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 information that can be used to adjust the behavior of a standard I/O function. This information includes a type and optionally some flags. To isolate the type from the flags, AND the xtype member with _IO_XTYPE_MASK:

if ((msg->i.xtype & _IO_XTYPE_MASK) == _IO_XTYPE_OFFSET) {
   ...
}

Most resource managers care about only a few types:

_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.

The following types are used for special purposes, so your resource manager probably doesn't need to handle them:

• _IO_XTYPE_MQUEUE
• _IO_XTYPE_REGISTRY
• _IO_XTYPE_TCPIP
• _IO_XTYPE_TCPIP_MSG
• _IO_XTYPE_TCPIP_MSG2

The xtype member may also include some flags. Your resource manager might be interested in the following:

_IO_XFLAG_DIR_EXTRA_HINT

This flag is valid only when you're 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.

The other defined flags are used for special purposes, so your resource manager probably doesn't need to handle them:

• _IO_XFLAG_BLOCK
• _IO_XFLAG_NONBLOCK