Re: [gentoo-embedded] gcc question - entry point problem in small, static app

[Arkadi Shishlov]

On Mon, 30 Mar 2009 11:15:30 +0100 Martin Guy <martinwguy@×××××.it> wrote:
> On 3/30/09, Christopher Friedt <chrisfriedt@×××××.com> wrote:
> >  I'm writing code in C for a very small static binary, and the code
> >  doesn't use any headers or rely on any other libraries.
>
> > I've noticed that there is a lot of static libc junk built into my
> > binary build my program with options like -nostdlib -nodefaultlibs
> >  -nostartfiles, I get an error that says
> >
> >  warning: cannot find entry symbol _start; defaulting to
> > 000000000000008074
>
> These days it is fairly hard, and system- and architecture-dependent,
> to get a minimal binary from a C program.

I don't know if arm is different, but I used the following on i386 to get flat binary that does not require any runtime support:
1. provide void _start(void) instead of main()
2. use LD and OBJCOPY directly:

IMAGE         = image
IMAGE_OFFSET  = 0x20000
CC            = gcc
LD            = ld
OBJCOPY       = objcopy
CFLAGS        = -march=i486 -O2 -fomit-frame-pointer -pipe -Wall
LDFLAGS       = -m elf_i386 -Ttext $(IMAGE_OFFSET) -T  
OBJCOPYFLAGS  = -O binary
LIBS          = ./../libucos.a /usr/lib/libc.a
INCL          = -I./../
OBJS          = main.o
all:            $(IMAGE)
$(IMAGE):       $(OBJS) $(LIBS) ./../_startup.o
                $(LD) $(LDFLAGS) $(LIBS) $(OBJS) -o $(IMAGE).linked
                $(OBJCOPY) $(OBJCOPYFLAGS) $(IMAGE).linked $(IMAGE)
                @chmod -x image
                @rm -f $(IMAGE).linked

3. ucos-ldscript linker script is stock ldscript modified to

SEARCH_DIR(/home/arkadi/work/ucos-gcc);
STARTUP(_startup.o) # my _start() is in _startup.c
comment out all ALIGN()

   use different offset (that depends on loader and memory layout)

SECTIONS
{
  /* Read-only sections, merged into text segment: */
  /*. = 0x08048000 + SIZEOF_HEADERS;*/
  . = 0x00020000 + SIZEOF_HEADERS;

Time has passed since than, so things may changed.