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how malloc works...

Started by Silverbolter 3 replies 3.1k views
Original Post
Silverbolter
Silverbolter
Hi, Just reading about memory allocators right now here: http://gee.cs.oswego.edu/dl/html/malloc.html . I understand that when you request say sizeof(char) with malloc(), on some machines the actual bytes returned isn''t the exact sizeof(char), but instead aligned to the machine''s word size. My question is, since malloc() will need extra bytes to do housekeeping, some information relating to the size of the block, whether its in use or not, etc. will need to be placed near the returned block of memory. However I have a question as to where this information is placed in relation to this returned block? Before? After? If its before (which means right at the pointer) then when you dereference the pointer you will get that housekeeping information too, which isn''t really what you want. If it''s after, then you don''t really know how far you must look? If anyone can point me to some articles or answer these questions it would be great. Thanks.
Fruny
Fruny
It indeed cannot be stored after the block. It can, on the other hand, be stored at the beginning of the block. malloc() would simply not return a pointer to the very beginning of the ''raw'' memory block, but a pointer offset by the size of the housekeeping information block. That way, you don''t *get* the housekeeping data, but the library can find it easily.


“Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.” — Brian W. Kernighan (C programming language co-inventor)
"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan
Sneftel
Sneftel
The housekeeping information is generally stored _before_ the block. Otherwise, free() would have no idea where to look for it, since it doesn't know the size of the block until after it already has the housekeeping information.

Let's say that malloc() has been asked to allocate 16 bytes of memory. Let's also say it uses 4 bytes for housekeeping info, placed immediately before the block. So that's a 20-byte block in all, with 16 bytes meant for use by the application. malloc() decides to put those 20 bytes at the location 100. So the block includes bytes 100 to 119.

Now, does malloc return "100"? No. It returns "104". It doesn't tell the program about the space for the housekeeping information; as the program never accesses bytes 100-103. But when "104" is later passed into free(), it knows to subtract 4 to find the actual location of the housekeeping information.

EDIT: Noooo! one second!

"Sneftel is correct, if rather vulgar." --Flarelocke

[edited by - sneftel on January 12, 2004 2:04:52 AM]
Fruny
Fruny
quote:
Original post by Sneftel
EDIT: Noooo! one second!


BOOYA !

“Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.” — Brian W. Kernighan (C programming language co-inventor)
"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan
Silverbolter
Silverbolter
Wow thanks for your replies! Makes lots of sense now.

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