2020-11-26

All about symbol versioning

Updated in 2026-01.

Many people just want to know how to define or reference versioned symbols properly. You may jump to Recommended usage below.

In 1995, Solaris' link editor and ld.so introduced the symbol versioning mechanism. Ulrich Drepper and Eric Youngdale borrowed Solaris' symbol versioning in 1997 and designed the GNU style symbol versioning for glibc.

gnu-gabi specification: https://sourceware.org/gnu-gabi/program-loading-and-dynamic-linking.txt#:~:text=versioning

When a shared object is updated and the behavior of a symbol changes, a DT_SONAME version bump is traditionally required to indicate ABI incompatibility (such as changing the type of parameters or return values). The DT_SONAME version bump can be inconvenient when there are many dependent applications. If we don't bump DT_SONAME, a dependent application/shared object built with the old version may run abnormally at run-time.

Symbol versioning provides a way to maintain backward compatibility without changing DT_SONAME.

The following part describes the representation, and then describes the behaviors from the perspectives of assembler, linker, and ld.so. One may wish to skip the representation part when reading for the first time.

Representation

In a shared object or executable file that uses symbol versioning, there are up to three sections. .gnu.version_r and .gnu.version_d are optional:

.gnu.version (type SHT_GNU_versym, pointed to by DT_VERSYM): A parallel table to .dynsym containing N uint16_t version IDs, one per dynamic symbol.
.gnu.version_r (type SHT_GNU_verneed, delimited by DT_VERNEED/DT_VERNEEDNUM): Describes versions required by undefined symbols.
.gnu.version_d (type SHT_GNU_verdef, delimited by DT_VERDEF/DT_VERDEFNUM): Describes versions of defined symbols.

The parallel table design makes symbol versioning optional - ld.so implementations that ignore it (like musl) treat all references as binding to default versions.

// Version definitions
typedef struct {
  Elf64_Half    vd_version;  // version: 1
  Elf64_Half    vd_flags;    // VER_FLG_BASE (index 1) or 0 (index != 1)
  Elf64_Half    vd_ndx;      // version index
  Elf64_Half    vd_cnt;      // number of associated aux entries, one plus number of parent versions
  Elf64_Word    vd_hash;     // SysV hash of the version name
  Elf64_Word    vd_aux;      // offset in bytes to the verdaux array
  Elf64_Word    vd_next;     // offset in bytes to the next verdef entry
} Elf64_Verdef;

typedef struct {
  Elf64_Word    vda_name;    // version name
  Elf64_Word    vda_next;    // offset in bytes to the next verdaux entry
} Elf64_Verdaux;

// Version needs
typedef struct {
  Elf64_Half    vn_version;  // version: 1
  Elf64_Half    vn_cnt;      // number of associated aux entries
  Elf64_Word    vn_file;     // .dynstr offset of the needed filename
  Elf64_Word    vn_aux;      // offset in bytes to vernaux array
  Elf64_Word    vn_next;     // offset in bytes to next verneed entry
} Elf64_Verneed;

typedef struct {
  Elf64_Word    vna_hash;    // SysV hash of vna_name
  Elf64_Half    vna_flags;   // usually 0; copied from vd_flags of the needed so
  Elf64_Half    vna_other;   // `Version:` in readelf -V output
  Elf64_Word    vna_name;    // .dynstr offset of the version name
  Elf64_Word    vna_next;    // offset in bytes to next vernaux entry
} Elf64_Vernaux;

Verdef entries describe version definitions in the module. Each Verdef has one or more Verdaux entries (vd_aux): the first gives the version name, and subsequent entries (if any, vda_next) name parent versions for version inheritance (e.g., v2 {} v1;). vd_cnt equals one plus the number of parent version definitions. However, it is not useful in practice—glibc and FreeBSD rtld ignore it, and ld.lld simply hard-codes it to 1.

Verneed entries describe version requirements from other shared objects (vn_file). Each Verneed has one or more Vernaux entries specifying the required version names (vna_name).

Version names are not globally unique. The following cases are valid:

A Verdef entry defines version v1 while a Verneed entry requires version v1 from a.so.
A Verneed entry requires version v1 from a.so while another Verneed entry requires version v1 from b.so.

`VER_FLG_WEAK`

GNU ld sets VER_FLG_WEAK in Verdef::vd_flags when a version node has no associated symbol, matching Solaris behavior. BZ24718#c15 proposed "set VER_FLG_WEAK on version reference if all symbols are weak" but was rejected. A 2026 comment requires to revisit whether VER_FLG_WEAK version references should support optional dependencies—e.g., an executable with a weak reference to foo links against libA.so.1 (which provides foo@@v1), but at runtime runs with a libA.so.1 that lacks the version definition for v1. Currently this causes an rtld error.

Version index values

Index 0 is called VER_NDX_LOCAL (misleading name, should have been VER_NDX_NONE). For defined symbols, the binding of the symbol will be changed to STB_LOCAL.
Index 1 is called VER_NDX_GLOBAL. It has no special effect and is used for unversioned symbols.
Index 2 to 0xffef are used for user defined versions.

Defined versioned symbols have two forms:

foo@@v2, the default version.
foo@v2, a non-default version (hidden version). The VERSYM_HIDDEN bit of the version ID is set.

Undefined versioned symbols have only the foo@v2 form.

There is a special case: a version symbol referenced by a copy relocation in an executable. The symbol acts as a definition in runtime relocation processing but its version ID references .gnu.version_r instead of .gnu.version_d. The resolution of PR28158 picks the form @.

Usually versioned symbols are only defined in shared objects, but executables can have defined versioned symbols as well. (When a shared object is updated, the old symbols are retained so that other shared objects do not need to be relinked, and executable files usually do not provide versioned symbols for other shared objects to reference.)

In the .gnu.version section (described by the DT_VERSYM tag), each symbol is assigned a version index:

Unversioned undefined symbols use index 0. However, LLD before 22 and GNU ld versions between 2.35 and 2.45 use index 1. See https://sourceware.org/bugzilla/show_bug.cgi?id=33577#c34
Versioned undefined symbols use index >= 2
Unversioned defined symbols use index 1
Versioned defined symbols use index >= 2

Defined symbols of index 0 are local and should not have an entry in .dynsym or .gnu.version.

Example

readelf -V can dump the symbol versioning tables.

In the .gnu.version_d output below:

Version index 1 (VER_NDX_GLOBAL) is the filename (soname if shared object). The VER_FLG_BASE flag is set.
Version index 2 is a user defined version. Its name is LUA_5.3.

In the .gnu.version_r output below, each of version indexes 3~10 represents a version in a needed shared object. The name GLIBC_2.2.5 appears thrice, each for a different shared object.

The .gnu.version table assigns a version index to each .dynsym entry. An entry (version ID) corresponds to a Index: entry in .gnu.version_d or a Version: entry in .gnu.version_r.

% readelf -V /usr/bin/lua5.3

Version symbols section '.gnu.version' contains 248 entries:
 Addr: 0x0000000000002af4  Offset: 0x002af4  Link: 5 (.dynsym)
  000:   0 (*local*)       3 (GLIBC_2.3)     4 (GLIBC_2.2.5)   4 (GLIBC_2.2.5)
  004:   5 (GLIBC_2.3.4)   4 (GLIBC_2.2.5)   4 (GLIBC_2.2.5)   4 (GLIBC_2.2.5)
  ...

Version definition section '.gnu.version_d' contains 2 entries:
 Addr: 0x0000000000002ce8  Offset: 0x002ce8  Link: 6 (.dynstr)
  000000: Rev: 1  Flags: BASE  Index: 1  Cnt: 1  Name: lua5.3
  0x001c: Rev: 1  Flags: none  Index: 2  Cnt: 1  Name: LUA_5.3

Version needs section '.gnu.version_r' contains 3 entries:
 Addr: 0x0000000000002d20  Offset: 0x002d20  Link: 6 (.dynstr)
  000000: Version: 1  File: libdl.so.2  Cnt: 1
  0x0010:   Name: GLIBC_2.2.5  Flags: none  Version: 9
  0x0020: Version: 1  File: libm.so.6  Cnt: 1
  0x0030:   Name: GLIBC_2.2.5  Flags: none  Version: 6
  0x0040: Version: 1  File: libc.so.6  Cnt: 6
  0x0050:   Name: GLIBC_2.11  Flags: none  Version: 10
  0x0060:   Name: GLIBC_2.14  Flags: none  Version: 8
  0x0070:   Name: GLIBC_2.4  Flags: none  Version: 7
  0x0080:   Name: GLIBC_2.3.4  Flags: none  Version: 5
  0x0090:   Name: GLIBC_2.2.5  Flags: none  Version: 4
  0x00a0:   Name: GLIBC_2.3  Flags: none  Version: 3

Symbol versioning in object files

The GNU scheme allows .symver directives to label the versions of the symbols in relocatable object files. The symbol names residing in .o contain @ or @@.

Assembler behavior

GNU as and LLVM integrated assembler provide implementation.

.symver foo, foo@v1
- If foo is undefined, produce foo@v1
- If foo is defined, produce foo and foo@v1 with the same binding (STB_LOCAL, STB_WEAK, or STB_GLOBAL) and st_other value (i.e. the same visibility). Personally I think this behavior is a design flaw {gas-copy}. The proposed V4 PATCH gas: Extend .symver directive can address this problem.
.symver foo, foo@@v1
- If foo is undefined, error
- If foo is defined, produce foo and foo@v1 with the same binding and st_other value.
.symver foo, foo@@@v1
- If foo is undefined, produce foo@v1
- If foo is defined, produce foo@@v1

With GNU as 2.35 (PR25295) or Clang 13:

.symver foo, foo@v1, remove
- If foo is undefined, produce foo@v1
- If foo is defined, produce foo@v1
- This is a recommended way to define a non-default version symbol.
- Unfortunately, in GNU as, foo cannot be used in a relocation (PR28157).

Linker behavior

The linker enters the symbol resolution stage after reading in object files, archive files, shared objects, LTO files, linker scripts, etc.

GNU ld uses indirect symbols to represent versioned symbols. There are complicated rules, and these rules are not documented. The symbol resolution rules that I personally derived:

Defined foo resolves undefined foo (traditional unversioned rule)
Defined foo@v1 resolves undefined foo@v1 (a non-default version symbol is like a separate symbol)
Defined foo@@v1 (default version) resolves both undefined foo and foo@v1

If there are multiple default version definitions (such as foo@@v1 foo@@v2), a duplicate definition error should be issued even if one is weak. Usually a symbol has zero or one default version (@@) definition, and an arbitrary number of non-default version (@) definitions.

If the linker sees undefined foo and foo@v1 first, it will treat them as two symbols. When the linker sees the definition foo@@v1, conceptually foo and foo@@v1 should be combined. If the linker sees foo@@v2 instead, foo@@v2 should resolve foo and foo@v1 should be a separate symbol.

Combining Versions describes the problem.
gold/symtab.cc Symbol_table::define_default_version uses a heuristic rule to solve this problem. It special cases on visibility, but I feel that this rule is unneeded.
Before 2.36, GNU ld reported a bogus multiple definition error for defined weak foo@@v1 and defined global foo@v1 PR ld/26978
Before 2.36, GNU ld had a bug that the visibility of undefined foo@v1 does not affect the output visibility of foo@@v1: PR ld/26979
I fixed the object file side problem of ld.lld 12.0 in https://reviews.llvm.org/D92259 foo Archive files and lazy object files may still have incompatibility issues.

When ld.lld sees a defined foo@@v, it adds both foo and foo@v1 into the symbol table, thus foo@@v1 can resolve both undefined foo and foo@v1. After processing all input files, a pass iterates symbols and redirects foo@v1 to foo@@v1. Because ld.lld treats them as separate symbols during input processing, a defined foo@v cannot suppress the extraction of an archive member defining foo@@v1, leading to a behavior incompatible with GNU ld. This probably does not matter, though.

If both foo and foo@v1 are defined (at the same position), foo will be removed. GNU ld has another strange behavior: if both foo and foo@v1 are defined, foo will be removed. I strongly believe it is an issue in GNU ld but the maintainer rejected PR ld/27210. I implemented a similar hack in ld.lld 13.0.0 (https://reviews.llvm.org/D107235) but hoped binutils can fix the assembler issue (https://sourceware.org/pipermail/binutils/2021-August/117677.html).

ld.lld assigns version indexes as follows: first, each Verdef entry gets an index starting from 2; then, for each dynamic symbol resolving to a shared object definition, a new Verneed/Vernaux index is assigned if the (file, version) pair hasn't been seen.

Version script

To define a versioned symbol in a shared object or an executable, a version script must be specified. If there is no defined versioned symbol, the version script can be omitted.

# Make all symbols other than foo and bar local.
{ global: foo; bar; local: *; };

# Assign version FBSD_1.0 to malloc and version FBSD_1.3 to mallocx,
# and make internal local.
FBSD_1.0 { malloc; local: internal; };
FBSD_1.3 { mallocx; };

A version script has three purposes:

Define versions.
Specify some patterns so that matched defined non-local symbols (which do not have @ in the name) are tied to the specified version.
Scope reduction
- for a matched symbol, its binding will be changed to STB_LOCAL and will not be exported to the dynamic symbol table.
- for a defined unversioned symbol, it can be matched by a local: pattern in any version node. E.g. foo can be matched by v1 { local: foo; };
- for a defined versioned symbol, it can be matched by a local: pattern in the associated version node. E.g. both foo@@v1 and foo@v1 can be matched by v1 { local: foo; };.

A version script consists of one anonymous version tag ({...};) or a list of named version tags (v1 {...};). If you use an anonymous version tag with other version tags, GNU ld will error: anonymous version tag cannot be combined with other version tags. A local: part can be placed in any version tag. Which version tag is used does not matter.

If a defined symbol is matched by multiple version tags, the following precedence rules apply (binutils-gdb/bfd/linker.c:find_version_for_sym):

The first version tag with an exact pattern (i.e. there is no wildcard) wins.
Otherwise, the last version tag with a non-* wildcard pattern in global: wins.
Otherwise, the last version tag with a non-* wildcard pattern in local: wins.
Otherwise, the last version tag with a * pattern wins.

In gold and ld.lld, the rules are like:

The first version tag with an exact pattern (i.e. there is no wildcard) wins.
Otherwise, the last version tag with a non-* wildcard pattern wins. If the version tag has non-* wildcard patterns in both global: and local:, the global: one wins.
Otherwise, the last version tag with a * pattern wins. (Prior to LLD 18, the first instead of the last)

For example, given v1 { local: p*;}; v2 { global: pq*;}; v3 { local: pqr*;};, local: pqr* is selected for a defined non-local symbol pqrs in gold and ld.lld while global: pq* is slected in GNU ld.

** is also a catch-all pattern, but its precedence is higher than *.

GNU ld reports an error when a pattern appears in both global: and local:.

Most patterns are exact so gold and ld.lld iterate patterns instead of symbols to improve performance.

GNU ld and gold add an absolute symbol (st_shndx=SHN_ABS) for each defined version to .symtab and .dynsym. ld.so does not need the symbol, so this behavior looks strange to me.

In a -r link, --version-script is ignored. Technically local: version nodes may be useful together with -r, but GNU ld and ld.lld just ignore --version-script.

How a versioned symbol is produced

An undefined symbol can be assigned a version if:

its name does not contain @ (.symver is unused) and a shared object provides a default version definition.
its name contains @ and a shared object defines the symbol. GNU ld errors if there is no such a shared object. After https://reviews.llvm.org/D92260, ld.lld will report an error as well.

A defined symbol can be assigned a version if:

its name does not contain @ and it is matched by a pattern in a named version tag in a version script.
its name contains @
- If -shared, the version should be defined by a version script, otherwise GNU ld errors version node not found for symbol. This exception looks strange to me so I have filed PR ld/26980.
- If -no-pie or -pie, a version definition is unneeded in GNU ld. This behavior is strange.

Recommended usage

Personal recommendation:

To define a default version symbol, don't use .symver. Just list the symbol name in a version node in the version script. If you really want to use .symver, use .symver foo, foo@@@v2 so that foo is not present. If you require binutils>=2.35 or Clang>=13, .symver foo, foo@@v2, remove works as well.

To define a non-default version symbol, add a suffix to the original symbol name (.symver foo_v1, foo@v1) to prevent conflicts with foo. This will however leave (usually undesirable) foo_v1. If you don't strip foo_v1 from the object file, you may localize it with a local: pattern in the version script. With the newer toolchain, you can use .symver foo_v1, foo@v1, remove.

cat > a.c <<e
__asm__(".symver foo_v1, foo@v1, remove");
void foo_v1() {}

void foo() {}
e
cat > a.ver <<e
v1 {};
v2 { foo; };
e

cc -fpic a.c -shared -Wl,--version-script=a.ver -o a.so

1
2
3

% readelf -W --dyn-syms a.so | grep @
     5: 0000000000001630     7 FUNC    GLOBAL DEFAULT   11 foo@@v2
     6: 0000000000001629     7 FUNC    GLOBAL DEFAULT   11 foo@v1

Most of the time, you want an undefined symbol to be bound to the default version symbol at link time. It is usually unnecessary to set the version with .symver.

If you really need to set a version, either .symver foo_v1, foo@@@v1 or .symver foo_v1, foo@v1 is fine.

cat > b.c <<e
__asm__(".symver foo_v1, foo@v1"); // foo@@@v1 and foo@v1, remove work as well
void foo_v1();

void bar() {
  foo_v1();
}
e

cc -fpic b.c ./a.so -shared -o b.so

The reference is bound to the non-default version foo@v1:

1 2	% readelf -W --dyn-syms b.so \| grep foo 5: 0000000000000000 0 FUNC GLOBAL DEFAULT UND foo@v1 (2)

If you omit .symver, the reference will be bound to the default version foo@@v2.

Why is `.symver xxx, foo@v1` bad with a defined symbol?

There are two cases.

First, xxx is not foo (the unadorned name of the versioned symbol). This is the most common usage. Without loss of generality, we use .symver foo_v1, foo@v1 as the example.

If the version script does not localize foo_v1, we will get foo_v1 in .dynsym. The extra symbol is almost always undesired.

Second, xxx is foo. The foo definition can satisfy unversioned references from other TUs. If you think about it, it is very rare for a non-default version definition to be used outside the TU.

# a.s
.symver foo, foo@v1
.globl foo
foo:

# b.s
# should reference foo@v1 instead of foo, if intended to use the non-default version symbol

If the version script contains v1 {};, the output will have just foo@v with GNU ld and ld.lld>=13.0.0. The output will have both foo and foo@v1 with gold and older ld.lld.

If the version script contains v1 { foo; };, the output will have just foo@v1 with GNU ld, gold, and ld.lld>=13.0.0. The output will have both foo and `foo@v1 with older ld.lld.

If the version script contains v2 { foo; };, the patterm will be ignored. Unfortunately no linker reports a warning for this error-prone case.

Having distinct behaviors is unfortunate. And the second case requires complexity in the linker internals.

rtld behavior

Linux Standard Base Core Specification, Generic Part describes the behavior of rtld (ld.so). Kan added symbol versioning support to FreeBSD rtld in 2005.

The DT_VERNEED and DT_VERNEEDNUM tags in the dynamic table delimiter the version requirement by a shared object/executable file: the required (needed) versions and required (needed) shared object names (Vernaux::vna_name).

When an object with DT_VERNEEDED is loaded, glibc rtld performs some checks (_dl_check_all_versions). For each Vernaux entry (a Verneed's auxiliary entry), glibc rtld checks whether the referenced shared object has a DT_VERDEF table. If no, ld.so handles the case as a graceful degradation and prints no version information available (required by %s); if yes and the table does not define the version, ld.so reports (if the Vernaux entry has the VER_FLG_WEAK bit) a warning or (otherwise) an error. [verneed-check]

Usually a minor release does not bump soname. Suppose that libB.so depends on libA 1.3 (soname is libA.so.1) and calls a function which does not exist in libA 1.2. If PLT lazy binding is used, libB.so may seem to work on a system with libA 1.2, until the PLT of the 1.3 symbol is called. If symbol versioning is not used and you want to solve this problem, you have to record the minor version number (libA.so.1.3) in the soname. However, bumping soname is all-or-nothing: all the dependent shared objects need to be relinked. If symbol versioning is used, you can continue to use the soname libA.so.1. ld.so will report an error if libA 1.2 is used, because the 1.3 version required by libB.so does not exist.

When searching a definition for foo,

for an object without DT_VERSYM
- it can be bound to foo
for an object with DT_VERSYM
- it can be bound to foo of version VER_NDX_GLOBAL. This takes precendence over the next two rules
- it can be bound to foo of any default version
- it can be bound to foo of non-default version index 2 in relocation resolving phase (not dlsym/dlvsym). The rule retains compatibility when a shared object becomes versioned.

Note (undefined foo binding to foo@v1 with version index 2) is allowed by ld.so but not allowed by the linker {reject-non-default}. The rtld behavior is to retains compatibility when a shared object becomes versioned: the symbols with the smallest version (index 2) indicate the previously unversioned symbols. If a new version of a shared object needs to deprecate an unversioned bar, you can remove bar and define bar@compat instead. Libraries using bar are unaffected but new linking against bar is disallowed.

When there are multiple versions of foo, dlsym(RTLD_DEFAULT, ...) returns the default version. On glibc before 2.36, dlsym(RTLD_NEXT, ...) returned the first version (BZ14932). This was because in elf/dl-sym.c:do_sym, the RTLD_NEXT branch did not pass the flags DL_LOOKUP_RETURN_NEWEST to dl_lookup_symbol_x. FreeBSD does not have the issue.

When searching a definition for foo@v1,

for an object without DT_VERSYM
- it can be bound to foo. In glibc, elf/dl-lookup.c:check_match asserts that the filename does not match the vn_file filename
for an object with DT_VERSYM
- it can be bound to foo@v1 or foo@@v1
- it can be bound to foo of version VER_NDX_GLOBAL in relocation resolving phase (not dlsym/dlvsym)

Say b.so references malloc@GLIBC_2.2.5. The executable defines an unversioned malloc due to linking in a malloc implementation. At run-time, malloc@GLIBC_2.2.5 in b.so will bind to the executable. For example, address/memory/thread sanitizers leverage this behavior: shared objects do not need to link in interceptors; having the interceptor in the executable is sufficient. libxml2 relied on the behavior to drop versioning on symbols while retaining compatibility for objects linking against older versions of libxml2.

When a versioned referenced is bound to a shared object without symbol versioning, in glibc versions before 2.41, elf/dl-lookup.c:check_match used to assert that the filename did not match the vn_file filename

echo 'void foo(); int main() { foo(); }' > a.c
echo 'v1 { foo; };' > c0.ver
echo 'void foo() {}' > c.c
sed 's/^        /\t/' > Makefile <<'eof'
.MAKE.MODE := meta curdirOk=1
CFLAGS := -fpic
LDFLAGS := -Wl,--no-as-needed

a: a.c c.so c0.so
        $(LINK.c) a.c c0.so -Wl,-rpath=$$PWD -o $@
c0.so: c.c c0.ver
        $(LINK.c) -shared -Wl,-soname=c.so,--version-script=c0.ver c.c -o $@
c.so: c.c
        $(LINK.c) -shared -Wl,-soname=c.so -nostdlib c.c -o $@
clean:
        rm -f a *.so *.o *.meta
eof

% bmake && ./a   # glibc<2.41
./a: /tmp/d/c.so: no version information available (required by ./a)
Inconsistency detected by ld.so: dl-lookup.c: 107: check_match: Assertion `version->filename == NULL || ! _dl_name_match_p (version->filename, map)' failed!

% bmake && ./a   # glibc>=2.41
./a: /tmp/d/c.so: no version information available (required by ./a)

This glibc<2.41 check is pretty dumb, as most shared objects have DT_VERSYM due to versioned references to libc like __cxa_finalize@GLIBC_2.2.5 (from GCC crtbeginS.o).

Aside from this assertion, vn_file is essentially ignored for symbol search since glibc 2.30 BZ24741. Previously during relocation resolving, after an object failed to provide a match, if it matched vn_file, rtld would report an error symbol %s version %s not defined in file %s with link time reference.

glibc 2.30 ld.so: Support moving versioned symbols between sonames [BZ #24741] has a side benefit for weak references. Previously, if b.so had a versioned weak reference foo@v1 (where v1 referenced c.so), rtld would error with symbol %s version %s not defined in file %s with link time reference when c.so lacked foo@@v1 or foo@v1—contrary to weak reference semantics. Newer rtld tolerates this as long as the runtime c.so defines version v1:

echo '#include <stdio.h>\nvoid fb(); int main() { fb(); puts("a"); }' > a.c
echo '__attribute__((weak)) void foo(); void fb() { if (foo) foo(); }' > b.c
echo 'v1 { foo; };' > c-link.ver
echo '#include <stdio.h>\nvoid foo() { puts("foo"); }' > c.c
echo 'v1 { };' > c.ver
echo 'v2 { };' > c2.ver
sed 's/^        /\t/' > Makefile <<'eof'
.MAKE.MODE := meta curdirOk=1
CFLAGS := -fpic

a: a.c b.so c-link.so c.so c2.so
	$(LINK.c) a.c b.so -Wl,-rpath=$$PWD -o $@
b.so: b.c c-link.so
	$(LINK.c) -shared -Wl,--no-as-needed $> -Wl,-rpath=$$PWD -o $@
c-link.so: c.c c.ver
	$(LINK.c) -shared -Wl,-soname=c.so,--version-script=c-link.ver c.c -o $@
c.so: c.c c.ver
	$(LINK.c) -shared -Wl,-soname=c.so,--version-script=c.ver -Dfoo=foo1 c.c -o $@
c2.so: c.c c2.ver
	$(LINK.c) -shared -Wl,-soname=c.so,--version-script=c2.ver -Dfoo=foo1 c.c -o $@
clean:
	rm -f a *.so *.o *.meta
eof

If c.so lacks the required version entirely, rtld still reports a fatal error:

% bmake
...
% ./a
a
% LD_PRELOAD=c2.so ./a
./a: /tmp/t/v2/c2.so: version `v1' not found (required by /tmp/t/v2/b.so)

The BZ24718#319 comment in 2026 requested a feature: When all undefined references to a version are weak, set vna_flags to VER_FLG_WEAK in the .gnu.version_r section. While this remains unimplemented, I have implemented this idea for lld in January 2026.

If the v1 Verneed in a has the VER_FLG_WEAK flag, we shall see the following instead:

1
2
3

% LD_PRELOAD=c2.so ./a
./a: /tmp/t/v2/c2.so: weak version `v1' not found (required by /tmp/t/v2/b.so)
a

Example

Run the following code to create a.c b.c b0.c Makefile, then run bmake.

cat > ./a.c <<'eof'
#define _GNU_SOURCE  // workaround before glibc 2.36 (commit 748df8126ac69e68e0b94e236ea3c2e11b1176cb)
#include <dlfcn.h>
#include <stdio.h>

int foo(int a);

int main(void) {
   int res = foo(0);
   printf ("foo(0) = %d, %s\n", res, res == 0 ? "ok" : "wrong");

   /* Resolve to foo@@v3 in b.so, instead of foo@v1 or foo@v2.  */
   int (*fp)(int) = dlsym(RTLD_DEFAULT, "foo");
   res = fp (0);
   printf ("foo(0) = %d, %s\n", res, res == 3 ? "ok" : "wrong");

   /* Resolve to foo@@v3 in b.so, instead of foo@v1 or foo@v2.  */
   fp = dlsym(RTLD_NEXT, "foo");
   res = fp(0);
   printf ("foo(0) = %d, %s\n", res, res == 3 ? "ok" : "wrong");
}
eof
echo 'int foo(int a) { return -1; }' > ./b0.c
cat > ./b.c <<'eof'

int foo_va(int a) { return 0; } asm(".symver foo_va, foo@va, remove");
int foo_v1(int a) { return 1; } asm(".symver foo_v1, foo@v1, remove");
int foo_v2(int a) { return 2; } asm(".symver foo_v2, foo@v2, remove");
int foo(int a) { return 3; } asm(".symver foo, foo@@@v3");
eof
echo 'va {}; v1 {} va; v2 {} v1; v3 {} v2;' > ./b.ver
sed 's/^        /\t/' > ./Makefile <<'eof'
.MAKE.MODE := meta curdirOk=1
CFLAGS = -fpic -g

a: a.o b0.so b.so
	$(CC) -Wl,--no-as-needed a.o b0.so -ldl -Wl,-rpath=$$PWD -o $@

b0.so: b0.o
	$(CC) $> -shared -Wl,--soname=b.so -o $@

b.so: b.o
	$(CC) $> -shared -Wl,--soname=b.so,--version-script=b.ver -o $@

clean:
	rm -f a *.so *.o *.meta
eof

Before glibc 2.36, the output is:

% ./a
foo(0) = 0, ok
foo(0) = 3, ok
foo(0) = 0, wrong

Since 2.36, the last line is correct.

Upgraded symbols in glibc

When to prevent execution of new binaries with old glibc has a summary about when a new symbol version is introduced.

Note that GNU nm before binutils 2.35 does not display @ or @@.

1
2
3

nm -D /lib/x86_64-linux-gnu/libc.so.6 | \
  awk '$2!="U" {i=index($3,"@"); if(i){v=substr($3,i); $3=substr($3,1,i-1); m[$3]=m[$3]" "v}} \
  END {for(f in m)if(m[f]~/@.+@/)print f, m[f]}'

The output on my x86-64 system:

pthread_cond_broadcast  @GLIBC_2.2.5 @@GLIBC_2.3.2
clock_nanosleep  @@GLIBC_2.17 @GLIBC_2.2.5
_sys_siglist  @@GLIBC_2.3.3 @GLIBC_2.2.5
sys_errlist  @@GLIBC_2.12 @GLIBC_2.2.5 @GLIBC_2.3 @GLIBC_2.4
quick_exit  @GLIBC_2.10 @@GLIBC_2.24
memcpy  @@GLIBC_2.14 @GLIBC_2.2.5
regexec  @GLIBC_2.2.5 @@GLIBC_2.3.4
pthread_cond_destroy  @GLIBC_2.2.5 @@GLIBC_2.3.2
nftw  @GLIBC_2.2.5 @@GLIBC_2.3.3
pthread_cond_timedwait  @@GLIBC_2.3.2 @GLIBC_2.2.5
clock_getres  @GLIBC_2.2.5 @@GLIBC_2.17
pthread_cond_signal  @@GLIBC_2.3.2 @GLIBC_2.2.5
fmemopen  @GLIBC_2.2.5 @@GLIBC_2.22
pthread_cond_init  @GLIBC_2.2.5 @@GLIBC_2.3.2
clock_gettime  @GLIBC_2.2.5 @@GLIBC_2.17
sched_setaffinity  @GLIBC_2.3.3 @@GLIBC_2.3.4
glob  @@GLIBC_2.27 @GLIBC_2.2.5
sys_nerr  @GLIBC_2.2.5 @GLIBC_2.4 @@GLIBC_2.12 @GLIBC_2.3
_sys_errlist  @GLIBC_2.3 @GLIBC_2.4 @@GLIBC_2.12 @GLIBC_2.2.5
sys_siglist  @GLIBC_2.2.5 @@GLIBC_2.3.3
clock_getcpuclockid  @GLIBC_2.2.5 @@GLIBC_2.17
realpath  @GLIBC_2.2.5 @@GLIBC_2.3
sys_sigabbrev  @GLIBC_2.2.5 @@GLIBC_2.3.3
posix_spawnp  @@GLIBC_2.15 @GLIBC_2.2.5
posix_spawn  @@GLIBC_2.15 @GLIBC_2.2.5
_sys_nerr  @@GLIBC_2.12 @GLIBC_2.4 @GLIBC_2.3 @GLIBC_2.2.5
nftw64  @GLIBC_2.2.5 @@GLIBC_2.3.3
pthread_cond_wait  @GLIBC_2.2.5 @@GLIBC_2.3.2
sched_getaffinity  @GLIBC_2.3.3 @@GLIBC_2.3.4
clock_settime  @GLIBC_2.2.5 @@GLIBC_2.17
glob64  @@GLIBC_2.27 @GLIBC_2.2.5

realpath@@GLIBC_2.3: the previous version returns EINVAL when the second parameter is NULL
memcpy@@GLIBC_2.14 BZ12518: the previous version guarantees a forward copying behavior. Shockwave Flash at that time had a "memcpy downward" bug which required the workaround.
quick_exit@@GLIBC_2.24 BZ20198: the previous version copies the destructors of thread_local objects.
glob64@@GLIBC_2.27: the previous version does not follow dangling symlinks.

How to remove symbol versioning

Imagine that you want to build an application with a prebuilt shared object which has versioned references, but you can only find shared objects providing the unversioned definitions. The linker will helpfully error:

1	ld.lld: error: undefined reference to foo@v1 [--no-allow-shlib-undefined]

As the diagnostic suggests, you can add --allow-shlib-undefined to get rid of the error. It is not recommended but the built application may happen to work.

For this case, an alternative hacky solution is:

# 64-bit
cp in.so out.so
rizin -wqc '/x feffff6f00000000 @ section..dynamic; w0 16 @ hit0_0' out.so
llvm-objcopy -R .gnu.version out.so

# 32-bit
cp in.so out.so
rizin -wqc '/x feffff6f @ section..dynamic; w0 8 @ hit0_0' out.so
llvm-objcopy -R .gnu.version out.so

With the removal of .gnu.version, the linker will think that out.so references foo instead of foo@v1. However, llvm-objcopy will zero out the section contents. At runtime, glibc ld.so will complain unsupported version 0 of Verneed record. To make glibc happy, you can delete DT_VER* tags from the dynamic table. The above code snippet uses an r2 command to locate DT_VERNEED(0x6ffffffe) and rewrite it to DT_NULL(a DT_NULL entry stops the parsing of the dynamic table). The difference of the readelf -d output is roughly:

  0x000000006ffffffb (FLAGS_1)            Flags: NOW
- 0x000000006ffffffe (VERNEED)            0x8ef0
- 0x000000006fffffff (VERNEEDNUM)         5
- 0x000000006ffffff0 (VERSYM)             0x89c0
- 0x000000006ffffff9 (RELACOUNT)          1536
  0x0000000000000000 (NULL)               0x0

ld.lld

If an undefined symbol is not defined by a shared object, GNU ld will report an error. ld.lld before 12.0 did not error (I fixed it in https://reviews.llvm.org/D92260).

GNU function attribute

There is a GNU function attribute which is lowers to a .symver assembly directive. The attribute is implemented by GCC but not by Clang.

1 2	extern "C" __attribute__((symver("foo@@v2"))) void foo() {} extern "C" __attribute__((symver("foo@v1"))) void foo_v1() {}

Unfortunately, @@@ and ,remove are not supported. Along with the reason that Clang does not implement the function attribute, I discourage using this feature.

Remarks

GCC/Clang supports asm specifier and #pragma redefine_extname renaming a symbol. For example, if you declare int foo() asm("foo_v1"); and then reference foo, the symbol in .o will be foo_v1.

For example, the biggest change in musl v1.2.0 is the time64 support for its supported 32-bit architectures. musl adopted a scheme based on asm specifiers:

// include/features.h
#define __REDIR(x,y) __typeof__(x) x __asm__(#y)

// API header include/sys/time.h
int utimes(const char *, const struct timeval [2]);
__REDIR(utimes, __utimes_time64);

// Implementation src/linux/utimes.c
int utimes(const char *path, const struct timeval times[2]) { ... }

// Internal header compat/time32/time32.h
int __utimes_time32() __asm__("utimes");

// Compat implementation compat/time32/utimes_time32.c
int __utimes_time32(const char *path, const struct timeval32 times32[2]) { ... }

In .o, the time32 symbol remains utimes and is compatible with the ABI required by programs linked against old musl versions; the time64 symbol is __utimes_time64.
The public header redirects utimes to __utimes_time64.
- cons: if the user declares utimes by themself, they will not link against the correct __utimes_time64.
The "good-looking" name utimes is used for the preferred time64 implementation internally and the "ugly" name __utimes_time32 is used for the legacy time32 implementation.
- If the time32 implementation is called elsewhere, the "ugly" name can make it stand out.

For the above example, here is an implementation with symbol versioning:

// API header include/sys/time.h
int utimes(const char *, const struct timeval [2]);

// Implementation src/linux/utimes.c
int utimes(const char *path, const struct timeval times[2]) { ... }

// Internal header compat/time32/time32.h
// Probably __asm__(".symver __utimes_time32, utimes@time32, rename"); if supported
__asm__(".symver __utimes_time32, utimes@time32");

// Implementation compat/time32/utimes_time32.c
int __utimes_time32(const char *path, const struct timeval32 times32[2])
{
  ...
}

Note that @@@ cannot be used. The header is included in a defining translation unit and @@@ will lead to a default version definition while we want a non-default version definition.

According to Assembler behavior, the undesirable __utimes_time32 is present. Be careful to use a version script to localize it.

So what is the significance of symbol versioning? I think carefully:

Refuse linking against old symbols while keeping compatibility with unversioned old libraries. {reject-non-default}
No need to label declarations.
The version definition can be delayed until link time. The version script provides a flexible pattern matching mechanism to assign versions.
Scope reduction. Arguably another mechanism like --dynamic-list might have been developed if version scripts did not provide local:.
There are some semantic issues in renaming builtin functions with asm specifiers in GCC and Clang (they do not know that the renamed symbol has built-in semantic). See 2020-10-15-intra-call-and-libc-symbol-renaming
[verneed-check]

For the first item, the asm specifier scheme uses conventions to prevent problems (users should include the header); and symbol versioning can be forced by ld.

Design flaws:

.symver foo, foo@v1 {gas-copy}
Verdaux is a bit redundant. In practice, one Verdef has only one auxiliary Verdaux entry.
This is arguably a minor problem but annoying for a framework providing multiple shared objects. ld.so requires "a versioned symbol is implemented in the same shared object in which it was found at link time", which disallows moving definitions between shared objects. Fortunately, glibc 2.30 BZ24741 relaxes this requirement, essentially ignoring Vernaux::vna_name.

Before that, glibc used a forwarder to move clock_* functions from librt.so to libc.so:

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2
3

// rt/clock-compat.c
__typeof(clock_getres) *clock_getres_ifunc(void) asm("clock_getres");
__typeof(clock_getres) *clock_getres_ifunc(void) { return &__clock_getres; }

libc.so defines __clock_getres and clock_getres. librt.so defines an ifunc called clock_getres which forwards to libc.so __clock_getres.

Representation

VER_FLG_WEAK

Version index values

Example

Symbol versioning in object files

Assembler behavior

Linker behavior

Version script

How a versioned symbol is produced

Recommended usage

Why is .symver xxx, foo@v1 bad with a defined symbol?

rtld behavior

Example

Upgraded symbols in glibc

How to remove symbol versioning

ld.lld

GNU function attribute

Remarks

Related links

`VER_FLG_WEAK`

Why is `.symver xxx, foo@v1` bad with a defined symbol?