Is the C++ `free` function thread-safe?

That is, if my C++ application allocates memory in one thread using malloc, will free开发者_开发问答 successfully de-allocate the memory, if called from another thread, or can I expect it to throw an exception? Both threads belong to the same process. I am using Visual Studio 2008. Thanks.

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The current standard doesn't make any guarantees about threads. On most implementations, malloc and free may be called from different threads. Visual C++ heap code also serializes access to heaps, so you should be fine.

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If malloc and free are properly synchronized, deallocating memory in another thread is perfectly fine and safe. Moreover, this claim is correct according to C++0x standard. As @Ashot mentioned, the current C++03 standard deals only with singlethreaded execution model

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It's thread-safe in that sense (the malloc pool is process-global, not thread-local), but not in the sense of behaving sanely if called while another thread is calling malloc() or free().

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The current C++ standard doesn't even know about threads. So in terms of standard I don't think you can tell whether it is ok or not. However all threads in a program share the same address space, so it must be Ok to free object in other thread.

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If the C compiler and OS support threads then it will be safe.

If your compiler is on Windows then you want to make sure you are linking with the multi-threaded runtime libraries. Or if your compiler supports a -pthread option or something similar then it's safe also.

If the compiler does have a flag like -pthread then don't assume things are thread-safe unless you use that flag. Using the flag will link in different libraries and set different preprocessor macros. It's entirely possible that a whole different thread-safe C runtime is linked in when that flag is given.