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Elaborate on how our ad-hack symbol versioning works. Tweak style.

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riastradh 2015-03-20 13:26:51 +00:00
parent 26ef86fddb
commit 934b3616ba
1 changed files with 42 additions and 9 deletions
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51
lib/libc/README
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@ -1,4 +1,4 @@
$NetBSD: README,v 1.1 2015/03/20 12:57:48 riastradh Exp $
$NetBSD: README,v 1.2 2015/03/20 13:26:51 riastradh Exp $
libc: The C library.
@ -56,16 +56,14 @@ create the ELF weak symbol alias.
`_consttime_memequal'.
If a library routine is internal to libc, it is defined as an ELF
global symbol with an underscore prefix.
Its name is declared in the appropriate internal header file.
global symbol with an underscore prefix. Its name is declared in the
appropriate internal header file.
=> Example: For the internal library routine _initdir, used by the
implementations of opendir and rewinddir, libc defines a global
symbol `_initdir'.
The name `_initdir' is declared normally in
src/lib/libc/gen/dirent_private.h.
symbol `_initdir'. The name `_initdir' is declared normally in
src/lib/libc/gen/dirent_private.h, and defined normally in
src/lib/libc/gen/initdir.c.
If the signature or semantics of a library routine foo changed in (for
example) NetBSD 6.0, then libc provides
@ -100,7 +98,7 @@ compiled with the old signature, will continue to use the old symbol.
(2) the ELF weak symbol `time' aliasing `_time', and
(3) the ELF global symbol `__time50' implementing the new signature.
The header file <time.h> declares
The header file <time.h> (src/include/time.h) declares
time_t time(time_t *) __RENAME(__time50);
@ -108,3 +106,38 @@ compiled with the old signature, will continue to use the old symbol.
use the __time50 symbol from libc. However, old programs that were
compiled against the 32-bit declaration will continue to use the
32-bit symbol from libc.
The header file "namespace.h" (src/lib/libc/include/namespace.h)
defines `time' as a macro expanding to `_time'.
The source file src/lib/libc/gen/time.c includes "namespace.h" and
<time.h> and defines `time' normally. The declaration of `time' in
<time.h> is replaced after macro expansion by a declaration of
`_time', and the definition in time.c is replaced by a definition of
`_time'. But the __RENAME directive causes the resulting ELF global
symbol to be `__time50'.
The header file <compat/include/time.h>
(src/lib/libc/compat/include/time.h) declares
int32_t time(int32_t *);
The source file src/lib/libc/compat/gen/compat_time.c includes
"namespace.h", <compat/include/time.h>, and <time.h>, but suppresses
the normal declaration of `time' in <time.h> by defining
__LIBC12_SOURCE__. Then compat_time.c defines `time' normally.
Again, the name is replaced after macro expansion by `_time', but
since there is no __RENAME directive in <compat/include/time.h>, the
resulting ELF global symbol is `_time'.
Finally, alongside the definition in compat_time.c is
__weak_alias(time,_time)
to provide `time' as an ELF weak symbol aliasing `_time'.
The net effect is that NetBSD 6's libc provides the same definitions
as NetBSD 5's libc for the symbols `time' and `_time', so that old
programs that were compiled in NetBSD 5 will continue to work with
NetBSD 6's libc. But programs compiled in NetBSD 6 will have 64-bit
time_t.