A dynamic object can have one or more internal version definitions associated with it. Each version definition is commonly associated with one or more symbol names. A symbol name can only be associated with one version definition. However, a version definition can inherit the symbols from other version definitions. Thus, a structure exists to define one or more independent, or related, version definitions within the object being created. As new changes are made to the object, new version definitions can be added to express these changes.
There are two consequences of providing version definitions within a shared object:
Dynamic objects that are built against a versioned shared object can record their dependency on the version definitions bound to. These version dependencies are verified at runtime to ensure that the appropriate interfaces, or functionality, are available for the correct execution of an application.
Dynamic objects can select the version definitions of a shared object to bind to during their link-edit. This mechanism enables developers to control their dependency on a shared object to the interfaces, or functionality, that provide the most flexibility.
Version definitions commonly consist of an association of symbol names to a unique version name. These associations are established within a mapfile and supplied to the final link-edit of an object using the link-editor's -M option. This technique is introduced in the section Reducing Symbol Scope.
A version definition is established whenever a version name is specified as part of the mapfile directive. In the following example, two source files are combined, together with mapfile directives, to produce an object with a defined public interface:
$ cat foo.c extern const char * _foo1; void foo1() { (void) printf(_foo1); } $ cat data.c const char * _foo1 = "string used by foo1()\n"; $ cat mapfile SUNW_1.1 { # Release X global: foo1; local: *; }; $ cc -o libfoo.so.1 -M mapfile -G foo.o data.o $ nm -x libfoo.so.1 | grep "foo.$" [33] |0x0001058c|0x00000004|OBJT |LOCL |0x0 |17 |_foo1 [35] |0x00000454|0x00000034|FUNC |GLOB |0x0 |9 |foo1 |
The symbol foo1 is the only global symbol defined to provide the shared object's public interface. The special auto-reduction directive “*” causes the reduction of all other global symbols to have local binding within the object being generated. This directive is introduced in Defining Additional Symbols. The associated version name, SUNW_1.1, causes the generation of a version definition. Thus, the shared object's public interface consists of the internal version definition SUNW_1.1, associated with the global symbol foo1.
Whenever a version definition, or the auto-reduction directive, are used to generate an object, a base version definition is also created. This base version is defined using the name of the file itself, and is used to associate any reserved symbols generated by the link-editor. See Generating the Output File for a list of these reserved symbols.
The version definitions contained within an object can be displayed using pvs(1) with the -d option:
$ pvs -d libfoo.so.1 libfoo.so.1; SUNW_1.1; |
The object libfoo.so.1 has an internal version definition named SUNW_1.1, together with a base version definition libfoo.so.1.
The link-editor's -z noversion option allows symbol reduction to be directed by a mapfile but suppresses the creation of version definitions.
Starting with this initial version definition, the object can evolve by adding new interfaces and updated functionality. For example, a new function, foo2, together with its supporting data structures, can be added to the object by updating the source files foo.c and data.c:
$ cat foo.c extern const char * _foo1; extern const char * _foo2; void foo1() { (void) printf(_foo1); } void foo2() { (void) printf(_foo2); } $ cat data.c const char * _foo1 = "string used by foo1()\n"; const char * _foo2 = "string used by foo2()\n"; |
A new version definition, SUNW_1.2, can be created to define a new interface representing the symbol foo2. In addition, this new interface can be defined to inherit the original version definition SUNW_1.1.
The creation of this new interface is important as it identifies the evolution of the object and enables users to verify and select the interfaces to which they bind. These concepts are covered in more detail in Binding to a Version Definition and in Specifying a Version Binding.
The following example shows the mapfile directives that create these two interfaces.
$ cat mapfile SUNW_1.1 { # Release X global: foo1; local: *; }; SUNW_1.2 { # Release X+1 global: foo2; } SUNW_1.1; $ cc -o libfoo.so.1 -M mapfile -G foo.o data.o $ nm -x libfoo.so.1 | grep "foo.$" [33] |0x00010644|0x00000004|OBJT |LOCL |0x0 |17 |_foo1 [34] |0x00010648|0x00000004|OBJT |LOCL |0x0 |17 |_foo2 [36] |0x000004bc|0x00000034|FUNC |GLOB |0x0 |9 |foo1 [37] |0x000004f0|0x00000034|FUNC |GLOB |0x0 |9 |foo2 |
The symbols foo1 and foo2 are both defined to be part of the shared object's public interface. However, each of these symbols is assigned to a different version definition; foo1 is assigned to SUNW_1.1, and foo2 is assigned to SUNW_1.2.
These version definitions, their inheritance, and their symbol association can be displayed using pvs(1) together with the -d, -v and -s options:
$ pvs -dsv libfoo.so.1 libfoo.so.1: _end; _GLOBAL_OFFSET_TABLE_; _DYNAMIC; _edata; _PROCEDURE_LINKAGE_TABLE_; _etext; SUNW_1.1: foo1; SUNW_1.1; SUNW_1.2: {SUNW_1.1}: foo2; SUNW_1.2 |
The version definition SUNW_1.2 has a dependency on the version definition SUNW_1.1.
The inheritance of one version definition by another is a useful technique that reduces the version information that will eventually be recorded by any object that binds to a version dependency. Version inheritance is covered in more detail in the section Binding to a Version Definition.
Any internal version definition has an associated version definition symbol created. As shown in the previous pvs(1) example, these symbols are displayed when using the -v option.
Internal changes to an object that do not require the introduction of a new interface definition can be defined by creating a weak version definition. Examples of such changes are bug fixes or performance improvements.
Such a version definition is empty, in that it has no global interface symbols associated with it.
For example, suppose the data file data.c, used in the previous examples, is updated to provide more detailed string definitions:
$ cat data.c const char * _foo1 = "string used by function foo1()\n"; const char * _foo2 = "string used by function foo2()\n"; |
A weak version definition can be introduced to identify this change:
$ cat mapfile SUNW_1.1 { # Release X global: foo1; local: *; }; SUNW_1.2 { # Release X+1 global: foo2; } SUNW_1.1; SUNW_1.2.1 { } SUNW_1.2; # Release X+2 $ cc -o libfoo.so.1 -M mapfile -G foo.o data.o $ pvs -dv libfoo.so.1 libfoo.so.1; SUNW_1.1; SUNW_1.2: {SUNW_1.1}; SUNW_1.2.1 [WEAK]: {SUNW_1.2}; |
The empty version definition is signified by the weak label. These weak version definitions enable applications to verify the existence of a particular implementation by binding to the version definition associated with that functionality. The section Binding to a Version Definition illustrates how these definitions can be used in more detail.
The previous examples show how new version definitions added to an object inherit any existing version definitions. You can also create version definitions that are unique and independent. In the following example, two new files, bar1.c and bar2.c, are added to the object libfoo.so.1. These files contribute two new symbols, bar1 and bar2, respectively:
$ cat bar1.c extern void foo1(); void bar1() { foo1(); } $ cat bar2.c extern void foo2(); void bar2() { foo2(); } |
These two symbols are intended to define two new public interfaces. Neither of these new interfaces are related to each other. However, each expresses a dependency on the original SUNW_1.2 interface.
The following mapfile definition creates this required association:
$ cat mapfile SUNW_1.1 { # Release X global: foo1; local: *; }; SUNW_1.2 { # Release X+1 global: foo2; } SUNW_1.1; SUNW_1.2.1 { } SUNW_1.2; # Release X+2 SUNW_1.3a { # Release X+3 global: bar1; } SUNW_1.2; SUNW_1.3b { # Release X+3 global: bar2; } SUNW_1.2; |
Again, the version definitions created in libfoo.so.1 when using this mapfile, and their related dependencies, can be inspected using pvs(1):
$ cc -o libfoo.so.1 -M mapfile -G foo.o bar1.o bar2.o data.o $ pvs -dv libfoo.so.1 libfoo.so.1; SUNW_1.1; SUNW_1.2: {SUNW_1.1}; SUNW_1.2.1 [WEAK]: {SUNW_1.2}; SUNW_1.3a: {SUNW_1.2}; SUNW_1.3b: {SUNW_1.2}; |
The following sections explore how these version definition recordings can be used to verify runtime binding requirements and control the binding of an object during its creation.
When a dynamic executable or shared object is built against other shared objects, these dependencies are recorded in the resulting object. See Shared Object Processing and Recording a Shared Object Name for more details. If these shared object dependencies also contain version definitions, then an associated version dependency is recorded in the object being built.
The following example takes the data files from the previous section and generates a shared object suitable for a compile time environment. This shared object, libfoo.so.1, is used in the succeeding binding examples.
$ cc -o libfoo.so.1 -h libfoo.so.1 -M mapfile -G foo.o bar.o \ data.o $ ln -s libfoo.so.1 libfoo.so $ pvs -dsv libfoo.so.1 libfoo.so.1: _end; _GLOBAL_OFFSET_TABLE_; _DYNAMIC; _edata; _PROCEDURE_LINKAGE_TABLE_; _etext; SUNW_1.1: foo1; SUNW_1.1; SUNW_1.2: {SUNW_1.1}: foo2; SUNW_1.2; SUNW_1.2.1 [WEAK]: {SUNW_1.2}: SUNW_1.2.1; SUNW_1.3a: {SUNW_1.2}: bar1; SUNW_1.3a; SUNW_1.3b: {SUNW_1.2}: bar2; SUNW_1.3b |
In effect, there are six public interfaces being offered by the shared object. Four of these interfaces, SUNW_1.1, SUNW_1.2, SUNW_1.3a, and SUNW_1.3b, define exported symbol names. One interface, SUNW_1.2.1, describes an internal implementation change to the shared object, and one interface, libfoo.so.1, defines several reserved labels. Dynamic objects created with this shared object as a dependency, record the version names of the interfaces the dynamic object binds to.
The following example creates an application that references symbols foo1 and foo2. The versioning dependency information recorded in the application can be examined using pvs(1) with the -r option.
$ cat prog.c extern void foo1(); extern void foo2(); main() { foo1(); foo2(); } $ cc -o prog prog.c -L. -R. -lfoo $ pvs -r prog libfoo.so.1 (SUNW_1.2, SUNW_1.2.1); |
In this example, the application prog has bound to the two interfaces SUNW_1.1 and SUNW_1.2. These interfaces provided the global symbols foo1 and foo2 respectively.
Because version definition SUNW_1.1 is defined within libfoo.so.1 as being inherited by the version definition SUNW_1.2, you also need to record the latter version dependency. This normalization of version definition dependencies reduces the amount of version information maintained within an object, and reduces the processing required at runtime.
Because the application prog was built against the shared object's implementation containing the weak version definition SUNW_1.2.1, this dependency is also recorded. Even though this version definition is defined to inherit the version definition SUNW_1.2, the version's weak nature precludes its normalization with SUNW_1.1, and results in a separate dependency recording.
Had there been multiple weak version definitions that inherited from each other, then these definitions will be normalized in the same manner as non-weak version definitions are.
The recording of a version dependency can be suppressed by the link-editor's -z noversion option.
Having recorded these version definition dependencies, the runtime linker validates the existence of the required version definitions in the objects that are bound to when the application is executed. This validation can be displayed using ldd(1) with the -v option. For example, by running ldd(1) on the application prog, the version definition dependencies are shown to be found correctly in the shared object libfoo.so.1:
$ ldd -v prog find object=libfoo.so.1; required by prog libfoo.so.1 => ./libfoo.so.1 find version=libfoo.so.1; libfoo.so.1 (SUNW_1.2) => ./libfoo.so.1 libfoo.so.1 (SUNW_1.2.1) => ./libfoo.so.1 .... |
ldd(1) with the -v option implies verbose output. A recursive list of all dependencies, together with all versioning requirements, is generated.
If a non-weak version definition dependency cannot be found, a fatal error occurs during application initialization. Any weak version definition dependency that cannot be found is silently ignored. For example, if the application prog is run in an environment in which libfoo.so.1 only contains the version definition SUNW_1.1, then the following fatal error occurs:
$ pvs -dv libfoo.so.1 libfoo.so.1; SUNW_1.1; $ prog ld.so.1: prog: fatal: libfoo.so.1: version `SUNW_1.2' not \ found (required by file prog) |
Had the application prog not recorded any version definition dependencies, the nonexistence of the required interface symbol foo2 would have manifested itself some time during the execution of the application as a fatal relocation error. This relocation error might occur at process initialization, during process execution, or might not occur at all if the execution path of the application did not call the function foo2. See Relocation Errors.
Recording version definition dependencies provides an alternative and immediate indication of the availability of the interfaces required by the application.
If the application prog is run in an environment in which libfoo.so.1 only contains the version definitions SUNW_1.1 and SUNW_1.2, then all non-weak version definition requirements will be satisfied. The absence of the weak version definition SUNW_1.2.1 is deemed nonfatal, and so no runtime error condition is generated. However, ldd(1) can be used to display all version definitions that cannot be found:
$ pvs -dv libfoo.so.1 libfoo.so.1; SUNW_1.1; SUNW_1.2: {SUNW_1.1}; $ prog string used by foo1() string used by foo2() $ ldd prog libfoo.so.1 => ./libfoo.so.1 libfoo.so.1 (SUNW_1.2.1) => (version not found) ........... |
If an object requires a version definition from a given dependency, and at runtime an implementation of that dependency is found that contains no version definition information, the version verification of the dependency will be silently ignored. This policy provides a level of backward compatibility as a transition from non-versioned to versioned shared objects occurs. ldd(1), however, can still be used to display any version requirement discrepancies.
Version definition symbols also provide a mechanism for verifying the version requirements of an object obtained by dlopen(3DL). Any object added to the process's address space using this function will have no automatic version dependency verification carried out by the runtime linker. Thus, the caller of this function is responsible for verifying that any versioning requirements are met.
The presence of a required version definition can be verified by looking up the associated version definition symbol using dlsym(3DL). The following example shows the shared object libfoo.so.1 being added to a process by dlopen(3DL) and verified to ensure that the interface SUNW_1.2 is available.
#include <stdio.h> #include <dlfcn.h> main() { void * handle; const char * file = "libfoo.so.1"; const char * vers = "SUNW_1.2"; .... if ((handle = dlopen(file, RTLD_LAZY)) == NULL) { (void) printf("dlopen: %s\n", dlerror()); exit (1); } if (dlsym(handle, vers) == NULL) { (void) printf("fatal: %s: version `%s' not found\n", file, vers); exit (1); } .... |
When creating a dynamic object against a shared object containing version definitions, you can instruct the link-editor to limit the binding to specific version definitions. Effectively, the link-editor enables you to control an object's binding to specific interfaces.
An object's binding requirements can be controlled using a file control directive. This directive is supplied using the link-editor's -M option and an associated mapfile. The syntax for these file control mapfile directives is:
name - version [ version ... ] [ $ADDVERS=version ]; |
name – Represents the name of the shared object dependency. This name should match the shared object's compilation environment name as used by the link-editor. See Library Naming Conventions.
version – Represents the version definition name within the shared object that should be made available for binding. Multiple version definitions can be specified.
$ADDVERS – Allows for additional version definitions to be recorded.
This binding control can be useful:
If a shared object has been versioned to define unique and independent versions, possibly defining different standards interfaces. The application can then ensure that its bindings meet the requirements of a specific interface.
If a shared object has been versioned over several software releases, application developers can restrict themselves to the interfaces that were available in a previous software release. Thus, an application can be built using the latest release of the shared object in the knowledge that the application's interface requirements can be met by a previous release of the shared object.
The following example illustrates the user of the version control mechanism. This example uses the shared object libfoo.so.1 containing the following version interface definitions:
$ pvs -dsv libfoo.so.1 libfoo.so.1: _end; _GLOBAL_OFFSET_TABLE_; _DYNAMIC; _edata; _PROCEDURE_LINKAGE_TABLE_; _etext; SUNW_1.1: foo1; foo2; SUNW_1.1; SUNW_1.2: {SUNW_1.1}: bar; |
The version definitions SUNW_1.1 and SUNW_1.2 represent interfaces within libfoo.so.1 that were made available in software Release X and Release X+1 respectively.
An application can be built to bind only to the interfaces available in Release X by using the following version control mapfile directive:
$ cat mapfile libfoo.so - SUNW_1.1; |
For example, suppose you develop an application, prog, and want to ensure that the application can run on Release X. The application can then only use the interfaces available in that release. If the application mistakenly references the symbol bar, then the application's noncompliance to the required interface will be signalled by the link-editor as an undefined symbol error:
$ cat prog.c extern void foo1(); extern void bar(); main() { foo1(); bar(); } $ cc -o prog prog.c -M mapfile -L. -R. -lfoo Undefined first referenced symbol in file bar prog.o (symbol belongs to unavailable \ version ./libfoo.so (SUNW_1.2)) ld: fatal: Symbol referencing errors. No output written to prog |
To be compliant with the SUNW_1.1 interface, you must remove the reference to bar. You can either rework the application to remove the requirement on bar, or add an implementation of bar to the creation of the application.
To record more version dependencies than would be produced from the normal symbol binding of an object, use the $ADDVERS file control directive. This section describes a couple of scenarios where this additional binding might be useful.
Continuing with the libfoo.so.1 example, assume that in Release X+2, the version definition SUNW_1.1 is subdivided into two standard releases, STAND_A and STAND_B. To preserve compatibility, the SUNW_1.1 version definition must be maintained. In this example, this version definition is expressed as inheriting the two standard definitions:
$ pvs -dsv libfoo.so.1 libfoo.so.1: _end; _GLOBAL_OFFSET_TABLE_; _DYNAMIC; _edata; _PROCEDURE_LINKAGE_TABLE_; _etext; SUNW_1.1: {STAND_A, STAND_B}: SUNW_1.1; SUNW_1.2: {SUNW_1.1}: bar; STAND_A: foo1; STAND_A; STAND_B: foo2; STAND_B; |
If the only requirement of application prog is the interface symbol foo1, the application will have a single dependency on the version definition STAND_A. This precludes running prog on a system where libfoo.so.1 is less than Release X+2. The version definition STAND_A did not exist in previous releases, even though the interface foo1 did.
The application prog can be built to align its requirement with previous releases by creating a dependency on SUNW_1.1 by using the following file control directive:
$ cat mapfile libfoo.so - SUNW_1.1 $ADDVERS=SUNW_1.1; $ cat prog extern void foo1(); main() { foo1(); } $ cc -M mapfile -o prog prog.c -L. -R. -lfoo $ pvs -r prog libfoo.so.1 (SUNW_1.1); |
This explicit dependency is sufficient to encapsulate the true dependency requirements and satisfy compatibility with older releases.
Creating a Weak Version Definition described how weak version definitions can be used to mark an internal implementation change. These version definitions are well suited to indicate bug fixes and performance improvements made to an object. If the existence of a weak version is required for the correct execution of an application, then an explicit dependency on this version definition can be generated.
Establishing such a dependency can be important when a bug fix, or performance improvement, is critical for the application to function correctly.
Continuing with the libfoo.so.1 example, assume a bug fix is incorporated as the weak version definition SUNW_1.2.1 in software Release X+3:
$ pvs -dsv libfoo.so.1 libfoo.so.1: _end; _GLOBAL_OFFSET_TABLE_; _DYNAMIC; _edata; _PROCEDURE_LINKAGE_TABLE_; _etext; SUNW_1.1: {STAND_A, STAND_B}: SUNW_1.1; SUNW_1.2: {SUNW_1.1}: bar; STAND_A: foo1; STAND_A; STAND_B: foo2; STAND_B; SUNW_1.2.1 [WEAK]: {SUNW_1.2}: SUNW_1.2.1; |
Normally, if an application is built against this shared object, the application records a weak dependency on the version definition SUNW_1.2.1. This dependency is informational only. This dependency does not cause termination of the application should the version definition not exist in the libfoo.so.1 used at runtime.
The file control directive $ADDVERS can be used to generate an explicit dependency on a version definition. If this definition is weak, then this explicit reference also causes the version definition to be promoted to a strong dependency.
The application prog can be built to enforce the requirement that the SUNW_1.2.1 interface be available at runtime by using the following file control directive:
$ cat mapfile libfoo.so - SUNW_1.1 $ADDVERS=SUNW_1.2.1; $ cat prog extern void foo1(); main() { foo1(); } $ cc -M mapfile -o prog prog.c -L. -R. -lfoo $ pvs -r prog libfoo.so.1 (SUNW_1.2.1); |
prog has been built with an explicit dependency on the interface STAND_A. Because the version definition SUNW_1.2.1 is promoted to a strong version, it is also normalized with the dependency STAND_A. At runtime, if the version definition SUNW_1.2.1 cannot be found, a fatal error is generated.
When working with one or two dependencies, you can use the link-editor's -u option to explicitly bind to a version definition by referencing the version definition symbol. However, a symbol reference is nonselective. When working with multiple dependencies, that might contain similarly named version definitions, this technique is insufficient to create explicit bindings.
The various models for binding to versions within an object only remain intact if the individual version definitions remain constant over the life time of the object.
Once a version definition for an object has been created and made public, it must exist in subsequent releases of that object unchanged. Both the version name and the symbols associated with it must remain constant. For this reason, wildcard expansion of the symbol names defined within a version definition is not supported. The number of symbols matching the wildcard might differ over the course of an objects evolution.
Version information can be recorded and used within dynamic objects. Relocatable objects can maintain versioning information in a similar manner. However, there are one or two subtle differences in how this information is used.
Any version definitions supplied to the link-edit of a relocatable object are recorded in the same format as they are when building dynamic executables or shared objects. However, by default, symbol reduction is not carried out on the object being created. Instead, when the relocatable object is finally used as input to the generation of a dynamic object, the version recording itself will be used to determine the symbol reductions to apply.
In addition, any version definitions found in relocatable objects are propagated to the dynamic object. For an example of version processing in relocatable objects, see Reducing Symbol Scope.