make _sbrk() weak (for malloc() from external memory)

[bmcdonnell-ionx]

Description

• Type: Bug | Enhancement
• Priority: Minor

---

Bug

Target
LPC40xx

Toolchain:
GCC_ARM

mbed-os sha:
(182bbd5)

---

Enhancement

Suggested enhancement

In the file mbed-os/platform/mbed_retarget.cpp, please add the attribute MBED_WEAK (or equivalent) to the function definition _sbrk(), to allow the user to override it.

extern "C" MBED_WEAK caddr_t _sbrk(int incr) {
    // [snip...]
}

Reason to enhance or problem with existing solution

An approach sometimes used to access external memory in embedded systems is to have all stack memory use internal RAM, and all dynamic memory (e.g. malloc()) allocate memory from external RAM. This is especially useful during development, to at least temporarily skip over the often difficult task of editing linker files.

Embedded Artists' EALib takes this approach to enable access to an external SDRAM chip. In sdram.cpp, it defines/overrides the _sbrk() function, which is used by malloc(), (etc.), to manage the heap.

This worked in older versions of mbed (at least through rev 120), but I cannot build it in the latest mbed-os, due to _sbrk() already being defined in mbed_retarget.cpp. Please make the mbed-os _sbrk() weak, to allow EALib to use the SDRAM as intended.

Pros

Described above.

Cons

Problematic if another library also defines _sbrk() weakly.

[0xc0170]

@c1728p9 @SenRamakri

[bmcdonnell-ionx]

#1819 was trying to do the same thing for a different platform, but it takes a different approach. I don't think it's the right approach here, because it's making mbed-os always use the external memory for dynamic memory for that platform, whereas I want to give the user the option to do so.

But I give details here for reference.

---

Upon closing that issue, @ccli8 said:

I overwrite _sbrk with conditional compile in retarget.cpp. This modification has merged and so I close the issue.

I think he's referring to these commits, in particular the changes to platform/retarget.cpp:
b954780
dfc3240

(EDIT: I think I'm wrong about which commits.)

[bmcdonnell-ionx]

PR merged.