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25Mapfiles and versioning in illumos
281.0 Objective of this README
30This README describes the engineering practices of creating and updating
31visible library interfaces.  It describes various kinds of actions that
32typically occur as libraries are evolved, and shows how interface
33specifications are affected or updated in accordance.  It tells you what
34you must do as a shared library developer if you:
36	1. Make interface additions to an existing library
37		- add a Public interface
38		- add a Private interface
39	2. Update an interface in an existing library
40		- remove an existing interface
41		- promote a Private interface to Public
42		- scope a Private interface to local
43		- move an interface from one library to another
44		- copy interfaces which are part of the standard to a new or
45		  existing library
46	3. Introduce a new library
47		- source directory hierarchy
48		- creation of the "mapfile-vers" file
49		- Makefiles
50	4. Make an entire library obsolete before end-of-life
51		- introduce SUNWobsolete to the "mapfile-vers" file
552.0 What's a mapfile?
57Mapfiles are used to tell the link-editor ("ld") all sorts of things about
58how to generate an executable file or a shared object from a collection of
59relocatable objects, such as generated by a compiler.  For all the gory
60details, see the Solaris Linker and Libraries Guide.
62There are two versions of the mapfile language accepted by the link-editor.
63Version 1 derives from AT&T System V Release 4 Unix. Version 2 is a newer
64syntax specific to Solaris and derivatives.  All mapfiles in illumos are
65required to use version 2 syntax. Note that every mapfile using version 2
66syntax must start with the line:
68        $mapfile_version 2
70Here, we are only concerned with specifying externally-visible interfaces
71for shared libraries (shared objects) and with specifying their versions
72for ABI (Application Binary Interface) purposes.  For these purposes, we
73only need to deal with a subset of the mapfile language.
75There should be a "mapfile-vers" file associated with every shared library
76and it should reside in the common source directory for that library, most
77often in a "common" directory.  This is the usual layout of a library's
78top-level directory (usr/src/lib/libwombat):
79	Makefile       amd64/         i386/          sparcv9/
80	Makefile.com   common/        sparc/
82The "common" directory contains the source files and other common files
83for the library:
84	bat.c              libwombat_impl.h   mapfile-vers       wom.c
85	libwombat.h        llib-lwombat       util.c             wombat.c
87The mapfile's name is, by convention, "mapfile-vers" because it is used
88for only two purposes: to specify externally-visible interface names while
89suppressing visibility of all other names, and to specify their respective
90unique version names.
943.0 Contents of mapfile-vers
96The structure of mapfile-vers is best explained by an example
97(the license notification and copyright notice is omitted here
98for brevity):
100$mapfile_version 2
102SYMBOL_VERSION ILLUMOS_0.2 {	# Second interface change in illumos
103    global:
104	wb_notify;
105} ILLUMOS_0.1;
107SYMBOL_VERSION ILLUMOS_0.1 {	# First interface change in illumos
108    global:
109	wb_poll;
110} SUNW_1.2;
112SYMBOL_VERSION SUNW_1.2 {	# update to libwombat, Solaris 10
113    global:
114	wb_readv;
115	wb_stat;
116	wb_writev;
117} SUNW_1.1;
119SYMBOL_VERSION SUNW_1.1 {	# first release of libwombat, Solaris 9
120    global:
121	wb_read;
122	wb_write;
125SYMBOL_VERSION SUNWprivate {	# private libwombat symbols
126    global:
127	wb_add;
128	wb_delete;
129	wb_search;
130    local:
131	*;
134Each of these sections is a version declaration describing an ABI version of
135the library containing the set of symbols exposed by the library to
136external callers.
138ABI versions must be constant, that is version ILLUMOS_0.2 in a given
139library must always describe the same interface such that applications may
140safely state their dependencies in terms of these versions and have a
141constant and predictable ABI be exposed.  This in effect means that once a
142version is publicly visible, it may never be removed or have symbols added
143to or removed from it.
145ABI versions with the same major number should be upward compatible, that is
146ILLUMOS_0.3 of a given library must contain all the interfaces in
147ILLUMOS_0.2, and they must be compatible.
149The private version, however, is special, and describes any private yet
150exposed symbols within a library, and may change at any time (and without
151notice).  It exists purely to allow certain symbols to be of global scope
152but not Public.  Similarly, any symbols scoped local are private and may
153change safely, even if the local statement happens to be within a public
156Interface changes made in illumos should be done with ILLUMOS_0.* versions,
157introducing one version per interface change.  In illumos, unlike Solaris,
158symbol versions are not release-specific because of the requirement that
159each be constant.  No change should be made to add or remove symbols from
160any pre-existing Public version.
162The SUNW_*.* names were the Public version names of the library in Solaris.
163There should be at most one version name for each release of Solaris, with
164the minor number incremented by one over the previous version.  No changes
165should ever be made to SUNW_1.* versions.
167So, for example, to add a new interface to libwombat in illumos one would add:
169SYMBOL_VERSION ILLUMOS_0.3 {	# Third update to libwombat in illumos
170    global:
171	wb_lseek;
172} ILLUMOS_0.2;
174Each version must inherit all symbols from its preceding version, specified at
175the ending "}" for each version. The initial public version does not inherit
176any symbols.  The private version named either "SUNWprivate" for libraries
177with private symbols pre-existing illumos, or "ILLUMOSprivate" otherwise
178stands alone, inheriting nothing and being inherited by nothing.
180The two lines in SUNWprivate:
181    local:
182	*;
183ensure that no symbols other than those listed in the mapfile are visible to
184clients of the library.  If there is no private version, these two lines should
185appear in the first public version.
187For maintainability, the list of names in each version block should
188be sorted in dictionary order (sort -d).  Please comply.
190The version 2 mapfile language supports a simple mechanism for conditional
191input, in which lines in the mapfile apply only to a specific platform or
192ELFCLASS (32/64-bit). This mechanism works very much like the #if/#endif
193feature of the C preprocessor. For instance, the following mapfile declares
194a version SUNW_1.1 that always exports a symbol foo, and also exports
195the symbol bar on 32-bit sparc platforms:
197$mapfile_version 2
199        foo;
200$if _sparc && _ELF32
201	bar;
205Conditional input can be used if there are ISA-specific library interfaces
206not common to all instances of the library. It is the preferred method for
207expressing platform specific items, as long as the differences are simple
208(which is almost always the case).  For example, see libproc, or, if you
209are masochistic, libc or libnsl.  In general, use of this feature should be
212In addition to conditional input, there is a second heavier weight mechanism
213for expressing ISA-specific differences. In addition to the common mapfile:
214	common/mapfile-vers
215some libraries may have ISA-specific supplemental mapfiles, one in each
216of the ISA directories:
217	amd64/mapfile-vers
218	i386/mapfile-vers
219	sparc/mapfile-vers
220	sparcv9/mapfile-vers
221The ISA-specific mapfiles look like the common mapfile, except that only
222the ISA-specific names appear.  The version names are the same as those
223in the common mapfile, but only non-empty version instances are present
224and no inheritance specification is present. The link-editor reads the
225information from the common and ISA-specific mapfiles and merges them
226in memory into a single description used to create the resulting object.
228ISA-specific mapfiles were used with the version 1 mapfile language, which
229lacked conditional input. Their use is rare now, as conditional input is
230generally preferred. However, it is important to use conditional input
231carefully, or the resulting mapfile can be extremly difficult to read.
2354.0 Making interface additions to an existing library
2374.1 Adding a Public interface
239Public interfaces should be added to a new ILLUMOS_ symbol version, with the
240minor number incremented by one from the current highest version name. If
241this is the first Public interface in the shared object, a new ILLUMOS_0.1
242version name must be introduced.
244The major revision number is incremented whenever an incompatible change is
245made to an interface.  This could be the case if an API changes so
246dramatically as to invalidate dependencies.  This should almost never occur
247in practice.  It also requires changing the suffix of the shared object
248from, say, .so.1 to .so.2 and introducing code to continue to ship the .so.1
249version of the library.
251The minor revision number is incremented whenever one or more new interfaces
252is added to a library.  Once a version comes to exist in illumos, it is from
253that point onward considered to be immutable.
255While strongly discouraged, if it is necessary to add global data symbols to a
256library (which become part of the ABI), an assertion that specifies the size
257of the object must be added.
2594.2 Adding a Private interface
261Private interfaces are the non-ABI interfaces of the library.  Unlike
262introducing a Public interface, a new entry is simply added to the
263private version.  No minor number increment is necessary.
265If this interface happens to be the first Private interface introduced into
266the library, the private version must be created (with no major.minor
267version numbers).  It inherits nothing, nothing inherits from it and it
268should be named ILLUMOSprivate.
270If the library already has Private interfaces in a SUNWprivate version, you
271should use that.  They may have numbered version names like SUNWprivate_m.n
272(due to errors of the past).  If so, just use the highest numbered private
273version name to version the new interface.  There is no need to introduce a
274new private version name.  Be careful not to use a lower numbered private
275version name; doing so can cause runtime errors (as opposed to load time
276errors) when running an application with older versions of the library.
278There are also libraries in illumos that contain only private interfaces. In
279such libraries, the private versions maybe legitimately be versioned and
280they may be incremented to ensure that the programs that depend on them are
281built and delivered as a integrated unit. A notable example of this is
282libld.so (usr/src/cmd/sgs/libld), which contains the implementation of the
283link-editor, the public interface to which is provided by the ld
284command. When making a modification to the interface of such a library, you
285should follow the convention already in place.
287As with public interfaces, the size of global objects must be asserted in
288private versions (though it will be less problematic to change them later).
2904.3 Historical handling of Solaris update releases.
292To aid the understanding of our existing mapfiles, it is useful to note how
293interface versions were handled as they interacted with update releases of
294Solaris.  Solaris update releases required careful coordination with the full
295release currently under development to keep symbol versions constant between
298Multiple update releases were generally shipped during the development of the
299next full release of Solaris.  It was impossible to know in advance the full
300set of new interfaces in the next full release until it was complete.  Some,
301though not all, new interfaces were included in the intervening update
302releases between full releases.
304Consequently, the new version number for an update cannot be a minor
305increment, but must be a micro increment to ensure that was a distinct version
306between the two releases.  For example, if Release N had version number
307SUNW_1.3 and Release N+1 had SUNW_1.4, then interfaces added to an update of
308Release N must have micro numbers such as SUNW_1.3.1, SUNW_1.3.2, etc.  (note
309that the micro number is not directly tied to the update number: SUNW_1.3.1
310may have appeared in Update 2).  The micro versions form an inheritance chain
311that is inserted between two successive minor versions.  For example, the
312mapfile-vers file for minor release "N+1" to reflect its inclusion of micro
313releases will look like the following:
315$mapfile_version 2
317SYMBOL_VERSION SUNW_1.4 {	# release N+1
318    global:
319	...
320} SUNW_1.3.2;
322SYMBOL_VERSION SUNW_1.3.2 {	# micro release 2 (e.g., release NU3)
323    global:
324	...
325} SUNW_1.3.1;
327SYMBOL_VERSION SUNW_1.3.1 {	# micro release 1 (e.g., release NU2)
328    global:
329	...
330} SUNW_1.3;
332SYMBOL_VERSION SUNW_1.3 {	# release N
333    global:
334	...
335} SUNW_1.2;
337SYMBOL_VERSION SUNW_1.2 {	# release N-1
338    global:
339	...
340} SUNW_1.1;
342SYMBOL_VERSION SUNW_1.1 {	# first release
343    global:
344	...
347SYMBOL_VERSION SUNW_private {	# same in all releases
348    global:
349	...
350    local:
351	*;
354The corresponding update/patch mapfile-vers file will be identical
355except for the exclusion of SUNW_1.4.
357Those interfaces which are only present in Release N+1 are always put
358into the next minor version set, SUNW_1.4.
360Thus when adding a new Public interface to an update release, both the mapfiles
361of the update release and next full release should have been modified to be
364There have been several cases of accidental deviation from this scheme, and
365existing mapfiles sometimes reflect this unfortunate state of affairs.
3695.0 How to update an interface in an existing library
3715.1 Removing an existing interface
3735.1.1 Moving a Public interface
375No Public interfaces should ever be removed from any mapfile, as this will
376break all existing consumers of that interface.
378To move an interface from one library to (say) libc, the code has to be
379deleted from the library and added to libc, then the mapfile for the
380library has to have the interface's entry changed from:
381	getfoobar;
383	getfoobar       { TYPE = FUNCTION; FILTER = libc.so.1 };
384This is an exception to the immutability of public symbol versions.  See,
385for example, libnsl's common/mapfile-vers file.
387Follow the rules for adding a new interface for the necessary changes
388to libc's mapfile to accommodate the moved interface, including creating a
389new version in libc for the symbol.
391When merging an entire library into libc, the mapfile is changed to specify
392the type of each public symbol similarly to the above:
393	getfoobar;
395	getfoobar       { TYPE = FUNCTION };
397But rather than specifying the library on which we filter for each symbol,
398the link-editor is invoked with '-F libc.so.1' to specify that our entire
399symbol table is a filter on libc.  For examples, see libaio and librt.
4015.1.2 Removing a Private interface
403Deletion of Private interfaces is allowed, but caution should be taken;
404it should first be established that the interface is not being used.
405To remove a Private interface, simply delete the corresponding entry
406for that symbol from the mapfile's private version section.
408Do not forget to delete these Public or Private interfaces from the library's
409header files as well as from the code that implements the interfaces.
4115.2 Promoting a Private interface to Public
413This is similar to what's done when adding a Public interface.  Promoting an
414existing Private interface to a Public one only requires a change to the
415existing interface definition.  Private interfaces have the symbol version
416name "ILLUMOSprivate" or "SUNWprivate" associated with them.  To make the
417interface a Public one, the interface must be added as if it were a new
418public symbol, following those same rules and removed from the private
421As an example, if we were modifying libwombat.so.1 and its existing latest
422version were ILLUMOS_0.3, any new ABI would be put into a version called
423ILLUMOS_0.4.  Therefore, whether you wish to promote an existing Private
424interface to Public, or to introduce a new Public interface, this (next
425successive minor numbered version level) would be the version that it would
426be associated with.
4285.3 Scoping a Private interface local
430Any interfaces not present in the mapfile-vers file will be scoped local
431due to the presence of the
432    local:
433	*;
434lines discussed earlier. This ensures that such interfaces will not be visible
435outside the library.  To move an interface from Private to local scope, simply
436remove the Private interface from the mapfile-vers file and the header file
437to prevent it from being exported.  This may require moving the Private
438interface into a library-private header file.  Scope reduction of Public
439interfaces is forbidden.
441For the interface to be used in more than one file within the library, it
442should be in a header file that can be included by each file in the library
443that uses the interface.  For example:
445	#include "libprivate.h"
4475.4 How to copy interfaces which are part of a standard to a new or existing
448    library
450SYSVABI and SISCD are reserved version names for interfaces listed in the
451System V Interface Definition and the Sparc Compliance Definition.  Avoid
452using these version names when copying the implementation of standard
453interfaces to another library.  Instead, use ILLUMOS_0.1 for a new library,
454and ILLUMOS_m.n for an existing library (where m.n is the next version; i.e.,
455if the last version was ILLUMOS_0.8, then you should version the interfaces
456with ILLUMOS_0.9).
4606.0 Introducing a new library
4626.1 Directories
464The normal discipline for introducing a new library in illumos is to create a
465new subdirectory of usr/src/lib.  The interface definition discipline is to
466create a common/mapfile-vers file for the new library.  If we were introducing
467a new foo library, libfoo, we'd create usr/src/lib/libfoo containing:
468	Makefile       amd64/         i386/          sparcv9/
469	Makefile.com   common/        sparc/
470The common subdirectory would contain the normal source files plus the
471mapfile-vers file.  See usr/src/lib/README.Makefiles for directions on
472how to organize the Makefiles.
4746.2 The mapfile
476The new common/mapfile-vers file would contain:
478$mapfile_version 2
480SYMBOL_VERSION ILLUMOS_0.1 {	# first release of libfoo
481    global:
482	...
486    global:
487	...
488    local:
489	*;
492If there are no Public interfaces, the ILLUMOS_0.1 section would be omitted.
493If there are no Private interfaces, the ILLUMOSprivate section would be
494omitted and the two lines:
495    local:
496	*;
497would be moved into ILLUMOS_0.1.
499To decide which interfaces are Public (part of the ABI) and which are
500Private (unstable interfaces not intended to be used by third parties), the
501heuristic which works to a first approximation is that if it has a man page
502then it's Public.
504For maintainability, the list of names in each version block should
505be sorted in dictionary order (sort -d).  Please comply.
5097.0 Make an entire library obsolete
5117.1 Introduce SUNWobsolete version
513Use this version name not for specific interfaces but for marking an entire
514library as obsolete.  The existing public/private version names are left
515unchanged, but a new SUNWobsolete version is created with no symbols in it.
516This becomes a tag by which the obsolescence of the library can be recognized.
517There is no numbering of this version name.
519$mapfile_version 2
522    global:
523	SUNWobsolete;	# This is the only way to do it.
524} SUNW_1.2;
529You should continue to use the name SUNWobsolete even in illumos.
5338.0 Documentation
535For further information, please refer to the following documents:
537	"Solaris Linker and Libraries Guide"