Is malloc thread-safe?

Question

Is the malloc() function re-entrant?

Answer 1

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.

Answer 2

No, it is not.

http://coding.derkeiler.com/Archive/C_CPP/comp.lang.c/2007-07/msg02658.html

Answer 3

Yes, under POSIX.1-2008 malloc is thread-safe.

2.9.1 Thread-Safety

All functions defined by this volume of POSIX.1-2008 shall be thread-safe, except that the following functions1 need not be thread-safe.

[ a list of functions that does not contain malloc ]

Answer 4

malloc and free are not reentrant, because they use a static data structure which records what memory blocks are free. As a result, no library functions that allocate or free memory are reentrant.

Answer 5

It depends on which implementation of the C runtime library you're using. If you're using MSVC for example then there's a compiler option which lets you specify which version of the library you want to build with (i.e. a run-time library that supports multi-threading by being tread-safe, or not).