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C programming : How does free know how much to free?
free() is called to deallocate the memory allocated by malloc() function call. From where does the free() find the information about the no. of bytes allocated by the malloc() function. I.e., how do you conform the no. of bytes allocated by the malloc() and where is this information stored.
-Surya
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So the question comes, that how the free() function know about the size of the block to deallocate? When we use the dynamic memory allocation techniques for memory allocations, then this is done in the actual heap section. It creates one word larger than the requested size. This extra word is used to store the size.
The free() function in C library allows you to release or deallocate the memory blocks which are previously allocated by calloc(), malloc() or realloc() functions. It frees up the memory blocks and returns the memory to heap. It helps freeing the memory in your program which will be available for later use.
In C, the library function malloc is used to allocate a block of memory on the heap. The program accesses this block of memory via a pointer that malloc returns. When the memory is no longer needed, the pointer is passed to free which deallocates the memory so that it can be used for other purposes.
For the case of malloc , the heap allocator stores a mapping of the original returned pointer, to relevant details needed for free ing the memory later.
Most implementations of C memory allocation functions will store accounting information for each block, either inline or separately.
One typical way (inline) is to actually allocate both a header and the memory you asked for, padded out to some minimum size. So for example, if you asked for 20 bytes, the system may allocate a 48-byte block:
The address then given to you is the address of the data area. Then, when you free the block, free will simply take the address you give it and, assuming you haven't stuffed up that address or the memory around it, check the accounting information immediately before it.
Keep in mind the size of the header and the padding are totally implementation defined (actually, the entire thing is implementation-defineda but the inline-accounting-info option is a common one).
The checksums and special markers that exist in the accounting information are often the cause of errors like "Memory arena corrupted" if you overwrite them. The padding (to make allocation more efficient) is why you can sometimes write a little bit beyond the end of your requested space without causing problems (still, don't do that, it's undefined behaviour and, just because it works sometimes, doesn't mean it's okay to do it).
a I've written implementations of malloc in embedded systems where you got 128 bytes no matter what you asked for (that was the size of the largest structure in the system) and a simple non-inline bit-mask was used to decide whether a 128-byte chunk was allocated or not.
Others I've developed had different pools for 16-byte chunks, 64-bytes chunks, 256-byte chunks and 1K chunks, again using a bitmask to reduce the overhead of the accounting information and to increase the speed of malloc and free (no need to coalesce adjacent free blocks), particularly important in the environment we were working in.
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This is implementation dependent. The heap stores that data in some manner that facilitates accessing it having a pointer returned by malloc() - for example, the block could store the number of bytes at the beginning and malloc() would return an offsetted pointer.
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