How to compile glibc for use without an operating system

I would like to compile the functions of glibc to an ob开发者_如何学运维ject file which will then be linked to a program which I am running on a computer without any operating system. Some functions, such as open, I want to just fail with ENOSYS. Other functions I will write myself, such as putchar, and then have glibc use those functions in it's own (like printf). I also want to use functions that don't need a file system or process management system or anything like that, such as strlen. How can I do this?

Most C libraries rely on the kernel heavily, so it's not reasonable to port 'em. But since most of it doesn't need to be implemented, you can get away easily with a few prototypes for stubs and gcc builtins.

You can implement stubs easily using weak symbols:

#define STUB __attribute__((weak, alias("__stub"))) int
#define STUB_PTR __attribute__((weak, alias("__stub0"))) void *
int __stub();
void *__stub0();

Then defining the prototypes becomes trivial:

STUB read(int, void*, int);
STUB printf(const char *, ...);
STUB_PTR mmap(void*, int, int, int, int, int);

And the actual functions could be:

int __stub()
{
    errno = ENOSYS;
    return -1;
}

void *__stub0()
{
    errno = ENOSYS;
    return NULL;
}

If you need some non-trivial function, like printf, take it from uClibc instead of glibc (or some other smaller implementation).