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malloc/free question

Started by WildWest Dec 29, 2002 at 9:04 AM 5 replies 250+ views

Original Post

Dec 29, 2002 at 9:04 AM

Hi! I have a following malloc/free problem: This seems to work:

//allocating memory n+1 doubles
double *pNew = (double*) malloc((n+1)*sizeof(double));

//free memory
free(pNew);

This seems not to free all memory:

//allocating memory n+1 doubles
double *pNew = (double*) malloc((n+1)*sizeof(double));

//temporary pointer
double *pTemp = pNew;

//free memory
free(pTemp);

My question is: How does free know, how much memory was allcocated? Or can I use only use example 1 to free the allocated memory? Does example 2 only free the memory for one double where pTemp points to or the memory block, which was allocated before? Thanks for your help! The Wild Wild West - Desperado!

civguy

Dec 29, 2002 at 9:17 AM

quote:
How does free know, how much memory was allcocated?

When you allocate memory, the amount of bytes you allocated is stored into memory too. But this is hidden from you.

quote:
can I use only use example 1 to free the allocated memory?

Example 2 frees all the memory too. When you use free(), you pass a pointer to the function. It can''t possibly distinguish between pTemp and pNew, since they have the same value.

Alpha_ProgDes

Dec 29, 2002 at 9:19 AM

well you are going to have a memory leak because:

1: pTemp and pNew now point to the same memory
--> double *pTemp = pNew;

pTemp-->-------[memory]---------<---pNew

2: now you deleted the memory pTemp points to
--> free(pTemp)

pTemp-->(nothing)

3: since you did that you have a dangling pointer (pNew)

pTemp-->(nothing) (?) <------------pNew

so if you want to solve that .... make pNew = NULL, then free pTemp.
all should be good.

civguy

Dec 29, 2002 at 11:01 AM

Alpha_ProgDes, not quite. More like this:

pTemp-->-------[valid memory]---------<---pNew

free(pTemp);

pTemp-->-------[invalid memory]---------<---pNew

They both still point to same memory location (which is now invalid). But dangling pointers aren''t memory leaks. Memory leak is if someone forgets to free allocated memory.

Alpha_ProgDes

Dec 29, 2002 at 11:04 AM

yeah you''re right!!!
thanks!!!

note: i wonder he said that the second example "didn''t free all the memory?" it seems from what you posted it does. hopefully he got figured out.

Qw3r7yU10p!

Dec 30, 2002 at 3:40 AM

wildwest: how do you know it doesn''t free all the memory?

WildWest

Dec 30, 2002 at 6:34 AM

petewood: I had a pointer which was not initialized to anything.
But when I read out the content to which the pointer pointed to I got something which should have been freed before.

But it seems you all are right and both examples work.
I found the error somewhere else.

The Wild Wild West - Desperado!

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