Memory allocation with malloc

[PSN]

Hello everyone,

int main()
{
char *pChar;
pChar = (char *)malloc(16);
memcpy(pChar, "AAAAAAAAAAAAAA A", 16);
printf ("***%s***\n ", pChar);

realloc(pChar, 28);
memcpy(pChar, "BBBBBBBBBBBBBB BBBBBBBBBBBBB", 28);
printf ("***%s***\n ", pChar);

free(pChar);
return 0;
}

Can someone please explain me why does this crash. what am i doing
wrong ??

Thank you for youri time

Prakash

[PSN]

Re: Memory allocation with malloc

Just to add a little more ...

1. Both the printfs print the strings correctly.
2. The crash occurs in the debug function, _CrtIsValidHeap Pointer,
when the memory is freed.

Thanks again.

On Nov 16, 12:27 am, PSN <prakash...@gma il.comwrote:

Hello everyone,
>
int main()
{
char *pChar;
pChar = (char *)malloc(16);
memcpy(pChar, "AAAAAAAAAAAAAA A", 16);
printf ("***%s***\n ", pChar);
>
realloc(pChar, 28);
memcpy(pChar, "BBBBBBBBBBBBBB BBBBBBBBBBBBB", 28);
printf ("***%s***\n ", pChar);
>
free(pChar);
return 0;
>
}
>
Can someone please explain me why does this crash. what am i doing
wrong ??
>
Thank you for youri time
>
Prakash

[peter koch]

Re: Memory allocation with malloc

On 16 Nov., 00:27, PSN <prakash...@gma il.comwrote:

Hello everyone,
>
int main()
{
char *pChar;
pChar = (char *)malloc(16);
memcpy(pChar, "AAAAAAAAAAAAAA A", 16);
printf ("***%s***\n ", pChar);
>
realloc(pChar, 28);
memcpy(pChar, "BBBBBBBBBBBBBB BBBBBBBBBBBBB", 28);
printf ("***%s***\n ", pChar);
>
free(pChar);
return 0;
>
}
>
Can someone please explain me why does this crash. what am i doing
wrong ??

Can you explain why you use malloc when you have the opportunity to
use std::string or std::vector (assuming you program in C++ as you
would otherwise be off-topic)?

realloc often allocates a new buffer, copies the content and frees the
old buffer - e.g.

void* realloc(void* ptr,size_t new_size)
{
void* new_area = malloc(new_size );
memcpy(new_area ,ptr,size_alloc ated(ptr)); // size_allocated is not
an official function
free(ptr);
return new_area;
}

And you forget to update pChar with the result of the realloc call.

/Peter

>
Thank you for youri time
>
Prakash

[Jim Langston]

Re: Memory allocation with malloc


"PSN" <prakash437@gma il.comwrote in message
news:55029d6d-8a06-4d0d-a403-fb75aea4b67e@w3 4g2000hsg.googl egroups.com...

Hello everyone,
>
int main()
{
char *pChar;
pChar = (char *)malloc(16);
memcpy(pChar, "AAAAAAAAAAAAAA A", 16);
printf ("***%s***\n ", pChar);
>
realloc(pChar, 28);

pChar = realloc(pChar, 28);

memcpy(pChar, "BBBBBBBBBBBBBB BBBBBBBBBBBBB", 28);
printf ("***%s***\n ", pChar);
>
free(pChar);
return 0;
}
>
Can someone please explain me why does this crash. what am i doing
wrong ??

Shown is the fix, but it would be better to use std::string or std::vector.

[Ron Natalie]

Re: Memory allocation with malloc

PSN wrote:

Hello everyone,
>
int main()
{
char *pChar;
pChar = (char *)malloc(16);

Malloc is not declared here.

memcpy(pChar, "AAAAAAAAAAAAAA A", 16);

you don't check to see if malloc fails.

printf ("***%s***\n ", pChar);

If I counted right above, You have exactly 16 A's there. Printf
%s expects a null-terminated string which you didn't provide.

>
realloc(pChar, 28);

Ditto.

memcpy(pChar, "BBBBBBBBBBBBBB BBBBBBBBBBBBB", 28);
printf ("***%s***\n ", pChar);

I'm not counting the B's but I bet you have the same problem.

>
free(pChar);
return 0;

[Peter]

Re: Memory allocation with malloc

On Nov 15, 3:27 pm, PSN <prakash...@gma il.comwrote:

Hello everyone,

use operators new and delete.
There is no need anymore to use malloc()/free().
The advantage of using new and delete is that you get error handling
via exceptions.
Of course you will have to write at least one try-catch-block.

[Jim Langston]

Re: Memory allocation with malloc

"Ron Natalie" <ron@spamcop.ne twrote in message
news:473cdb42$0 $802$9a6e19ea@n ews.newshosting .com...

PSN wrote:

>Hello everyone,
>>
>int main()
>{
>char *pChar;
>pChar = (char *)malloc(16);

>
Malloc is not declared here.
>

>memcpy(pChar , "AAAAAAAAAAAAAA A", 16);

>
you don't check to see if malloc fails.

>printf ("***%s***\n ", pChar);

>
If I counted right above, You have exactly 16 A's there. Printf
%s expects a null-terminated string which you didn't provide.

You counted wrong. he has 15 A's there.

>realloc(pCha r, 28);

Ditto.
>

>memcpy(pChar , "BBBBBBBBBBBBBB BBBBBBBBBBBBB", 28);
>printf ("***%s***\n ", pChar);

I'm not counting the B's but I bet you have the same problem.

27 B's That was the first thing I had looked at in the code. He's just not
assiging the pointer returned from realloc anywhere.

>free(pChar);
>return 0;

[Default User]

Re: Memory allocation with malloc - TPA

PSN wrote:

Just to add a little more ...

Please don't top-post. Your replies belong following or interspersed
with properly trimmed quotes. See the majority of other posts in the
newsgroup, or the group FAQ list:
<http://www.parashift.c om/c++-faq-lite/how-to-post.html>