How to prevent inclusion of C library destructors and atexit()?

Question

Using arm-none-eabi-gcc for Cortex-M4 (baremetal application), the code for malloc is also emitted even though I never use malloc in my code.

S

Answer 1

Probably you need -fno-use-cxa-atexit argument for compiler.

Look on this simple example to get working C++ code on pure bare-metal: https://github.com/cortexm/baremetal

Answer 2

-nostartfiles and -nostdlib worked for me in a similar context.

Answer 3

So here's the workaround that I'm using. Not perfect, but good enough.

Using the --wrap option of ld, I can provide my own definition for atexit that doesn't do anything.

int __wrap_atexit(void __attribute__((unused)) (*function)(void)) {
    return -1;
}

and then link with --wrap=atexit option.

Now, atexit does not call any other functions and the -ffunction-sections and --gc-sections options ensure that malloc is not included in the output file.

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Additionally, to enforce that malloc isn't included, a --wrap=malloc option can be passed while linking without defining __wrap_malloc anywhere. This will fail with a link error if some other function happens to use malloc.

Answer 4

In an environment with limited memory such as the Cortex M4, another option is to use newlib-nano. It provides a __register_exitproc() that is weakly linked. Therefore it is easy to override with your own empty function that avoids calling malloc(). This will have the additional benefit of removing __call_exitprocs() from your binary as well.

• Add the flag --specs=nano.specs to your compiler and linker options.
• Create the following function somewhere in your compiled code: void __register_exitproc(void) { }

Note that this will prevent destructors being called for static instances of classes. This seems appropriate in your example.

See the comments in the newlib source for more details.