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prefer using visibility=hidden for __libc internal data, rather than
an accessor function, if the compiler has visibility.
optimize with -O3 for PIC targets (shared library). without heavy
inlining, reloading the GOT register in small functions kills
performance. 20-30% size increase for a single libc.so is not a big
deal, compared to comparaible size increase in every static binaries.
use -Bsymbolic-functions, not -Bsymbolic. global variables are subject
to COPY relocations, and thus binding their addresses in the library
at link time will cause library functions to read the wrong (original)
copies instead of the copies made in the main program's bss section.
add entry point, _start, for dynamic linker.
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prior to this change, a large portion of libc was unusable prior to
relocation by the dynamic linker, due to dependence on the global data
in the __libc structure and the need to obtain its address through the
GOT. with this patch, the accessor function __libc_loc is now able to
obtain the address of __libc via PC-relative addressing without using
the GOT. this means the majority of libc functionality is now
accessible right away.
naturally, the above statements all depend on having an architecture
where PC-relative addressing and jumps/calls are feasible, and a
compiler that generates the appropriate code.
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note that this presently does not handle consistency of the libc's own
global state during forking. as per POSIX 2008, if the parent process
was threaded, the child process may only call async-signal-safe
functions until one of the exec-family functions is called, so the
current behavior is believed to be conformant even if non-ideal. it
may be improved at some later time.
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