GDB Debugger - "malloc" not defined. Trying to assign C array with debugger - c

Question

I've got some software compiled to run on an embedded NRF24 target, using a gcc-arm-none-eabi toolchain from here (a custom one that provides gdb with support for python3) . I'm trying essentially, malloc an array from the GDB debugger console at runtime, then fill it with elements that I provide.
I have a pointer defined in a .c file like: static float32_t *array;.
I want to then call a cmd like: call (void*) malloc(num_of_elements*sizeof(float32_t)) from inside the GDB console to allocate an array at runtime, and then fill it with elements with something like maybe: call (void*) memcpy(array, {var1, var2... var n}, n)
My issue is the GDB debugger cannot find the malloc stdlib function. If I do something like:
break malloc
Function "malloc" not defined.
Make breakpoint pending on future shared library load? (y or [n]) [answered N; input not from terminal]
It can't find this function, although it is fine with finding <string.h> fns, like memcpy for example and I can't quite work out why this is.
I have a feeling it could be something to do with linking, the program is built with a Makefile, the flags towards the end may be of interest:
LIB_FILES += \
$(SDK_ROOT)/components/toolchain/cmsis/dsp/GCC/libarm_cortexM4lf_math.a \
# Optimization flags
OPT = -O0 -g3
# Uncomment the line below to enable link time optimization
#OPT += -flto
# C flags common to all targets
CFLAGS += $(OPT)
CFLAGS += -DBOARD_PCA10056
CFLAGS += -DARM_MATH_CM4
CFLAGS += -DBSP_DEFINES_ONLY
CFLAGS += -DCONFIG_GPIO_AS_PINRESET
CFLAGS += -DFLOAT_ABI_HARD
CFLAGS += -DNRF52840_XXAA
CFLAGS += -mcpu=cortex-m4
CFLAGS += -mthumb -mabi=aapcs
CFLAGS += -mfloat-abi=hard -mfpu=fpv4-sp-d16
# keep every function in a separate section, this allows linker to discard unused ones
CFLAGS += -ffunction-sections -fdata-sections -fno-strict-aliasing
CFLAGS += -fno-builtin -fshort-enums
CFLAGS += -DDEV8_PINOUT
CFLAGS += -DNUM_FLASH_BLOCKS=128
CFLAGS += -DDEBUG
CFLAGS += -DNRF_LOG_ENABLED=1
CFLAGS += -DNRF_LOG_BACKEND_UART_ENABLED=1
# C++ flags common to all targets
CXXFLAGS += $(OPT)
# Assembler flags common to all targets
ASMFLAGS += $(OPT)
ASMFLAGS += -mcpu=cortex-m4
ASMFLAGS += -mthumb -mabi=aapcs
ASMFLAGS += -mfloat-abi=hard -mfpu=fpv4-sp-d16
ASMFLAGS += -DBOARD_PCA10056
ASMFLAGS += -DBSP_DEFINES_ONLY
ASMFLAGS += -DCONFIG_GPIO_AS_PINRESET
ASMFLAGS += -DFLOAT_ABI_HARD
ASMFLAGS += -DNRF52840_XXAA
ASMFLAGS += -DARM_MATH_CM4
# Linker flags
LDFLAGS += $(OPT)
LDFLAGS += -mthumb -mabi=aapcs -L$(SDK_ROOT)/modules/nrfx/mdk -T$(LINKER_SCRIPT)
LDFLAGS += -mcpu=cortex-m4
LDFLAGS += -mfloat-abi=hard -mfpu=fpv4-sp-d16
# let linker dump unused sections
LDFLAGS += -Wl,--gc-sections
# use newlib in nano version
LDFLAGS += --specs=nano.specs
LDFLAGS += -Wl,--print-memory-usage
nrf52840_xxaa: CFLAGS += -D__HEAP_SIZE=8192
nrf52840_xxaa: CFLAGS += -D__STACK_SIZE=8192
nrf52840_xxaa: ASMFLAGS += -D__HEAP_SIZE=8192
nrf52840_xxaa: ASMFLAGS += -D__STACK_SIZE=8192
# Add standard libraries at the very end of the linker input, after all objects
# that may need symbols provided by these libraries.
LIB_FILES += -lc -lnosys -lm
To debug I'm using GDB with a Jlink server type setup.

Answer 1

Your code needs to explicitly reference the symbol to force it to link, and you need to prevent link optimisation from removing unused references. Rather then doing a dummy call with potential unwanted side effects, you can simply refer to the symbol via a function pointer instantiation thus:
void* (*volatile force_malloc_link)(size_t) = &malloc ;
Or more simply since you will not actually invoke the function through the pointer:
volatile void* force_link_malloc = &malloc ;
You could make it generic with a macro for any symbol you wish to link:
#define FORCE_LINK( sym ) volatile void* force_link_ ## sym = &sym
You can also force the entire library to be linked (if you have the space) - How to force gcc to link an unused static library. One of the answers there explains how you can also unpack the static library and link individual object files.

Answer 2

It can't find this function
The function may not be linked into your binary.
Does your binary call malloc elsewhere?
Do you link against libc.so or libc.a?
Does nm a.out | grep ' malloc' find it?
it is fine with finding <string.h> fns, like memcpy
If your binary calls memcpy and you link against libc.a, then memcpy implementation will be linked in. Using the same nm command from above will show that the memcpy symbol is present and malloc is not.
If you want to call malloc at runtime, you need to make sure it's linked in. One way to achieve this is to add:
const char *argv0;
int main(int argc, char *argv[])
{
argv0 = strdup(argv[0]); // This should guarantee that malloc is linked in.
// rest of the program
}