About thread safety in malloc and free

Question

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Malloc thread-safe?

I heard that glibc malloc() was not thread safe, since several threads of a process calling malloc() simultaneously will lead to undefined behaviour. And my question is if a thread calls free() will another thread is calling malloc(), will this lead to undefined behaviour as well?

Answer 1

If you link with -pthreads, malloc() will be threadsafe in glibc.

Without that, the linker doesn't link in a threadsafe malloc, which will lead to undefined behavior.

Answer 2

Check here http://stackoverflow.com/questions/855763/malloc-thread-safe

Answer 3

It really depends on the memory allocator you're using, however, I think by default, malloc and free are non-reentrant as they maintain the list of blocks of memory in a static list.

This could lead to complications if you're malloc'ing and freeing simultaneously.

I know that ptmalloc, however, is threadsafe, so you could use that instead.

These links were also useful:

• http://www.ibm.com/developerworks/linux/library/l-reent.html
• http://kernelstudy.net/2009/01/07/malloc-and-free-are-thread-safe/

Answer 4

It depends upon your glibc implementation. A simple "man malloc" on your system might tell you. In general if you tell the compiler that you will be using threads then it will link in a thread safe version of the c runtime library including a thread-safe malloc().