From: Andrew J Bromage (ajb_at_[hidden])
Date: 2006-02-08 22:40:59
G'day Ion.
Ion Gaztañaga <igaztanaga <at> gmail.com> writes:
> I hope you can find to do a review, since it seems you have experience
> with memory mappings.
I hope so too. :-)
> If I understand this correctly, you want a class that can map more than
> a fragment of a file holding a single file descriptor, is that right?
Well... I want a way to do that, whether it's a single class or not.
When I first thought about it, the approach that I considered was to use
two classes. mmaped_file represents a file which has zero or more mmaped
regions, and mmaped_region which represents a single region. Off the top
of my head, something like this might be appropriate:
class mmapped_file : private boost::noncopyable
{
public:
/* A slightly richer set of access modes. If the operating system does
not support some mode, the library should pick a mode that at least
allows the required mode. (e.g. if the OS doesn't support write-only
mode, read-write mode is an appropriate substitute since it allows
writing. */
typedef enum {
none_mode = 0x00, ro_mode = 0x01, wo_mode = 0x02, rw_mode =
0x03, access_mode_mask = 0xff
} accessmode;
/* Open a file, RAII style. Throws an exception on failure. */
mmapped_file(const char* filename, accessmode mode);
/* Closes the file. Does not throw. */
~mmapped_file();
/* XXX Support for iterating through the mapped regions? */
};
class mmaped_region : private boost::noncopyable
{
public:
/* Map a region from an mmaped_file. Throws an exception on failure. */
mmaped_region(shared_ptr<mmaped_file>& file, fileoff_t offset,
size_t size, mmaped_file::accessmode mode);
/* Unmap the region. Does not throw. */
~mmapped_region();
/* Get the size of the mapped region. Does not throw. */
std::size_t get_size() const;
/* Get a pointer to the base of the mapped region. Does not throw. */
void* get_base() const;
/* Get the file offset of the beginning of the mapped region. Does not
throw. */
fileoff_t get_file_offset() const;
/* Flush an area of the mapped region. Does not throw. */
bool flush(size_t mapping_offset = 0, size_t numbytes = 0);
};
mmaped_region holds a shared_ptr to an mmaped_file, so that when the
last mapped region is destroyed, the file is closed (assuming someone
else isn't holding onto if, of course).
Cheers,
Andrew Bromage