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
--
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Robert P. J. Day Waterloo, Ontario, CANADA
Top-notch, inexpensive online Linux/OSS/kernel courses
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