c++ global object methods not linked

Question

I am experiencing a problem with a C++ project for an STM32 ARM micro. I created a class that implement a state machine. This class has some methods that are not called directly by other methods, but just by function pointers:

// Foo.h
class Foo
{
public:
    typedef void(Foo::*State)(void);
    State state;

    void init();
    virtual void poll();

    void state1();
    void state2();

    Foo();
    virtual ~Foo() {}
};

// Foo.cpp
#include "Foo.h"

#define ASSIGN_STATE(X,Y) \
X = &Foo::state ## Y;

void Foo::init()
{
    ASSIGN_STATE(state, 1);
}

void Foo::state1()
{
    ASSIGN_STATE(state, 2);
}

void Foo::state2()
{
    ;
}

void Foo::poll()
{
    (this->*state)();
}

Foo::Foo()
{
    this->init();
}

I instanciate an object of this class with global scope:

Foo foo;
int main()
{
    foo.init();
    while(1)
        foo.poll();
    return(1);
}

If I compile the project the methods that are not directly called by other methods (state1() and state2()) are not linked and the firmware crashes when it calls them.

If instead I instantiate them inside main(), the methods are linked and everything works correctly.

int main()
{
    Foo foo;
    foo.init();
    while(1)
        foo.poll();
    return(1);
}

Compiler and linker flags:

COMPILER_FLAGS = $(DEBUG_FLAGS) -mcpu=cortex-m3 -mthumb -Wall -mlittle-endian -MD -MP -MF $(DEPS)/$(@F:%.o=%.d) -fno-strict-aliasing -fsigned-char -ffunction-sections -fdata-sections $(DEFINES) $(INCLUDES)
CPPCOMPILER_FLAGS =  $(DEBUG_FLAGS) -mcpu=cortex-m3 -mthumb -Wall -mlittle-endian -MD -MP -MF $(DEPS)/$(@F:%.o=%.d) -fno-strict-aliasing -fsigned-char -ffunction-sections -fdata-sections -fno-rtti -fno-exceptions $(DEFINES) $(INCLUDES)
LINKER_FLAGS = -mcpu=cortex-m3 -mthumb -specs=nano.specs -lnosys -L$(LINKERSCRIPT_DIR) -T$(LINK_SCRIPT) -Wl,--gc-sections $(DEFINES)

The project is C/C++ mixed. I user the GCC ARM TOOLS toolchain to compile it, under the Windows 7 OS.

I tried with different compiler flags and different optimization options, but nothing changed. Any idea on why I get this behavior, or how I could investigate its causes?

Answer 1

What's happening is that part of your code is getting garbage-collected by the linker because it thinks it's not referenced anywhere. I've seen this happen with code only accessed from an ISR because gcc couldn't see any reference between the main code and the ISR.

When you specify -ffunction-sections and -fdata-sections an automatically named section is created for every function and data item in your project. Then, when it gets to the linker you've got -Wl,--gc-sections. This causes the linker to drop all unreferenced sections from the output. On an MCU this is a Very Good Thing because it minimises your program memory usage. Unfortunately, as I've described, the compiler is not infallible in its detection of unused code.

In the past I've hacked around this by declaring a pointer to the missing object inside code that's definitely OK and ensuring that I initialise it in the main code. The compiler sees the outgoing reference and the function/data doesn't get garbage collected.

You could probably also solve it by moving the data/function that's going missing to a section that never gets garbage collected using the gcc section attribute. For example, to move a function you could use __attribute__((section(".text"))) on it.