Is it possible (within reason) to build a "toy" OS on a mac using llmv/clang (and the other "normal" build tools)? By "toy" OS, I mean the simple, "Hello, World" examples found on OSDev (http://wiki.osdev.org/Bare_Bones) and x86 Bare Metal (https://github.com/cirosantilli/x86-bare-metal-examples).
My main problem is I can't figure out how to specify precisely where the linker should place the code (i.e., that the starting point should be 0x7c00, that bytes 510 and 511 need to be 0xaa55, etc.).
I would say yes it is possible within reason, at least if you consider waiting for a build of lld (and its dependency llvm) reasonable. Instructions to build lld can be found on their website or as part of this answer.
Compiling and linking for a different target than the host is relatively easy with clang. You just have to set a target, for example -target i386-none-elf for an ELF binary. Cross-compilation using clang is explained in more detail here.
As for macOS, as Micheal Petch noted, you have to use another linker than the standard ld installed. You could in theory install binutils to get an ELF ld but then you have to compile it yourself to set the target. My recommendation is to use lld which can target many architectures without the need to recompile.
With clang and a lld in place we can compile sources with
clang -c -o file.o file.c -target i386-none-elf # freestanding flags omitted
and then link them with
clang -o kernel.bin file.o -target i386-linux-elf -nostdlib -Wl,linkerscript.ld -fuse-ld=lld
Note that for linking I am using i386-linux-elf because there is a bug in clang where they just forward their input to gcc. But when using -nostdlib it is essentially the same.
If you want to see a complete example ready to build, you can take a look at https://github.com/Henje/x86-Toy-OS.
Related
I have a code running on a PowerPC e500v2 embedded Linux and I want to measure its performance since it is running in an infinite loop. I tried gcc's gprof which was simply by adding -pg option to gcc. When I run the binary on the target device I get this:
./main: /lib/libc.so.6: version GLIBC_2.16 not found (required by ./main)
I am using ELDK 5.6 toolchain with the default CFLAGS and LDFLAGS and these flags: -Wall -lrt -pthread -D_GNU_SOURCE nothing else. Some article suggested defining FORTIFY_SOURCE along with an optimization level but it did not work. I searched for some gcc's feature test macros and tried defining some GLIBC 2.16 specific macros but it did not work.
I faced similar issue with GLIBC 2.17 when I used some structures and functions from <sched.h>, adding _GNU_SOURCE resolved it. Any idea on how to resolve it?
When I run the binary on the target device I get this
Your tool chain targets a version of GLIBC that is newer than what is installed on the target.
This doesn't bite you in non-pg compiles only by accident. An "innocent" change to your source can cause the same problem.
You need to upgrade your target to the version of GLIBC which your toolchain actually builds for.
I'm using the Windows version of Clang (LLVM) 8 under Windows.
I'm compiling a code which uses OpenMP.
Under the lib folder of Clang there are 2 files which are OpenMP related:
libomp.lib.
libiomp5md.dll.
My questions are:
When I compile the code I use the flags -Xclang -fopenmp for the compiler. In in GCC and ICC using the flags tell the compiler to link the OpenMP library automatically. What about Clang? Does it do it automatically or must I link with libomp.lib manually? Is there a way to trigger automatic linking to the OpenMP library?
Answer: This was answered in Michael Klemm's answer below - Use the clang driver both for compiling and linking and then the -fopenmp will work as in GCC.
When I link with libomp.lib manually (Defining as a library for the linker) the output exe requires libomp.dll while the supplied OpenMP Dynamic Library is libiomp5md.dll. Is that a bug or is it because I link manually?
Answer: The libomp.dll is supplied in the bin folder and not the lib folder.
What's the proper way to utilize OpenMP in Clang under Windows? The clang-cl driver doesn't work with /openmp or -openmp as the MSVC's cl compiler.
Answer: Currently it can be done either with clang -fopenmp ..., clang-cl -Xclang -fopenmp ... or clang-cl /clang:-fopenmp ... (Which is equivalent of -Xclang -fopenmp).
Remark
On Windows I use Windows Driver of Clang using clang-cl.
Adding clarity to what the OpenMP libraries actually are, and how to use them on Windows with clang-cl
libomp.dll and libiomp5md.dll ARE THE SAME FILES!
When compiling for Windows, you link against libomp.lib OR libiomp5md.lib which will link to the same-named DLL at runtime, i.e. libomp.dll OR libiomp5md.dll respectively.
If you load 2 files that use the "different-name DLL," the interpreter will crash and give you a nasty error like: OMP: Error #15: Initializing libiomp5md.dll, but found libomp.dll already initialized.
Why? Because the program has no idea they are the same DLL, they have different names, so it assumes they are different. And it crashes. For this reason only, you can choose to swap which OpenMP DLL you link to in your program.
If your program doesn't crash and give you an error, you can keep using the same link to OpenMP. Otherwise, to silence the error, link to the one that is loaded by another program already.
If using clang-cl.exe which is the "drop-in" Clang replacement for MSVC cl.exe you should pass a compiler argument such as -Xclang -fopenmp which will convert the argument over to "Clang language." Don't forget to still pass to the linker the OpenMP LIB you chose, because on Windows, it won't be automatic.
That's all I've learned as brief as possible about OpenMP linking on Windows.
To compile and link OpenMP code with clang on Windows, you will have to pass -fopenmp to both the compiler and the linker:
clang -fopenmp -o bla.obj -c bla.c
clang -fopenmp -o bla.exe bla.obj
I'm working through a text on linking, and wanted to work along with some examples in said text.
To better understand whats going on when I invoke the gcc driver, I was looking into doing all the compilation old-school by hand;
preprocessing using cpp
compiling with cc1
assembling with as
linking using ld
Unfortunately, on my Mac I don't seem to be able to reference cc1directly (no listing of cc1 in man). What are my options?
Read some material about GCC internals. First the wikipage on GCC. Then, you could play with the MELT probe (you may want a Linux system to use it).
Then read the GCC manual. And the GCC resource center
Use gcc -v -Wall to compile, it will show what cc1 is running (and -Wall is always useful). So try compiling with gcc -v -Wall hello.c -o helloworld.bin
the cc1 program don't sit in your PATH or in /usr/bin/ but -on my system- in /usr/lib/gcc/x86_64-linux-gnu/4.8/cc1
The command gcc -print-prog-name=cc1 will tell you which cc1 is used by your gcc. See GCC debugging options for more.
The preprocessing is now inside cc1.
The gcc program is just a driver, starting cc1 (which does most of the compiling work), then as, ld etc...
The MELT site contains some documentation, and some slides explaining GCC, which you could find interesting. MELT is a domain specific language to extend GCC.
See also the picture on http://starynkevitch.net/Basile/gcc-melt/cc1-internals.svg and the below picture
picture from http://starynkevitch.net/Basile/gcc-melt/gcc-MELT-inside.png, done by me, CC BY SA
The cc1 is producing a .s assembly file; the as (started by gcc) is transforming it into .o. The ld linker (started by gcc) will produce a library or an executable.
PS. I have a Linux system, but things are very similar on MacOSX.
reference on linking
A good book about linking is Levine's Linkers & loaders book.
PS. MELT is obsolete in 2021, but I am working on the Bismon static source code analyzer and on RefPerSys (which generates C++ code).
For reference, I installed gcc-10 with brew on my macOS (Catalina).
While cc1 is not directly accessible (via PATH), it can be found in
/usr/local/Cellar/gcc/10.2.0/libexec/gcc/x86_64-apple-darwin19/10.2.0
If you are lost, try gcc -v and from the verbose information you may find where your cc1 is.
Is it (easily) possible to use software floating point on i386 linux without incurring the expense of trapping into the kernel on each call? I've tried -msoft-float, but it seems the normal (ubuntu) C libraries don't have a FP library included:
$ gcc -m32 -msoft-float -lm -o test test.c
/tmp/cc8RXn8F.o: In function `main':
test.c:(.text+0x39): undefined reference to `__muldf3'
collect2: ld returned 1 exit status
It is surprising that gcc doesn't support this natively as the code is clearly available in the source within a directory called soft-fp. It's possible to compile that library manually:
$ svn co svn://gcc.gnu.org/svn/gcc/trunk/libgcc/ libgcc
$ cd libgcc/soft-fp/
$ gcc -c -O2 -msoft-float -m32 -I../config/arm/ -I.. *.c
$ ar -crv libsoft-fp.a *.o
There are a few c files which don't compile due to errors but the majority does compile. After copying libsoft-fp.a into the directory with our source files they now compile fine with -msoft-float:
$ gcc -g -m32 -msoft-float test.c -lsoft-fp -L.
A quick inspection using
$ objdump -D --disassembler-options=intel a.out | less
shows that as expected no x87 floating point instructions are called and the code runs considerably slower as well, by a factor of 8 in my example which uses lots of division.
Note: I would've preferred to compile the soft-float library with
$ gcc -c -O2 -msoft-float -m32 -I../config/i386/ -I.. *.c
but that results in loads of error messages like
adddf3.c: In function '__adddf3':
adddf3.c:46: error: unknown register name 'st(1)' in 'asm'
Seems like the i386 version is not well maintained as st(1) points to one of the x87 registers which are obviously not available when using -msoft-float.
Strangely or luckily the arm version compiles fine on an i386 and seems to work just fine.
Unless you want to bootstrap your entire toolchain by hand, you could start with uclibc toolchain (the i386 version, I imagine) -- soft float is (AFAIK) not directly supported for "native" compilation on debian and derivatives, but it can be used via the "embedded" approach of the uclibc toolchain.
GCC does not support this without some extra libraries. From the 386 documentation:
-msoft-float Generate output containing library calls for floating
point. Warning: the requisite
libraries are not part of GCC.
Normally the facilities of the
machine's usual C compiler are used,
but this can't be done directly in
cross-compilation. You must make your
own arrangements to provide suitable
library functions for
cross-compilation.
On machines where a function returns
floating point results in the 80387
register stack, some floating point
opcodes may be emitted even if
-msoft-float is used
Also, you cannot set -mfpmath=unit to "none", it has to be sse, 387 or both.
However, according to this gnu wiki page, there is fp-soft and ieee. There is also SoftFloat.
(For ARM there is -mfloat-abi=softfp, but it does not seem like something similar is available for 386 SX).
It does not seem like tcc supports software floating point numbers either.
Good luck finding a library that works for you.
G'day,
Unless you're targetting a platform that doesn't have inbuilt FP support, I can't think of a reason why you'd want to emulate FP support.
Doesn't your x386 platform have external FPU support? Pity it's not a x486 with the FPU built in!
In my experience, any soft emulation is bound to be much slower than its hardware equivalent.
That's why I finished up writing a package in Ada to taget the onboard 68k FPU instead of using the soft emulation provided by the compiler manufacturer at the time. They finished up bundling it in their compiler as a matter of fact.
Edit: Just seen your comment below. Hmmm, if you don't need a full suite of FP support is it possible to roll your own for the few math functions you do need? That how the Ada package I mentioned got started.
HTH
cheers,
The code I'm working on is supposed to be possible to build for both hosted and freestanding environments, providing private implementations for some stdlib functions for the latter case.
Can I reliably test this with just GCC on a normal workstation/build server? Compile for freestanding environment with GCC
The "-ffreestanding" option looked promising, but it seems that it "only" disables built-ins and sets the STDC_HOSTED macro properly, it still provides all system headers.
The option "-nostdinc" is too restrictive; I still want to use the headers required for a freestanding implementation (in particular stddef.h and limits.h).
What am I missing here?
Oh, and I'm using GCC 4.4.3 for the moment, will upgrade to 4.5.0 "soon".
Well, since no answer is given yet I'd might as well describe how I made this work. It's pretty simple although depending on the target system it can be tedious.
Using "-nostdinc" means that the standard system include paths will be skipped; other include-paths given with "-I" will of course still be searched for headers.
So, for the freestanding build target I create a folder 'include-freestanding-c89' and link the relevant system headers -- float.h, iso646.h, limits.h, stdarg.h and stddef.h -- there. Other headers might be included in these, depending on your platform, so you might have to do some research and set up more links (hence the tediousness if you need to do this for several target platforms).
The C89 directory can then be used as base for 'include-freestanding-c99', the extra headers to link are stdbool.h and stdint.h
The command-line to use is then
gcc -std=c89 -nostdinc -nostdlib -ffreestanding -I include-freestanding-c89
or
gcc -std=c99 -nostdinc -nostdlib -ffreestanding -I include-freestanding-c99
This Xen Makefile uses gcc -print-search-dirs to get the directory with stddef.h and similar, adds it with -isystem, then uses -nostdinc to build:
https://github.com/mirage/xen/blob/2676bc915157ab474ee478d929b0928cf696b385/stubdom/Makefile#L35