io bufinfo_t Structure

`io bufinfo_t` Structure

The bufinfo_t structure is the abstraction that describes an I/O request. The buffer corresponding to an I/O request is pointed to by args[0] in the start, done, wait-start, and wait-done probes. The bufinfo_t structure definition is as follows:

typedef struct bufinfo {
        int b_flags;                    /* flags */
        size_t b_bcount;                /* number of bytes */
        caddr_t b_addr;                 /* buffer address */
        uint64_t b_blkno;               /* expanded block # on device */
        uint64_t b_lblkno;              /* block # on device */
        size_t b_resid;                 /* # of bytes not transferred */
        size_t b_bufsize;               /* size of allocated buffer */ 
        caddr_t b_iodone;               /* I/O completion routine */
        int b_error;                    /* expanded error field */
        dev_t b_edev;                   /* extended device */
 } bufinfo_t;

The b_flags member indicates the state of the I/O buffer, and consists of a bitwise-or of different state values. The valid state values are the following:

B_DONE: Indicates that the data transfer has completed.
B_ERROR: Indicates an I/O transfer error. It is set in conjunction with the b_error field.
B_PAGEIO: Indicates that the buffer is being used in a paged I/O request. See the description of the b_addr field for more information.
B_PHYS: Indicates that the buffer is being used for physical (direct) I/O to a user data area.
B_READ: Indicates that data is to be read from the peripheral device into main memory.
B_WRITE: Indicates that the data is to be transferred from main memory to the peripheral device.
B_ASYNC: The I/O request is asynchronous, and will not be waited upon. The wait-start and wait-done probes do not fire for asynchronous I/O requests. Note that some I/Os directed to be asynchronous might not have B_ASYNC set: the asynchronous I/O subsystem might implement the asynchronous request by having a separate worker thread perform a synchronous I/O operation.

The following describes the other fields in the bufinfo structure:

b_bcount: Is the number of bytes to be transferred as part of the I/O request.
b_addr: Is the virtual address of the I/O request, unless B_PAGEIO is set. The address is a kernel virtual address unless B_PHYS is set, in which case it is a user virtual address. If B_PAGEIO is set, the b_addr field contains kernel private data. Exactly one of B_PHYS and B_PAGEIO can be set, or neither flag will be set.
b_lblkno: Identifies which logical block on the device is to be accessed. The mapping from a logical block to a physical block (such as the cylinder, track, and so on) is defined by the device.
b_resid: Is set to the number of bytes not transferred because of an error.
b_bufsize: Contains the size of the allocated buffer.
b_iodone: Identifies a specific routine in the kernel that is called when the I/O is complete.
b_error: May hold an error code returned from the driver in the event of an I/O error. b_error is set in conjunction with the B_ERROR bit set in the b_flags member.
b_edev: Contains the major and minor device numbers of the device accessed. Consumers may use the D subroutines getmajor and getminor to extract the major and minor device numbers from the b_edev field.