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man pages section 7: Device and Network Interfaces

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Updated: July 2017



uscsi - user SCSI command interface


#include <sys/scsi/impl/uscsi.h>

ioctl(int fildes, int request, struct uscsi_cmd *cmd);


The uscsi command is very powerful and somewhat dangerous; therefore it has some permission restrictions. See WARNINGS for more details.

Drivers supporting this ioctl(2) provide a general interface allowing user-level applications to cause individual SCSI commands to be directed to a particular SCSI or ATAPI device under control of that driver. The uscsi command is supported by the sd driver for SCSI disks and ATAPI CD-ROM drives, and by the st driver for SCSI tape drives. uscsi may also be supported by other device drivers; see the specific device driver manual page for complete information.

Applications must not assume that all Solaris disk device drivers support the uscsi ioctl command. The SCSI command may include a data transfer to or from that device, if appropriate for that command. Upon completion of the command, the user application can determine how many bytes were transferred and the status returned by the device. Also, optionally, if the command returns a Check Condition status, the driver will automatically issue a Request Sense command and return the sense data along with the original status. See the USCSI_RQENABLE flag below for this Request Sense processing. The uscsi_cmd structure is defined in <sys/scsi/impl/uscsi.h> and includes the following members:

int uscsi_flags;               /* read, write, etc. see below */
short uscsi_status;           /* resulting status */
short uscsi_timeout;          /* Command Timeout */
caddr_t uscsi_cdb             /* CDB to send to target */
caddr_t uscsi_bufaddr;        /* i/o source/destination */
size_t uscsi_buflen;          /* size of i/o to take place*/
size_t uscsi_resid;           /* resid from i/o operation */
uchar_t uscsi_cdblen;         /* # of valid CDB bytes */
uchar_t uscsi_rqlen;          /* size of uscsi_rqbuf */
uchar_t uscsi_rqstatus;       /* status of request sense cmd */
uchar_t uscsi_rqresid;        /* resid of request sense cmd */
caddr_t uscsi_rqbuf;          /* request sense buffer */
void *uscsi_reserved_5;       /* Reserved for future use */

The fields of the uscsi_cmd structure have the following meanings:


The I/O direction and other details of how to carry out the SCSI command. Possible values are described below.


The SCSI status byte returned by the device is returned in this field.


Time in seconds to allow for completion of the command.


A pointer to the SCSI CDB (command descriptor block) to be transferred to the device in command phase.


The user buffer containing the data to be read from or written to the device.


The length of uscsi_bufaddr.


If a data transfer terminates without transferring the entire requested amount, the remainder, or residue, is returned in this field.


The length of the SCSI CDB to be transferred to the device in command phase.


The length of uscsi_rqbuf, the application's Request Sense buffer.


The SCSI status byte returned for the Request Sense command executed automatically by the driver in response to a Check Condition status return.


The residue, or untransferred data length, of the Request Sense data transfer (the number of bytes, less than or equal to uscsi_rqlen, which were not filled with sense data).


Points to a buffer in application address space to which the results of an automatic Request Sense command are written.


Reserved for future use.

The uscsi_flags field defines the following:

USCSI_WRITE                   /* send data to device */
USCSI_SILENT                  /* no error messages */
USCSI_DIAGNOSE                /* fail if any error occurs */
USCSI_ISOLATE                 /* isolate from normal commands */
USCSI_READ                    /* get data from device */
USCSI_ASYNC                   /* set bus to asynchronous mode */
USCSI_SYNC                    /* return bus to sync mode if possible */
USCSI_RESET                   /* reset target */
USCSI_RESET_TARGET            /* reset target */
USCSI_RESET_LUN               /* reset logical unit */
USCSI_RESET_ALL               /* reset all targets */
USCSI_RQENABLE                /* enable request sense extensions */
USCSI_RENEGOT                 /* renegotiate wide/sync on next I/O */

The uscsi_flags bits have the following interpretation:


Data will be written from the initiator to the target.


The driver should not print any console error messages or warnings regarding failures associated with this SCSI command.


The driver should not attempt any retries or other recovery mechanisms if this SCSI command terminates abnormally in any way.


This SCSI command should not be executed with other commands.


Data will be read from the target to the initiator.


Set the SCSI bus to asynchronous mode before running this command.


Set the SCSI bus to synchronous mode before running this command.


Send a SCSI bus device reset message to this target.


Same as USCSI_RESET. Use this flag to request TARGET RESET. (USCSI_RESET is maintained only for compatibility with old applications).


Send a SCSI logical unit reset message to this target.


USCSI_RESET_ALL, USCSI_RESET/USCSI_RESET_TARGET and USCSI_RESET_LUN are mutually exclusive options and issuing them in any simultaneous combination will result in implementation-dependent behavior

When a USCSI reset request is combined with other SCSI commands, the following semantics take effect:

If the USCSI RESET flag is specified, the other fields (other than uscsi_flags) in the uscsi_cmd are ignored. The uscsi_cdblen must be set to zero.


Enable Request Sense extensions. If the user application is prepared to receive sense data, this bit must be set, the fields uscsi_rqbuf and uscsi_rqbuflen must be non-zero, and the uscsi_rqbuf must point to memory writable by the application.


Tells USCSI to renegotiate wide mode and synchronous transfer speed before the transmitted SCSI command is executed. This flag in effects tells the target driver to pass the FLAG_RENEGOTIATE_WIDE_SYNC flag in the SCSI packet before passing the command to an adapter driver for transport.

See the scsi_pkt(9S) flag FLAG_RENEGOTIATE_WIDE_SYNC for more information.


The ioctl supported by drivers providing the uscsi interface is:


The argument is a pointer to a uscsi_cmd structure. The SCSI device addressed by that driver is selected, and given the SCSI command addressed by uscsi_cdb. If this command requires a data phase, the uscsi_buflen and uscsi_bufaddr fields must be set appropriately; if data phase occurs, the uscsi_resid is returned as the number of bytes not transferred. The status of the command, as returned by the device, is returned in the uscsi_status field. If the command terminates with Check Condition status, and Request Sense is enabled, the sense data itself is returned in uscsi_rqbuf. The uscsi_rqresid provides the residue of the Request Sense data transfer.



A parameter has an incorrect, or unsupported, value.


An error occurred during the execution of the command.


A process without root credentials tried to execute the USCSICMD ioctl.


The uscsi_cmd itself, the uscsi_cdb, the uscsi_buf, or the uscsi_rqbuf point to an invalid address.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also

ioctl(2), attributes(5), sd(7D), st(7D)

ANSI Small Computer System Interface-2 (SCSI-2)


The uscsi command is very powerful, but somewhat dangerous, and so its use is restricted to processes running as root, regardless of the file permissions on the device node. The device driver code expects to own the device state, and uscsi commands can change the state of the device and confuse the device driver. It is best to use uscsi commands only with no side effects, and avoid commands such as Mode Select, as they may cause damage to data stored on the drive or system panics. Also, as the commands are not checked in any way by the device driver, any block may be overwritten, and the block numbers are absolute block numbers on the drive regardless of which slice number is used to send the command.

The uscsi interface is not recommended for very large data transfers (typically more than 16MB). If the requested transfer size exceeds the maximum transfer size of the DMA engine, it will not be broken up into multiple transfers and DMA errors may result.

The USCSICMD ioctl associates a struct uscsi_cmd with a device by using an open file descriptor to the device. Other APIs might provide the same struct uscsi_cmd programming interface, but perform device association in some other manner.