I'm reading the chapter on the Unix System Interface from K&R The C Programming Language, and I'm trying to understand how malloc, morecore, and free interact with the free list.
What happens to the header of a free block when the block is allocated? Does it "dissolve" into the free space returned to the caller of malloc?
Malloc returns a pointer p
p - some amount is where malloc "hides" all the information needed for when you call free()
p + some amount is where you store your data.
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hello evrybody,
I'm new here, I'm from Italy and i learn to program in C some years ago.
After a long time without programming I'm approaching C another time.
I have some trouble understanding the right way to use free().
There is a structure like this:
and in the main a pointer to this structure like this:Code:struct item_t{ int value; char *name; }
if i makeCode:item_t *values;
and in some other function getting as imput *values (or in the same main) I will allocate a name for values[i].nameCode:values=(item_t *) malloc (10*sizeof(item_t));
do i free evrything just with the command free(values) or not?
Sorry for my bad english and if I do not write in a good way my trouble, I hope you undestrand what I mean.
Many thanks
ZaC
Yeah, you free everything. It is a good question though, not very clear in the beggining.
As a rule, you free() EXACTLY what you malloc.
So just lookup whatever malloc you have and put the appropriate free(). You might also want to free() int the opposite order you malloc. That will save you some error in some memory allocation situations.
Consider the following. You want to malloc memory for each array of characters (string) that *name will point at:
You do the opposite order for obvious reasons. If you de-allocate values then values[i].name won't be a valid accessed pointer because values would have already be freedCode:MALLOC: values=(item_t *) malloc (10*sizeof(item_t)); for (int i=0; i < 10; ++i) values[i].name = (char *)malloc(100*sizeof(char)); FREE: for (int i=0; i < 10; ++i) free(values[i].name); //opposite order, first the last malloc free(values); //opposite order, second the first malloc //Two mallocs, two frees
Last edited by C_ntua; 06-20-2008 at 06:03 PM.
Thanks for the fast replay.
Sorry but I still do not understand if with free(values) I'll free values[i].name too (even if it was "mallocated" separately).
Last edited by ZaC; 06-20-2008 at 06:12 PM.
Except you don't. free(values) frees ONLY the memory allocated with the malloc call of values -- it does NOT free values[i] for any i.Originally Posted by ZaC
Thanks I replayed before C_ntua edited his post with the full explaing
EDIT: I was writting more explaining...but, nevermind, i thought you asked for values[i].value
Last edited by C_ntua; 06-20-2008 at 06:17 PM.
Yes thanks
Thank you very much
another question
Is possible to set free just portion of values?
E.G.: i'm tring to load in values n-item_t from a file. For some problem in file reading I loaded only n-5 items. May i free from n-4 to n the array values? How?
EDIT: perhaps freeing the item in excess is not important.
Last edited by ZaC; 06-21-2008 at 07:47 AM.
Hmm, good question.
You use realloc() http://www.cplusplus.com/reference/c...b/realloc.html
You can free values[n-4], values[n-3], values[n-2], and values[n-1] separately, sure (just modify the for loop to start at n-4). You just have to remember that you did it, so that you don't free them again later.
thanks tabstop and C_ntua
realloc is what i was looking for
for(i=n-4;i<=n;i++) free(values[i]);
realloc do what i want, if i understand well
PS: always sorry for my english
