Handling the _IO_WRITE message

The io_write handler is responsible for writing data bytes to the media after receiving a client's _IO_WRITE message. Examples of functions that send this message are write() and fflush(). Here's the message:

struct _io_write {
    uint16_t            type;
    uint16_t            combine_len;
    uint32_t            nbytes;
    uint32_t            xtype;
    uint32_t            zero;
    /* unsigned char    data[nbytes]; */
};

struct _io_write64 {
    uint16_t            type;
    uint16_t            combine_len;
    uint32_t            nbytes;
    uint32_t            xtype;
    uint32_t            nbytes_hi;
    /* unsigned char    data[nbytes]; */
};

typedef union {
    struct _io_write    i;
    struct _io_write    i64;
    /*  nbytes is returned with MsgReply  */
} io_write_t;

As with the io_read_t, we have a union of an input and an output message, with the output message being empty (the number of bytes actually written is returned by the resource manager library directly to the client's MsgSend()).

Note: You can use the _IO_WRITE_GET_NBYTES() macro (defined in <sys/iofunc.h>) to determine the number of bytes. You pass it a pointer to the message:

num_bytes = _IO_WRITE_GET_NBYTES(msg);

The data being written by the client almost always follows the header message stored in struct _io_write. The exception is if the write was done using pwrite() or pwrite64(). More on this when we discuss the xtype member.

To access the data, we recommend that you reread it into your own buffer. Let's say you had a buffer called inbuf that was “big enough” to hold all the data you expected to read from the client (if it isn't big enough, you'll have to read the data piecemeal).