Internal Versioning

A dynamic object can have one or more internal version definitions associated with the object. 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.

Version definitions within a shared object provide two facilities.

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.

Creating a Version Definition

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
#include <stdio.h>
extern  const char *_foo1;

void foo1()
{
        (void) printf(_foo1);
}

$ cat data.c
const char *_foo1 = "string used by foo1()\n";

$ cat mapfile
$mapfile_version 2
SYMBOL_VERSION SUNW_1.1 {                  # Release X
        global:
                foo1;
        local:
                *;
};
$ cc -c -Kpic foo.c data.c
$ cc -o libfoo.so.1 -M mapfile -G foo.o data.o
$ elfdump -sN.symtab libfoo.so.1 | grep 'foo.$'
      [32]  0x0001074c 0x00000004  OBJT LOCL  H    0 .data          _foo1
      [53]  0x00000560 0x00000038  FUNC GLOB  D    0 .text          foo1

The symbol foo1 is the only global symbol that is 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. The auto-reduction directive is described in SYMBOL_SCOPE / SYMBOL_VERSION Directives. The associated version name, SUNW_1.1, causes the generation of a version definition. Thus, the shared object's public interface consists of the global symbol foo1 associated to the internal version definition SUNW_1.1.

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 object being built. This base version is used to associate any reserved symbols generated by the link-editor. See Generating the Output File for a list of reserved symbols.

The version definitions that are 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.

Note - The link-editor's -z noversion option allows symbol reduction to be directed by a mapfile but suppresses the creation of version definitions.

From this initial version definition, the object can evolve by adding new interfaces together with 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
#include <stdio.h>
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 the interface describes the evolution of the object. These interfaces enable users to verify and select the interfaces to bind with. 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
$mapfile_version 2
SYMBOL_VERSION SUNW_1.1 {                   # Release X
        global:
                foo1;
        local:
                *;
};

SYMBOL_VERSION SUNW_1.2 {                   # Release X+1
        global:
                foo2;
} SUNW_1.1;

$ cc -o libfoo.so.1 -M mapfile -G foo.o data.o
$ elfdump -sN.symtab libfoo.so.1 | grep 'foo.$'
      [28]  0x000107a4 0x00000004  OBJT LOCL  H    0 .data          _foo1
      [29]  0x000107a8 0x00000004  OBJT LOCL  H    0 .data          _foo2
      [48]  0x000005e8 0x00000020  FUNC GLOB  D    0 .text          foo1
      [51]  0x00000618 0x00000020  FUNC GLOB  D    0 .text          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 version SUNW_1.1. foo2 is assigned to version 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 version definition is a useful technique. This inheritance reduces the version information that is eventually 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.

A version definition symbol is created and associated with a version definition. As shown in the previous pvs(1) example, these symbols are displayed when using the -v option.

Creating a Weak Version Definition

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. The version definition has no global interface symbols associated with the definition.

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
$mapfile_version 2
SYMBOL_VERSION SUNW_1.1 {                   # Release X
        global:
                foo1;
        local:
                *;
};
  
SYMBOL_VERSION SUNW_1.2 {                   # Release X+1
        global:
                foo2;
} SUNW_1.1;

SYMBOL_VERSION 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 detail. An application can bind to the version definition that is associated with an implementation detail that the application requires. The section Binding to a Version Definition illustrates how these definitions can be used in more detail.

Defining Unrelated Interfaces

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 interface expresses a dependency on the original SUNW_1.2 interface.

The following mapfile definition creates the required association.

$ cat mapfile
$mapfile_version 2
SYMBOL_VERSION SUNW_1.1 {                   # Release X
        global:
                foo1;
        local:
                *;
};
 
SYMBOL_VERSION SUNW_1.2 {                   # Release X+1
        global:
                foo2;
} SUNW_1.1;
 
SYMBOL_VERSION SUNW_1.2.1 { } SUNW_1.2;     # Release X+2
 
SYMBOL_VERSION SUNW_1.3a {                  # Release X+3
        global:
                bar1;
} SUNW_1.2;
 
SYMBOL_VERSION SUNW_1.3b {                  # Release X+3
        global:
                bar2;
} SUNW_1.2;

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};

Version definitions can be used to verify runtime binding requirements. Version definitions can also be used to control the binding of an object during the objects creation. The following sections explore these version definition usages in more detail.

Binding to a Version Definition

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 a dependency also contain version definitions, then an associated version dependency is recorded in the object being built.

The following example uses the data files from the previous section to generate a shared object, libfoo.so.1, which is suitable for a compile time environment.

$ 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

Six public interfaces are offered by the shared object libfoo.so.1. 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 object. One interface, libfoo.so.1, defines several reserved labels. Dynamic objects created with libfoo.so.1 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 that is 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 only need to record the one dependency. This inheritance provides for the normalization of version definition dependencies. This normalization reduces the amount of version information that is maintained within an object. This normalization also reduces the version verification processing that is 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. A weak version definition results in a separate dependency recording.

Had there been multiple weak version definitions that inherited from each other, then these definitions are normalized in the same manner as non-weak version definitions are.

Note - The recording of a version dependency can be suppressed by the link-editor's -z noversion option.

The runtime linker validates the existence of any recorded version definitions from 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 dependency 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
   ....

Note - 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)

If prog had not recorded any version definition dependencies, the nonexistence of the symbol foo2 could result in a fatal relocation error a runtime. This relocation error might occur at process initialization, or during process execution. An error condition might not occur at all if the execution path of the application did not call the function foo2. See Relocation Errors.

A version definition dependency provides an alternative and immediate indication of the availability of the interfaces required by the application.

For example, prog might run in an environment in which libfoo.so.1 only contains the version definitions SUNW_1.1 and SUNW_1.2. In this event, all non-weak version definition requirements are satisfied. The absence of the weak version definition SUNW_1.2.1 is deemed nonfatal. In this case, no runtime error condition is generated.

$ 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(1) can be used to display all version definitions that cannot be found.

$ ldd prog
        libfoo.so.1 =>   ./libfoo.so.1
        libfoo.so.1 (SUNW_1.2.1) =>          (version not found)
        ...........

At runtime, if an implementation of a dependency contains no version definition information, then any version verification of the dependency is silently ignored. This policy provides a level of backward compatibility as a transition from non-versioned to versioned shared objects occurs. ldd(1) can always be used to display any version requirement discrepancies.

Note - The environment variable LD_NOVERSION can be used to suppress all runtime versioning verification.

Verifying Versions in Additional Objects

Version definition symbols also provide a mechanism for verifying the version requirements of an object obtained by dlopen(3C). An object that is added to the process's address space by using dlopen(3C) receives no automatic version dependency verification. Thus, the caller of dlopen(3C) 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(3C). The following example adds the shared object libfoo.so.1 to a process using dlopen(3C). The availability of the interface SUNW_1.2 is then verified.

#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 | RTLD_FIRST))) == NULL) {
                (void) printf("dlopen: %s\n", dlerror());
                return (1);
        }
 
        if (dlsym(handle, vers) == NULL) {
                (void) printf("fatal: %s: version `%s' not found\n",
                    file, vers);
                return (1);
        }
        ....

Note - The use of the dlopen(3C) flag RTLD_FIRST ensures that the dlsym(3C) search is restricted to libfoo.so.1.

Specifying a Version Binding

When creating a dynamic object that is linked 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 DEPEND_VERSIONS mapfile directive. This directive is supplied using the link-editor's -M option and an associated mapfile. The DEPEND_VERSIONS directive uses the following syntax.

        $mapfile_version 2
        DEPEND_VERSIONS objname {
                ALLOW   = version_name;
                REQUIRE = version_name;
                ...
        };

objname 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.
The ALLOW attribute is used to specify version definition names within the shared object that should be made available for binding. Multiple ALLOW attributes can be specified.
The REQUIRE attribute allows additional version definitions to be recorded. Multiple REQUIRE attributes can be specified.

The control of version binding can be useful in the following scenarios.

When a shared object defines independent, unique versions. This versioning is possible when defining different standards interfaces. An object can be built with binding controls to ensure the object only binds to a specific interface.
When a shared object has been versioned over several software releases. An object can be built with binding controls to restrict its binding to the interfaces that were available in a previous software release. Thus, an object can run with an old release of the shared object dependency, after being built using the latest release of the shared object.

The following example illustrates the use 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
$mapfile_version 2
DEPEND_VERSIONS libfoo.so {
        ALLOW = 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 must only use the interfaces available in Release X. If the application mistakenly references the symbol bar, then the application is not compliant with the required interface. This condition is 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.

Note - By default, shared object dependencies encountered as part of a link-edit, are also verified against any file control directives. Use the environment variable LD_NOVERSION to suppress the version verification of any shared object dependencies.

Binding to Additional Version Definitions

To record more version dependencies than would be produced from the normal symbol binding of an object, use the REQUIRE attribute to the DEPEND_VERSIONS mapfiile directive. The following sections describe scenarios where this additional binding can be useful.

Redefining an Interface

One scenario is the consumption of an ISV specific interface into a public standard interface.

From the previous 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.

$ cat mapfile
$mapfile_version 2
DEPEND_VERSIONS libfoo.so {
        ALLOW = SUNW_1.1;
        REQURE = 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. This dependency satisfies compatibility with older releases.

Binding to a Weak Version

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, an explicit dependency on this version definition can be generated. The creation of such a dependency can be important when a bug fix, or performance improvement, is critical for the object to function correctly.

From the previous 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 libfoo.so.1, 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 implementation of libfoo.so.1 that is used at runtime.

The REQUIRE attribute to the DEPEND_VERSIONS mapfile directive can be used to generate an explicit dependency on a version definition. If this definition is weak, then this explicit reference also 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
$mapfile_version 2
DEPEND_VERSIONS libfoo.so {
        ALLOW = SUNW_1.1;
        REQUIRE = 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 an explicit dependency on the interface STAND_A. Because the version definition SUNW_1.2.1 is promoted to a strong version, the version SUNW_1.2.1 is normalized with the dependency STAND_A. At runtime, if the version definition SUNW_1.2.1 cannot be found, a fatal error is generated.

Note - When working with a small number of dependencies, you can use the link-editor's -u option to explicitly bind to a version definition. Use this option to reference the version definition symbol. However, a symbol reference is nonselective. When working with multiple dependencies, that contain similarly named version definitions, this technique might be insufficient to create explicit bindings.

Version Stability

Various models have been described that provide for binding to a version definition within an object. These models allow for the runtime validation of interface requirements. This verification only remains valid if the individual version definitions remain constant over the life time of the object.

A version definition for an object can be created for other objects to bind with. This version definition must continue to exist in subsequent releases of the object. Both the version name and the symbols associated with the version must remain constant. To help enforce these requirements, wildcard expansion of the symbol names defined within a version definition is not supported. The number of symbols that can match a wildcard might differ over the course of an objects evolution. This difference can lead to accidental interface instability.

Relocatable Objects

The previous sections have described how version information can be recorded within dynamic objects. Relocatable objects can maintain versioning information in a similar manner. However, subtle differences exist regarding how this information is used.

Any version definitions supplied to the link-edit of a relocatable object are recorded in the object. These definitions follow the same format as version definitions recorded in dynamic objects. However, by default, symbol reduction is not carried out on the relocatable object being created. Symbol reductions that are defined by the versioning information are applied to the relocatable object when the object is used to create a dynamic object.

In addition, any version definition found in a relocatable object is propagated to the dynamic object. For an example of version processing in relocatable objects, see Reducing Symbol Scope.

Note - Symbol reduction that is implied by a version definition can be applied to a relocatable object by using the link-editors -B reduce option.

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