Is malloc thread-safe?

This is quite old question and I want to bring freshness according current state of things.

Yes, currently malloc() is thread-safe.

From the GNU C Library Reference Manual of glibc-2.20 [released 2014-09-07]:

void * malloc (size_t size)

Preliminary: MT-Safe | ...

... 1.2.2.1 POSIX Safety Concepts:

... MT-Safe or Thread-Safe functions are safe to call in the presence of other threads. MT, in MT-Safe, stands for Multi Thread.

Being MT-Safe does not imply a function is atomic, nor that it uses any of the memory synchronization mechanisms POSIX exposes to users. It is even possible that calling MT-Safe functions in sequence does not yield an MT-Safe combination. For example, having a thread call two MT-Safe functions one right after the other does not guarantee behavior equivalent to atomic execution of a combination of both functions, since concurrent calls in other threads may interfere in a destructive way.

Whole-program optimizations that could inline functions across library interfaces may expose unsafe reordering, and so performing inlining across the GNU C Library interface is not recommended. The documented MT-Safety status is not guaranteed underwhole-program optimization. However, functions defined in user-visible headers are designed to be safe for inlining.

I read somewhere that if you compile with -pthread, malloc becomes thread safe. I´m pretty sure its implementation dependant though, since malloc is ANSI C and threads are not.

If we are talking gcc:

Compile and link with -pthread and malloc() will be thread-safe, on x86 and AMD64.

http://groups.google.com/group/comp.lang.c.moderated/browse_thread/thread/2431a99b9bdcef11/ea800579e40f7fa4

Another opinion, more insightful

{malloc, calloc, realloc, free, posix_memalign} of glibc-2.2+ are thread safe

http://linux.derkeiler.com/Newsgroups/comp.os.linux.development.apps/2005-07/0323.html

Question: "is malloc reentrant"?
Answer: no, it is not. Here is one definition of what makes a routine reentrant.

None of the common versions of malloc allow you to re-enter it (e.g. from a signal handler). Note that a reentrant routine may not use locks, and almost all malloc versions in existence do use locks (which makes them thread-safe), or global/static variables (which makes them thread-unsafe and non-reentrant).

All the answers so far answer "is malloc thread-safe?", which is an entirely different question. To that question the answer is it depends on your runtime library, and possibly on the compiler flags you use. On any modern UNIX, you'll get a thread-safe malloc by default. On Windows, use /MT, /MTd, /MD or /MDd flags to get thread-safe runtime library.