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    <p>Most implementations don't bother identifying those (relatively rare) cases where entire "blocks" (of whatever size suits the OS) have been freed and could be returned, but there are of course exceptions. For example, and I quote from <a href="http://en.wikipedia.org/wiki/Malloc#OpenBSD.27s_malloc" rel="noreferrer">the wikipedia page</a>, in OpenBSD:</p> <blockquote> <p>On a call to <code>free</code>, memory is released and unmapped from the process address space using munmap. This system is designed to improve security by taking advantage of the address space layout randomization and gap page features implemented as part of OpenBSD's <code>mmap</code> system call, and to detect use-after-free bugs—as a large memory allocation is completely unmapped after it is freed, further use causes a segmentation fault and termination of the program.</p> </blockquote> <p>Most systems are not as security-focused as OpenBSD, though.</p> <p>Knowing this, when I'm coding a long-running system that has a known-to-be-transitory requirement for a large amount of memory, I always try to <code>fork</code> the process: the parent then just waits for results from the child [[typically on a pipe]], the child does the computation (including memory allocation), returns the results [[on said pipe]], then terminates. This way, my long-running process won't be uselessly hogging memory during the long times between occasional "spikes" in its demand for memory. Other alternative strategies include switching to a custom memory allocator for such special requirements (C++ makes it reasonably easy, though languages with virtual machines underneath such as Java and Python typically don't).</p>
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    1. POWill malloc implementations return free-ed memory back to the system?
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    1. USAlex Martelli
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    1. COCan I use fork in multithreaded app? So I really CANT use fork.
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    2. CO@osgx: Yes you can can fork in a multithreaded application as long as you only use it to exec a new process. Well, actually "... the child process may only execute async-signal-safe operations until such time as one of the exec functions is called"
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      1. USZan Lynx
    3. CO@Zan: Where did you get that idea? `fork` is allowed in multi-threaded processes, and you can do whatever you like as long as you don't corrupt the state of your own synchronization objects by using it. `pthread_atfork` gives you the tools to avoid doing so.
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