Handling the xtype member

The message structures passed to the io_read, io_write, and io_openfd handlers contain a member called xtype. From struct _io_read:

struct _io_read {
    ...
    uint32_t            xtype;
    ...
}

Basically, the xtype contains extended type information that can be used to adjust the behavior of a standard I/O function. Most resource managers care about only a few values:

_IO_XTYPE_NONE

No extended type information is being provided.

_IO_XTYPE_OFFSET

If clients are calling pread(), pread64(), pwrite(), or pwrite64(), then they don't want you to use the offset in the OCB. Instead, they're providing a one-shot offset. That offset follows the struct _io_read or struct _io_write headers that reside at the beginning of the message buffers.

For example:

struct myread_offset {
    struct _io_read        read;
    struct _xtype_offset   offset;
}   
      

Some resource managers can be sure that their clients will never call pread*() or pwrite*(). (For example, a resource manager that's controlling a robot arm probably wouldn't care.) In this case, you can treat this type of message as an error.

_IO_XTYPE_READCOND

If a client is calling readcond(), they want to impose timing and return buffer size constraints on the read. Those constraints follow the struct _io_read or struct _io_write headers at the beginning of the message buffers. For example:

struct myreadcond {
    struct _io_read        read;
    struct _xtype_readcond cond;
}   
      

As with _IO_XTYPE_OFFSET, if your resource manager isn't prepared to handle readcond(), you can treat this type of message as an error.

_IO_XFLAG_DIR_EXTRA_HINT

This flag is valid only when reading from a directory. The filesystem should normally return extra directory information when it's easy to get. If this flag is set, it is a hint to the filesystem to try harder (possibly causing media lookups) to return the extra information. The most common use is to return _DTYPE_LSTAT information.