Thread: How much will free(x) free?

How does free(x) know when to stop?
I'm not sure if my question even makes sense, cause I don't know how free works (I've just learned that it needs to be used after we're done with anything that we malloc'ed).

If you malloc(some number of bytes), will the system put some kind of marker at the end of the box of memory it reserved, so free() knows how much memory was associated?

Thanks

Also, if you have a block of memory, and a pointer (call it ptr) to somewhere in the middle of that block, what happens if you free(ptr)?

[edit] silly typo, irrelevant though

Originally Posted by Jang

How does free(x) know when to stop?

The information is available to it.

Originally Posted by Jang

If you malloc(some number of bytes), will the system put some kind of marker at the end of the box of memory it reserved, so free() knows how much memory was associated?

The method is implementation defined. A marker at the end probably won't work very well though, since the same series of bits could appear anywhere in between.

Originally Posted by Jang

Also, if you have a block of memory, and a pointer (call it ptr) to somewhere in the middle of that block, what happens if you free(prt)?

Undefined behaviour. The program will tend to "work" when you test it, but will mysteriously fail when you demonstrate the program to the customer in front of the boss.

Originally Posted by laserlight

Undefined behaviour. The program will tend to "work" when you test it, but will mysteriously fail when you demonstrate the program to the customer in front of the boss.

Excellent!