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Add small README on how to add additional storage backends.
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Marco van Wieringen authored and pstorz committed Mar 27, 2015
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Adding additional storage backends:

- Creating a new backend:
- Create a new derived class of DEVICE e.g.

class whatever_device: public DEVICE {
private:
POOLMEM *m_virtual_filename;
boffset_t m_offset;

public:
whatever_device();
~whatever_device();

/*
* Interface from DEVICE
*/
int d_close(int);
int d_open(const char *pathname, int flags, int mode);
int d_ioctl(int fd, ioctl_req_t request, char *mt = NULL);
boffset_t d_lseek(DCR *dcr, boffset_t offset, int whence);
ssize_t d_read(int fd, void *buffer, size_t count);
ssize_t d_write(int fd, const void *buffer, size_t count);
bool d_truncate(DCR *dcr);
};

In file src/stored/backends/whatever_device.h
- Create a new class implementing the pure virtual methods.
In file src/stored/backends/whatever_device.c

There are plenty of examples.
- Add build rules to src/stored/backends/Makefile.in
- Add new backend to AVAILABLE_DEVICE_API_SRCS in src/stored/Makefile.in for non dynamic loading.

- Glue code for loading and using the new backend
- Add new enum value to Device types enum in src/stored/dev.h
- Add new enum value to is_file() method of DEVICE class.
- Add new enum mapping to dev_types array in src/stored/stored_conf.c
- For static allocation of new backend add code to m_init_dev() in src/stored/dev.c
(In switch (device->dev_type) which dispatches based on device type)
- Add mapping for dynamic loading of backend to src/stored/sd_backends.h

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