Re: [MiNT] '-e' parameter in gcc

[Vincent Rivière <vincent.riviere@freesbee.fr>]

Miro Kropáček wrote:
> m68k-atari-mint-gcc -nostdlib -e main conftest.c
> /home/mikro/gnu-tools/lib/gcc/m68k-atari-mint/4.5.1/../../../../m68k-atari-mint/bin/ld:
> warning: cannot find entry symbol main; defaulting to 000000e4
> /tmp/cc4HfuQ9.o:/tmp/cc4HfuQ9.o:(.text+0x6): undefined reference to `___main'

There are 2 differences between Linux and MiNT, and both are related to ELF 
vs a.out.

1) cannot find entry symbol main
Traditionally, on a.out targets, the public C symbols are prefixed by '_'. 
It is some kind of namespace, to differentiate function names and register 
names which could have the same name.
Also traditionally, on ELF targets, the public C symbols are not prefixed.

For your information, the preprocessor automatically defines 
__USER_LABEL_PREFIX__ to the character used as prefix for public C functions 
on the current target ( _ or nothing).

So Eero is right, to get rid of the first warning you have to write "-e 
_main" on MiNT.

2) Calling code before main (constructors of global C++ variables and GCC 
extensions)
On a.out targets, GCC silently inserts a call to __main() at the very 
beginning of the user's main(). That function is implemented inside -lgcc. 
Of course, as other functions, the public symbol is prefixed to an 
additional underscore, so the undefined symbol is ___main with 3 underscores.
On ELF targets, things are more sophisticated. Especially, the startup code 
is split into several object files. crtbegin.o is linked before the user's 
objects, and crtend.o is linked at the end. One of the jobs of crtbegin.o is 
to call the constructors, there is no need for the __main() hack.

So what happens ?
On Linux, "gcc -nostdlib conftest.c -e main" do exactly what you think. The 
final executable contains only the main() function from conftest.c.
But on MiNT (a.out), you also get an undefined reference to __main (inserted 
by GCC at the beginning of main()), and it can't be resolved by libgcc 
because of -nostdlib !

What to do ?

A) Add -lgcc -lc -lgcc to the end of the link command line. Two times -lgcc 
due to circular dependencies. But this will fail if either libgcc or libc is 
missing on your system.

B) In conftest.c, name your function differently from main, for example 
main2. Then do: "m68k-atari-mint-gcc conftest.c -nostdlib -e _main2"
This will work as expected, since GCC adds a call to __main() only in the 
very special case of a function named main().

Back to the real world:
You're damned because Linux people don't care about things other than ELF. 
However, xBSD still use a.out and the same issues would appear.

--
Vincent Rivière