How does free know how much to free?

c size pointers free
Joshua Cheek picture Joshua Cheek · Oct 5, 2009 · Viewed 68.1k times · Source

In C programming, you can pass any kind of pointer you like as an argument to free, how does it know the size of the allocated memory to free? Whenever I pass a pointer to some function, I have to also pass the size (ie an array of 10 elements needs to receive 10 as a parameter to know the size of the array), but I do not have to pass the size to the free function. Why not, and can I use this same technique in my own functions to save me from needing to cart around the extra variable of the array's length?

Answer

Gary McGill picture Gary McGill · Oct 5, 2009

When you call malloc(), you specify the amount of memory to allocate. The amount of memory actually used is slightly more than this, and includes extra information that records (at least) how big the block is. You can't (reliably) access that other information - and nor should you :-).

When you call free(), it simply looks at the extra information to find out how big the block is.

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