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The Linux SCSI Generic (sg) HOWTO
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7.2. write()

write(int sg_fd, const void * buffer, size_t count). The action of write() with a control block based on struct sg_header is discussed in the earlier document: www.torque.net/sg/p/scsi-generic.txt (i.e the sg version 2 documentation). This section describes the action of write() when it is given a control block based on struct sg_io_hdr.

The 'buffer' should point to an object of type sg_io_hdr_t and 'count' should be sizeof(sg_io_hdr_t) [it can be larger but the excess is ignored]. If the write() call succeeds then the 'count' is returned as the result.

Up to SG_MAX_QUEUE (16) write()s can be queued up before any finished requests are completed by read(). An attempt to queue more than that will result in an EDOM error. [1] The write() command should return more or less immediately. [2]

The version 2 sg driver defaulted the maximum queue length to 1 (and made available the SG_SET_COMMAND_Q ioctl() to switch it to SG_MAX_QUEUE). So for backward compatibility a file descriptor that only receives sg_header structures in its write() will have a default "max" queue length of 1. As soon as a sg_io_hdr_t structure is seen by a write() then the maximum queue length is switched to SG_MAX_QUEUE on that file descriptor.

The "const" on the 'buffer' pointer is respected by the sg driver. Data is read in from the sg_io_hdr object that is pointed to. Significantly this is when the 'sbp' and the 'dxferp' are recorded internally (i.e. not from the sg_io_hdr object given to the corresponding read() ).

Notes

[1]

The command queuing capabilities of the SCSI device and the adapter driver should also be taken into account. To this end the sg_scsi_id::h_cmd_per_lun and sg_scsi_id::d_queue_depth values returned bu ioctl(SG_GET_SCSI_ID) may be useful. Also some devices that indicate in their INQUIRY response that they can accept command queuing react badly when queuing is actually attempted.

[2]

There is a small probability it will spend some time waiting for a command block to become available. In this case the wait is interruptible. If O_NONBLOCK is active then this scenario will cause a EAGAIN.

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