Under what circumstances can malloc return NULL?

Question

It has never happened to me, and I\'ve programming for years now.

Can someone give me an example of a non-trivial program in which malloc will actually

Answer 1

Yes. Malloc will return NULL when the kernel/system lib are certain that no memory can be allocated.

The reason you typically don't see this on modern machines is that Malloc doesn't really allocate memory, but rather it requests some “virtual address space” be reserved for your program so you might write in it. Kernels such as modern Linux actually over commit, that is they let you allocate more memory than your system can actually provide (swap + RAM) as long as it all fits in the address space of the system (typically 48bits on 64bit platforms, IIRC). Thus on these systems you will probably trigger an OOM killer before you will trigger a return of a NULL pointer. A good example is a 512MB RAM in a 32bit machine: it's trivial to write a C program that will be eaten by the OOM killer because of it trying to malloc all available RAM + swap.

(Overcomitting can be disabled at compile time on Linux, so it depends on the build options whether or not a given Linux kernel will overcommit. However, stock desktop distro kernels do it.)