This involves renaming the descriptor_from_fd function to the more appropriately named
open_descriptor_from_fd (since we check if the descriptor was opened and error out otherwise),
and creating a new function that does not verify that the file descriptor was opened.
Some pages, such as framebuffer pages, are not physical memory frames reserved for the current process.
Some, such as the framebuffer, may be shared between all processes.
Yet, on exit() or on exec(), a process frees all frames mapped into its address spaces.
And on fork(), it copies all data between frames. So how could we map framebuffers.
Simple: we use one of the bits in page table entries which are available to the OS, and mark whether that page is owned by the current process.
If it is owned, it will be:
- Freed on address space destruction
- Its data will be copied to a new page owned by the child process on fork()
If it is not owned, it will be:
- Left alone on address space destruction
- On fork(), the child's virtual page will be mapped to the same physical frame as the parent
This still needs a bit more work, such as keeping a reference of how many processes use a page to free it when all processes using it exit/exec.
This should be done for MAP_SHARED mappings, for example, since they are not permanent forever,
unlike the framebuffer for example.
Kernel: Add an errno.h header with definitions for each header,
and return those, negated, from syscalls when there is an error.
mmap() returns an invalid address with errno encoded, instead of
returning a negated errno; this address is encoded as ffffffffffffffEE
where EE is errno in hex.
libc: make syscall() return -1 and set errno on error, instead of
returning the raw return value of the system call. Also, add mmap()
and munmap() wrappers in sys/mman.h :).
userspace: make the memeater program show the value of errno
when allocating memory fails.
Things to improve: add perror() and strerror() to make the errno
experience even better! >.<
