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Re: Document __foo symbol variants and their uses in the library sources.

On Mon, 14 Apr 2014, Carlos O'Donell wrote:

> In response to some questions on libc-help here is a 
> first attempt at documenting some of the conventions
> behind calling the double-underscore versions of the
> public API functions from within the library (to avoid
> the PLT indirect and maintain internal consistency).
> I would appreciate any review.

That appears to confuse two things.

* hidden_proto / hidden_def (etc.) are about PLT avoidance (and may also 
enable better code generation in some cases because the compiler knows 
it's calling a hidden symbol which must be in the same library).  They may 
be used both on names starting with __ and names that aren't so-prefixed.  
They are irrelevant when a function in one library is calling a function 
in another library.

* __-prefixed names are about being namespace-clean, where the user calls 
a function in one standard and that function internally uses a function 
from another standard, or not in any standard.  In such cases, you need to 
use __-prefixed names, whether or not the function being called is in the 
same library, to avoid conflicts with the user's own symbols in the static 
linking case.  If the function called is in another library, the 
__-prefixed function *also* needs to be exported at GLIBC_PRIVATE so that 
the call can work in the dynamic linking case - unless there's a reason 
for it to be exported at a public symbol version.  (For example, if a 
macro definition of a public function in an installed header uses the 
__-prefixed function, or libstdc++ should use it, or redirection for 
_FILE_OFFSET_BITS=64 should use it, then it may be necessary to export at 
a public version, in which case you don't need a redundant GLIBC_PRIVATE 
export.)  And if the function is in the same library you should still use 
hidden_proto / hidden_def.

Joseph S. Myers

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