=== As a guess and only a guess: === POSIX / UNIX SVR4 probably defines minor and major as 8-bits each. So to be posix compatible, the Linux ABI has to be 8-bits each. But internal to the kernel, they needed more bits for devices not exposed to userspace. As a guess as to what kind of device that is: SCSI uses the minor device to encode both the drive and the partition. Something like: minor = ((drive << 4) | partition); You can see that only allows a handful of partitions, but the old IDE driver supported at least 64 partitions (iirc). So last year a SCSI extended partition setup was created. In user space it uses multiple major device numbers. Hopefully in kernel space it only uses one! Greg On Thu, Jul 8, 2010 at 4:14 PM, Robert P. J. Day <rpjday@xxxxxxxxxxxxxx> wrote: > > over the next little while, i'm going to ask some fairly trivial > questions that i really should know the answer to but i just don't, > just because i want to fill in those little gaps. > > for instance, consider the kernel header file > include/linux/kdev_t.h, which defines a kernel space device file > major/minor combination thusly: > > #define MINORBITS 20 > #define MINORMASK ((1U << MINORBITS) - 1) > > #define MAJOR(dev) ((unsigned int) ((dev) >> MINORBITS)) > #define MINOR(dev) ((unsigned int) ((dev) & MINORMASK)) > #define MKDEV(ma,mi) (((ma) << MINORBITS) | (mi)) > ... etc etc ... > > so it's clear that, these days, a device file is identified by a > 32-bit value, (major,minor) = (12 bits, 20 bits). > > in addition, later definitions in that file define *old* device > files as 16-bit, (8,8). > > but at the bottom of that file, you read: > > #else /* __KERNEL__ */ > > /* > Some programs want their definitions of MAJOR and MINOR and MKDEV > from the kernel sources. These must be the externally visible ones. > */ > #define MAJOR(dev) ((dev)>>8) > #define MINOR(dev) ((dev) & 0xff) > #define MKDEV(ma,mi) ((ma)<<8 | (mi)) > #endif /* __KERNEL__ */ > > > that means that's what exported to user space in the file > /usr/include/linux/kdev_t.h is the *old* format. so what's the value > of that? what's the point of having macros in user space that don't > match the actual structure of a kernel space device file identifier? > > rday > > -- > > ======================================================================== > Robert P. J. Day Waterloo, Ontario, CANADA > > Top-notch, inexpensive online Linux/OSS/kernel courses > http://crashcourse.ca > > Twitter: http://twitter.com/rpjday > LinkedIn: http://ca.linkedin.com/in/rpjday > ======================================================================== > > -- > To unsubscribe from this list: send an email with > "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx > Please read the FAQ at http://kernelnewbies.org/FAQ > > -- Greg Freemyer Head of EDD Tape Extraction and Processing team Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer CNN/TruTV Aired Forensic Imaging Demo - http://insession.blogs.cnn.com/2010/03/23/how-computer-evidence-gets-retrieved/ The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
