how can I prevent arm-none-eabi compiler generating main symbol

Question

I'm using arm-none-eabi to compile source file. after compiling and generating elf file. I got the following symbols using nm command

00021da8 T ISR_Init
         U main
         U malloc
010008b0 D MASTER_AHB_MAP

I'm using gdb to debug, but I have problem with main symbol which is not defined. gdb generate following error :

Function "main" not defined.

when I change my entry point to main, it works fine. I'm developing bare metal program, so I didn't define main anywhere in my program.

I linked my program with those following libraries

(GNU_ARM_TOOL)/lib/gcc/arm-none-eabi/4.8.4/armv7-ar/thumb/fpu
(GNU_ARM_TOOL)/arm-none-eabi/lib/armv7-ar/thumb/fpu

for my understanding, the main symbol is generated from one of the above libraries. my question is how can I or how can I avoid the compiler generating the undefined symbol main, or at least delete the undefined main symbol in the final elf file to avoid gdb error.

Answer 1

To avoid gcc generating references to main, link your program with the -nostdlib gcc option:

-nostdlib: Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See Interfacing to GCC Output, for more discussion of libgcc.a.) In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines.

To avoid gcc generating calls to memcmp, memset, memcpy etc compile with gcc's -ffreestanding option. Or use "function attributes" syntax, e.g.:

/* defined in the linker script gcc.ld */
extern int __etext, __data_start__, __data_end__, __bss_start__, __bss_end__;

/* make gcc not translate the data copy loop into a memcpy() call
 *
 * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888
 * Note that just passing optimize("freestanding", "no-builtin")
 * as a function attribute here doesn't work on
 * gcc-arm-embedded 2014 (gcc 4.9.3) */
__attribute__((optimize("freestanding", "no-builtin",
                        "no-tree-loop-distribute-patterns")))
void Reset_Handler()
{
        int *src, *dst;
        for (src = &__etext, dst = &__data_start__;
                        dst != &__data_end__;
                        src++, dst++)
                *dst = *src;
        for (dst = &__bss_start__; dst < &__bss_end__; dst++)
            *dst = 0;

        main();
}