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All rights reserved. 23# 24 25Mapfiles and versioning in illumos 26================================== 27 281.0 Objective of this README 29 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: 35 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 52 53------------------------------------------------------------------------------- 54 552.0 What's a mapfile? 56 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. 61 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: 67 68 $mapfile_version 2 69 70Here, we are only concerned with specifying externally-visible interfaces 71for shared libraries (shared objects) and with specifying their versions 72for ABI (Application Binary Interface) purposes. For these purposes, we 73only need to deal with a subset of the mapfile language. 74 75There should be a "mapfile-vers" file associated with every shared library 76and it should reside in the common source directory for that library, most 77often in a "common" directory. This is the usual layout of a library's 78top-level directory (usr/src/lib/libwombat): 79 Makefile amd64/ i386/ sparcv9/ 80 Makefile.com common/ sparc/ 81 82The "common" directory contains the source files and other common files 83for the library: 84 bat.c libwombat_impl.h mapfile-vers wom.c 85 libwombat.h llib-lwombat util.c wombat.c 86 87The mapfile's name is, by convention, "mapfile-vers" because it is used 88for only two purposes: to specify externally-visible interface names while 89suppressing visibility of all other names, and to specify their respective 90unique version names. 91 92------------------------------------------------------------------------------- 93 943.0 Contents of mapfile-vers 95 96The structure of mapfile-vers is best explained by an example 97(the license notification and copyright notice is omitted here 98for brevity): 99 100$mapfile_version 2 101 102SYMBOL_VERSION ILLUMOS_0.2 { # Second interface change in illumos 103 global: 104 wb_notify; 105} ILLUMOS_0.1; 106 107SYMBOL_VERSION ILLUMOS_0.1 { # First interface change in illumos 108 global: 109 wb_poll; 110} SUNW_1.2; 111 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; 118 119SYMBOL_VERSION SUNW_1.1 { # first release of libwombat, Solaris 9 120 global: 121 wb_read; 122 wb_write; 123}; 124 125SYMBOL_VERSION SUNWprivate { # private libwombat symbols 126 global: 127 wb_add; 128 wb_delete; 129 wb_search; 130 local: 131 *; 132}; 133 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. 137 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. 144 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. 148 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 154version. 155 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. 161 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. 166 167So, for example, to add a new interface to libwombat in illumos one would add: 168 169SYMBOL_VERSION ILLUMOS_0.3 { # Third update to libwombat in illumos 170 global: 171 wb_lseek; 172} ILLUMOS_0.2; 173 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. 179 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. 186 187For maintainability, the list of names in each version block should 188be sorted in dictionary order (sort -d). Please comply. 189 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: 196 197$mapfile_version 2 198SYMBOL_VERSION SUNW_1.1 { 199 foo; 200$if _sparc && _ELF32 201 bar; 202$endif 203}; 204 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 210minimal. 211 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. 227 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. 232 233------------------------------------------------------------------------------- 234 2354.0 Making interface additions to an existing library 236 2374.1 Adding a Public interface 238 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. 243 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. 250 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. 254 2554.2 Adding a Private interface 256 257Private interfaces are the non-ABI interfaces of the library. Unlike 258introducing a Public interface, a new entry is simply added to the 259private version. No minor number increment is necessary. 260 261If this interface happens to be the first Private interface introduced into 262the library, the private version must be created (with no major.minor 263version numbers). It inherits nothing, nothing inherits from it and it 264should be named ILLUMOSprivate. 265 266If the library already has Private interfaces in a SUNWprivate version, you 267should use that. They may have numbered version names like SUNWprivate_m.n 268(due to errors of the past). If so, just use the highest numbered private 269version name to version the new interface. There is no need to introduce a 270new private version name. Be careful not to use a lower numbered private 271version name; doing so can cause runtime errors (as opposed to load time 272errors) when running an application with older versions of the library. 273 274There are also libraries in illumos that contain only private interfaces. In 275such libraries, the private versions maybe legitimately be versioned and 276they may be incremented to ensure that the programs that depend on them are 277built and delivered as a integrated unit. A notable example of this is 278libld.so (usr/src/cmd/sgs/libld), which contains the implementation of the 279link-editor, the public interface to which is provided by the ld 280command. When making a modification to the interface of such a library, you 281should follow the convention already in place. 282 2834.3 Historical handling of Solaris update releases. 284 285To aid the understanding of our existing mapfiles, it is useful to note how 286interface versions were handled as they interacted with update releases of 287Solaris. Solaris update releases required careful coordination with the full 288release currently under development to keep symbol versions constant between 289releases. 290 291Multiple update releases were generally shipped during the development of the 292next full release of Solaris. It was impossible to know in advance the full 293set of new interfaces in the next full release until it was complete. Some, 294though not all, new interfaces were included in the intervening update 295releases between full releases. 296 297Consequently, the new version number for an update cannot be a minor 298increment, but must be a micro increment to ensure that was a distinct 299version between the two releases. For example, if Release N had version 300number SUNW_1.3 and Release N+1 had SUNW_1.4, then interfaces added to 301an update of Release N must have micro numbers such as SUNW_1.3.1, 302SUNW_1.3.2, etc. (note that the micro number is not directly tied to the 303update number: SUNW_1.3.1 may have appeared in Update 2). The micro versions form 304an inheritance chain that is inserted between two successive minor versions. 305For example, the mapfile-vers file for minor release "N+1" to reflect its 306inclusion of micro releases will look like the following: 307 308$mapfile_version 2 309 310SYMBOL_VERSION SUNW_1.4 { # release N+1 311 global: 312 ... 313} SUNW_1.3.2; 314 315SYMBOL_VERSION SUNW_1.3.2 { # micro release 2 (e.g., release NU3) 316 global: 317 ... 318} SUNW_1.3.1; 319 320SYMBOL_VERSION SUNW_1.3.1 { # micro release 1 (e.g., release NU2) 321 global: 322 ... 323} SUNW_1.3; 324 325SYMBOL_VERSION SUNW_1.3 { # release N 326 global: 327 ... 328} SUNW_1.2; 329 330SYMBOL_VERSION SUNW_1.2 { # release N-1 331 global: 332 ... 333} SUNW_1.1; 334 335SYMBOL_VERSION SUNW_1.1 { # first release 336 global: 337 ... 338}; 339 340SYMBOL_VERSION SUNW_private { # same in all releases 341 global: 342 ... 343 local: 344 *; 345}; 346 347The corresponding update/patch mapfile-vers file will be identical 348except for the exclusion of SUNW_1.4. 349 350Those interfaces which are only present in Release N+1 are always put 351into the next minor version set, SUNW_1.4. 352 353Thus when adding a new Public interface to an update release, both the mapfiles 354of the update release and next full release should have been modified to be 355consistent. 356 357There have been several cases of accidental deviation from this scheme, and 358existing mapfiles sometimes reflect this unfortunate state of affairs. 359 360------------------------------------------------------------------------------- 361 3625.0 How to update an interface in an existing library 363 3645.1 Removing an existing interface 365 3665.1.1 Moving a Public interface 367 368No Public interfaces should ever be removed from any mapfile, as this will 369break all existing consumers of that interface. 370 371To move an interface from one library to (say) libc, the code has to be 372deleted from the library and added to libc, then the mapfile for the 373library has to have the interface's entry changed from: 374 getfoobar; 375to: 376 getfoobar { TYPE = FUNCTION; FILTER = libc.so.1 }; 377This is an exception to the immutability of public symbol versions. See, 378for example, libnsl's common/mapfile-vers file. 379 380Follow the rules for adding a new interface for the necessary changes 381to libc's mapfile to accommodate the moved interface, including creating a 382new version in libc for the symbol. 383 384When merging an entire library into libc, the mapfile is changed to specify 385the type of each public symbol similarly to the above: 386 getfoobar; 387to: 388 getfoobar { TYPE = FUNCTION }; 389 390But rather than specifying the library on which we filter for each symbol, 391the link-editor is invoked with '-F libc.so.1' to specify that our entire 392symbol table is a filter on libc. For examples, see libaio and librt. 393 3945.1.2 Removing a Private interface 395 396Deletion of Private interfaces is allowed, but caution should be taken; 397it should first be established that the interface is not being used. 398To remove a Private interface, simply delete the corresponding entry 399for that symbol from the mapfile's private version section. 400 401Do not forget to delete these Public or Private interfaces from the library's 402header files as well as from the code that implements the interfaces. 403 4045.2 Promoting a Private interface to Public 405 406This is similar to what's done when adding a Public interface. Promoting an 407existing Private interface to a Public one only requires a change to the 408existing interface definition. Private interfaces have the symbol version 409name "ILLUMOSprivate" or "SUNWprivate" associated with them. To make the 410interface a Public one, the interface must be added as if it were a new 411public symbol, following those same rules and removed from the private 412version. 413 414As an example, if we were modifying libwombat.so.1 and its existing latest 415version were ILLUMOS_0.3, any new ABI would be put into a version called 416ILLUMOS_0.4. Therefore, whether you wish to promote an existing Private 417interface to Public, or to introduce a new Public interface, this (next 418successive minor numbered version level) would be the version that it would 419be associated with. 420 4215.3 Scoping a Private interface local 422 423Any interfaces not present in the mapfile-vers file will be scoped local 424due to the presence of the 425 local: 426 *; 427lines discussed earlier. This ensures that such interfaces will not be visible 428outside the library. To move an interface from Private to local scope, simply 429remove the Private interface from the mapfile-vers file and the header file 430to prevent it from being exported. This may require moving the Private 431interface into a library-private header file. Scope reduction of Public 432interfaces is forbidden. 433 434For the interface to be used in more than one file within the library, it 435should be in a header file that can be included by each file in the library 436that uses the interface. For example: 437 438 #include "libprivate.h" 439 4405.4 How to copy interfaces which are part of a standard to a new or existing 441 library 442 443SYSVABI and SISCD are reserved version names for interfaces listed in the 444System V Interface Definition and the Sparc Compliance Definition. Avoid using 445these version names when copying the implementation of standard interfaces to 446another library. Instead, use ILLUMOS_0.1 for a new library, and ILLUMOS_m.n for 447an existing library (where m.n is the next version; i.e., if the 448last version was ILLUMOS_0.8, then you should version the interfaces with 449ILLUMOS_0.9). 450 451------------------------------------------------------------------------------- 452 4536.0 Introducing a new library 454 4556.1 Directories 456 457The normal discipline for introducing a new library in illumos is to create a 458new subdirectory of usr/src/lib. The interface definition discipline is to 459create a common/mapfile-vers file for the new library. If we were introducing 460a new foo library, libfoo, we'd create usr/src/lib/libfoo containing: 461 Makefile amd64/ i386/ sparcv9/ 462 Makefile.com common/ sparc/ 463The common subdirectory would contain the normal source files plus the 464mapfile-vers file. See usr/src/lib/README.Makefiles for directions on 465how to organize the Makefiles. 466 4676.2 The mapfile 468 469The new common/mapfile-vers file would contain: 470 471$mapfile_version 2 472 473SYMBOL_VERSION ILLUMOS_0.1 { # first release of libfoo 474 global: 475 ... 476}; 477 478SYMBOL_VERSION ILLUMOSprivate { 479 global: 480 ... 481 local: 482 *; 483}; 484 485If there are no Public interfaces, the ILLUMOS_0.1 section would be omitted. 486If there are no Private interfaces, the ILLUMOSprivate section would be 487omitted and the two lines: 488 local: 489 *; 490would be moved into ILLUMOS_0.1. 491 492To decide which interfaces are Public (part of the ABI) and which are 493Private (unstable interfaces not intended to be used by third parties), the 494heuristic which works to a first approximation is that if it has a man page 495then it's Public. 496 497For maintainability, the list of names in each version block should 498be sorted in dictionary order (sort -d). Please comply. 499 500------------------------------------------------------------------------------- 501 5027.0 Make an entire library obsolete 503 5047.1 Introduce SUNWobsolete version 505 506Use this version name not for specific interfaces but for marking an entire 507library as obsolete. The existing public/private version names are left 508unchanged, but a new SUNWobsolete version is created with no symbols in it. 509This becomes a tag by which the obsolescence of the library can be recognized. 510There is no numbering of this version name. 511 512$mapfile_version 2 513 514SYMBOL_VERSION SUNWobsolete { 515 global: 516 SUNWobsolete; # This is the only way to do it. 517} SUNW_1.2; 518 519SYMBOL_VERSION ILLUMOS_0.2 { 520... 521 522You should continue to use the name SUNWobsolete even in illumos. 523 524------------------------------------------------------------------------------- 525 5268.0 Documentation 527 528For further information, please refer to the following documents: 529 530 "Solaris Linker and Libraries Guide" 531