before
> free() is the same. Or in other words, in_buffer is a valid pointer
> allocated by malloc.
>
> And the SF only happens when the file is large, although block size
> could be small.
> Means that for both small (2KB) and large(5MB) files, block size are
> the same. However, only large files could cause SF.
>
> Could anyone please point me out what could possibly be the reason?
> BTW, pls cc me when you reply, since I am not able to receive emails
> from this list. Thanks a lot!
>
> TIA!
> Lei
> -
> To unsubscribe from this list: send the line "unsubscribe
> linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
^ permalink raw reply [flat|nested] 6+ messages in thread* RE: Segmentation fault from free()
2004-08-24 20:34 Segmentation fault from free() Lei Yang
2004-08-24 21:12 ` Robert Schiele
2004-08-25 3:05 ` joy
@ 2004-08-27 20:26 ` Mariano Moreyra
2 siblings, 0 replies; 6+ messages in thread
From: Mariano Moreyra @ 2004-08-27 20:26 UTC (permalink / raw)
To: 'Lei Yang', linux-c-programming, linux-gcc
[-- Attachment #1: Type: text/plain, Size: 3490 bytes --]
Are you writting exactly 2048 chars to in_buffer?? Or are you writting
2047??
If you want to write 2048 chars into in_buffer, you have to malloc 2049
bytes, to store the '\0' char.
I know it seems a little stupid observation, but sometimes we let this
stupid errors happen.
Mariano Moreyra
-----Mensaje original-----
De: linux-c-programming-owner@vger.kernel.org
[mailto:linux-c-programming-owner@vger.kernel.org]En nombre de Lei Yang
Enviado el: Martes, 24 de Agosto de 2004 17:34
Para: linux-c-programming@vger.kernel.org; linux-gcc@vger.kernel.org
CC: Lei Yang
Asunto: Segmentation fault from free()
Hi friends,
I am writing a c code and have been bugged by this segmentation fault
for a while.
What I did is simply like:
----------------------------------------------------------------------------
-------------------------
.........
unsigned long blocksize = 2048;
char *in_buffer
char *out_buffer;
if(( in_buffer= malloc(blocksize)) == NULL)
{
fprintf(stderr, "*** Can't malloc(%ld) forbuffer.\n",blocksize);
return NULL;
}
if(( out_buffer= malloc(2*blocksize)) == NULL)
{
fprintf(stderr, "*** Can't malloc(%ld) forbuffer.\n",blocksize);
free(in_buffer);
return NULL;
}
loop: until all the data are read from file
{
//read a block of data from a file to in_buffer
// do some data processing with in_buffer
//write the result to out_buffer
//memcpy out_buffer to list
}
free(in_buffer);
free(out_buffer);
return list;
......
----------------------------------------------------------------------------
-------------------------------------
I've debugged with gdb to see where the segmentation fault happens, it
is at free(in_buffer).
I've verified that the value for in_buffer after malloc() and before
free() is the same. Or in other words, in_buffer is a valid pointer
allocated by malloc.
And the SF only happens when the file is large, although block size
could be small.
Means that for both small (2KB) and large(5MB) files, block size are the
same. However, only large files could cause SF.
Could anyone please point me out what could possibly be the reason?
BTW, pls cc me when you reply, since I am not able to receive emails
from this list. Thanks a lot!
TIA!
Lei
-
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linux-c-programming" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
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^ permalink raw reply [flat|nested] 6+ messages in thread