Dynamic allocation in uClinux

Question

I'm new to embedded development, and the big differences I see between traditional Linux and uClinux is that uClinux lacks the MMU.

From this article:

Without VM, each process must be located at a place in memory where it can be run. In the simplest case, this area of memory must be contiguous. Generally, it cannot be expanded as there may be other processes above and below it. This means that a process in uClinux cannot increase the size of its available memory at runtime as a traditional Linux process would.

To me, this sounds like all data must reside on the stack, and that heap allocation is impossible, meaning malloc() and/or "new" are out of the question... is that accurate? Perhaps there are techniques/libraries which allow for managing a "static heap" (i.e. a stack based area from which "dynamic" allocations can be requested)?

Or am I over thinking it? Or over simplifying it?

Answer 1

Under regular Linux, the programmer does not need to deal with physical resources. The kernel takes care of this, and a user space process sees only its own address space. As the stack grows, or malloc-type requests are made, the kernel will map free memory into the process's virtual address space.

In uClinux, the programmer must be more concerned with physical memory. The MMU and VM are not available, and all address space is shared with the kernel. When a user space program is loaded, the process is allocated physical memory pages for the text, stack, and variables. The process's program counter, stack pointer, and data/bss table pointers are set to physical memory addresses. Heap allocations (via malloc-type calls) are made from the same pool.

You will not have to get rid of heap allocation in programs. You will need to be concerned with some new issues. Since the stack cannot grow via virtual memory, you must size it correctly during linking to prevent stack overflows. Memory fragmentation becomes an issue because there's no MMU to consolidate smaller free pages. Errant pointers become more dangerous because they can now cause unintended writes to anywhere in physical memory.

Answer 2

It's been a while since I've worked with uCLinux (it was before it was integrated into the main tree), but I thought malloc was still available as part of the c library. There was a lot higher chance of doing Very Bad Things (tm) in memory since the heap wasn't isolated, but it was possible.

Answer 3

yes you can use malloc in user space applications on uclinux ,but then you have to increase the size of stack of user space application(before running the program cause stack size would be static),so that when malloc runs it will get the space it needs. for e.g. uclinux on arm-cortex arm toolchain provides command to find and change size of stack used by binary of user application then you can tranfer it to your embedded system and run ----- > arm-uclinuxeabi-flthdr