malloc() is non-reentrant but thread-safe?

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Malloc thread-safe?

I am not a bit confused while I am reading "The Linux Programming Interface".

From the book it says that malloc is non-reentrant since it manipulates the global linked list data structure but is made thread-safe by using mutex.

I am a bit confused about this: since it's thread-safe with using mutex and thus can be invoked by more than one threads at the same time, why it isn't a reentrant function? (if we say that reentrant means that it can be invoked by more than one caller at the same time)

Another question is that, since malloc is thread-safe, can we put it in a signal handler? I think the answer is yes but I am not sure since according to this book, it says that only a reentrant or async-signal-safe function can be put in the signal handler.

Can anyone explain this to me?

Is the mutex included in malloc's implementation? – Bingo Jan 15 '12 at 12:33 — Bingo, Jan 15 '12 at 12:33

score 25 · Accepted Answer · answered Jan 15 '12 at 12:34

if we say that reentrant means that it can be invoked by more than one caller at the same time

Wrong. Reentrant means you can interrupt it and call it again before the previous incarnation ended. Imagine malloc looks like this:

lock(mutex);

/* Stuff. */

unlock(mutex):

What happens if it is interrupted in the middle, before unlocking and someone else calls malloc ?

The first context cannot continue until the second one is done
The second context blocks on the mutex and cannot continue until the first unlocks the mutex

That's a deadlock.

Another question is that, since malloc is thread-safe, can we put it in a signal handler? I think the answer is yes

Wrong again. See the example above. Imagine the main program is doing a malloc and before the function actually ends your handler calls malloc.

@kai: Note that the problem (confusing these concepts) is exacerbated by the fact that older versions of POSIX (pre-2008) wrongly used the word reentrant to mean thread-safe... — R.. GitHub STOP HELPING ICE, Jan 15 '12 at 15:29

score 2 · Answer 2 · answered Jan 15 '12 at 12:52

2

Reentrancy and thread-safety are two different concepts. A reentrant function can be non-thread -safe and a thread-safe function can be non-reentrant.

Library functions in C are not guaranteed to be reentrant and only reentrant functions can be called from signal handlers.

answered Jan 15 '12 at 12:52

ouah

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`A reentrant function can be non-thread-safe` ... Can it? – Iulius Curt May 23 '12 at 22:40
5

@iuliux: Yes. On some machines, static variables are faster than stack-based local variables. Code which on entry copies its static variables to the stack, uses the static variables, and then restores the variables from the stack will be re-entrant but not thread-safe. – supercat Jul 17 '13 at 22:33

malloc() is non-reentrant but thread-safe?

2 Answers2