How to convert between a dev_t and major/minor device numbers?

Question

I'm trying to write a portable program that deals with ustar archives. For device files, these archives store the major and minor device numbers. However, the struct stat as laid out in POSIX only contains a single st_rdev member of type dev_t described with “Device ID (if file is character or block special).”

How can I convert between a pair of major and minor device numbers and a single st_rdev member as returned by stat() in a portable manner?

Answer 1

While all POSIX programming interfaces use the device number (of type dev_t) as is, FUZxxl pointed out in a comment to this answer that the common UStar file format -- most common tar archive format -- does split the device number into major and minor. (They are typically encoded as seven octal digits each, so for compatibility reasons one should limit to 21-bit unsigned major and 21-bit unsigned minor. This also means that mapping the device number to just the major or just the minor is not a reliable approach.)

The following include file expanding on Jonathon Reinhart's answer, after digging on the web for the various systems man pages and documentation (for makedev(), major(), and minor()), plus comments to this question.

#if defined(custom_makedev) && defined(custom_major) && defined(custom_minor)
/* Already defined */
#else

#undef custom_makedev
#undef custom_major
#undef custom_minor

#if defined(__linux__) || defined(__GLIBC__)
/* Linux, Android, and other systems using GNU C library */
#ifndef _BSD_SOURCE
#define _BSD_SOURCE 1
#endif
#include <sys/types.h>
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)

#elif defined(_WIN32)
/* 32- and 64-bit Windows. VERIFY: These are just a guess! */
#define custom_makedev(dmajor, dminor) ((((unsigned int)dmajor << 8) & 0xFF00U) | ((unsigned int)dminor & 0xFFFF00FFU))
#define custom_major(devnum)           (((unsigned int)devnum & 0xFF00U) >> 8)
#define custom_minor(devnum)           ((unsigned int)devnum & 0xFFFF00FFU)

#elif defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__) || defined(__DragonFly__)
/* FreeBSD, OpenBSD, NetBSD, and DragonFlyBSD */
#include <sys/types.h>
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)

#elif defined(__APPLE__) && defined(__MACH__)
/* Mac OS X */
#include <sys/types.h>
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)

#elif defined(_AIX) || defined (__osf__)
/* AIX, OSF/1, Tru64 Unix */
#include <sys/types.h>
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)

#elif defined(hpux)
/* HP-UX */
#include <sys/sysmacros.h>
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)

#elif defined(sun)
/* Solaris */
#include <sys/types.h>
#include <sys/mkdev.h>
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)

#else
/* Unknown OS. Try a the BSD approach. */
#ifndef _BSD_SOURCE
#define _BSD_SOURCE 1
#endif
#include <sys/types.h>
#if defined(makedev) && defined(major) && defined(minor)
#define custom_makedev(dmajor, dminor) makedev(dmajor, dminor)
#define custom_major(devnum)           major(devnum)
#define custom_minor(devnum)           minor(devnum)
#endif
#endif

#if !defined(custom_makedev) || !defined(custom_major) || !defined(custom_minor)
#error Unknown OS: please add definitions for custom_makedev(), custom_major(), and custom_minor(), for device number major/minor handling.
#endif

#endif

One could glean additional definitions from existing UStar-format -capable archivers. Compatibility with existing implementations on each OS/architecture is, in my opinion, the most important thing here.

The above should cover all systems using GNU C library, Linux (including Android), FreeBSD, OpenBSD, NetBSD, DragonFlyBSD, Mac OS X, AIX, Tru64, HP-UX, and Solaris, plus any that define the macros when <sys/types.h> is included. Of the Windows part, I'm not sure.

As I understand it, Windows uses device 0 for all normal files, and a HANDLE (a void pointer type) for devices. I am not at all sure whether the above logic is sane on Windows, but many older systems put the 8 least significant bits of the device number into minor, and the next 8 bits into major, and the convention seems to be that any leftover bits would be put (without shifting) into minor, too. Examining existing UStar-format tar archives with references to devices would be useful, but I personally do not use Windows at all.

If a system is not detected, and the system does not use the BSD-style inclusion for defining the macros, the above will error out stopping the compilation. (I would personally add compile-time machinery that could help finding the correct header definitions, using e.g. find, xargs, and grep, in case that happens, with a suggestion to send the addition upstream, too. touch empty.h ; cpp -dM empty.h ; rm -f empty.h should show all predefined macros, to help with identifying the OS and/or C library.)

Originally, POSIX stated that dev_t must be an arithmetic type (thus, theoretically, it might have been some variant of float or double on some systems), but IEEE Std 1003.1, 2013 Edition says it must be an integer type. I would wager that means no known POSIX-y system ever used a floating-point dev_t type. It would seem that Windows uses a void pointer, or HANDLE type, but Windows is not POSIX-compliant anyway.

Answer 2

Use the major() and minor() macros after defining BSD_SOURCE.

The makedev(), major(), and minor() functions are not specified in POSIX.1, but are present on many other systems.

http://man7.org/linux/man-pages/man3/major.3.html