POSIX Shared Memory API.

## Documentation

The `posix-shm` allows the creation and access of POSIX shared
memory objects. A POSIX shared memory object is in effect a file
handle which can be used by several processes to access the same
region of shared memory.

<procedure>posix-shm? :: BOOLEAN</procedure>

`#t` if the current platform supports POSIX shared memory, `#f` otherwise.

<procedure>shm-open:: PATH * OFLAGS  [* MODE] -> FD</procedure>

`shm-open` is a wrapper around `shm_open`, which analogous to the
UNIX system call `open(2)`.

Argument `PATH` specifies the shared memory object to be created or
opened. For portable use, name should have an initial slash (/) and
contain no embedded slashes.

`OFLAGS` is a list of bit masks from the `chicken file posix` unit which will
be ORed together and passed to `shm_open`. It must contains exactly
one of `open/rdonly` or `open/rdwr` and any of the other flags
listed here:

- `open/creat` : Creates the shared memory object if it does not exist. The user and group ownership of the object are taken from the corresponding effective IDs of the calling process, and the object's permission bits are set according to the low-order 9 bits of mode, except that those bits set in the process file mode creation mask (see `umask(2)`) are cleared for the new object.  A new shared memory object initially has zero length. The size of the object can be set using `file-truncate`. The newly allocated bytes of a shared memory object are automatically initialised to 0.
- `open/excl` : If `open/create` was also specified, and a shared memory object with the given name already exists, returns an error. 
- `open/trunc` : If the shared memory object already exists, truncate it to zero bytes.

`MODE` should be a bitmask composed of one or more permission
values like `perm/irusr` and is only relevant when a new file is
created. The default mode is `perm/irwxu | perm/irgrp | perm/iroth`.

On successful completion `shm-open` returns a new file descriptor
referring to the shared memory object.

<procedure>shm-unlink:: PATH -> STATUS</procedure>

The operation of `shm-unlink` is analogous to `unlink(2)`: it
removes a shared memory object name, and, once all processes have
unmapped the object, de-allocates and destroys the contents of the
associated memory region. After a successful `shm-unlink`, attempts
to `shm-open` an object with the same name will fail (unless
`open/creat` was specified, in which case a new, distinct object is
created).

## Examples

```scheme
  (let* ((str "Hello, world!")
         (path (sprintf "/shmtest~A" (random 100)))
         (fd (shm-open path (list open/rdwr open/creat))))
    (file-truncate fd (string-length str))
    (file-write fd str)
    (file-close fd)
    (let ((fd (shm-open path (list open/rdonly open/excl ))))
      (file-read fd (string-length str))
      (file-close fd)
      (shm-unlink path)))
```

## License

>
> Copyright 2011-2020 Ivan Raikov
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