The Linux SCSI Generic (sg) HOWTO
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# 7.5. close()

close(int sg_fd). Preferably a close() should be done after all issued write()s have had their corresponding read() calls completed. Unfortunately this is not always possible (e.g. the user may choose to send a kill signal to a running process). The sg driver implements "fast" close semantics and thus will return more or less immediately (i.e. not wait on any event). This is application friendly but requires the sg driver to arrange for an orderly cleanup of those packets that are still "in flight".

When close() leaves outstanding SCSI commands still awaiting responses, the sg driver maintains its internal structures for the now defunct file descriptor. These internal structures are maintained until all outstanding responses (some might be timeouts) are received. When the sg driver is loaded as a module and has any open file descriptors or "defunct" file descriptors then it cannot be unloaded. An attempt to call rmmod sg will report the driver is busy. Defunct file descriptors that remain for some time, perhaps awaiting a timeout, can be observed with the cat /proc/scsi/sg/debug command. In this case "closed=1" will be set on the defunct file descriptor [see Section 11.1]. Defunct file descriptors do not impede attempts by applications to open() new file descriptors on the same SCSI device.

The kernel arranges for only the last close() on a file descriptor to be seen by a driver (and to emphasize this, the corresponding sg driver call is named sg_release() rather than sg_close()). This is only significant when an application uses fork() or dup().

Returns 0 if successful, otherwise -1 implies an error.

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