library porting problem

[Norman Feske]

Hello Paramesh,

On 04/22/2015 12:20 PM, Paramesh Oddepally wrote:
>         But, in my library(bionic), the same thing is not working after
> the same changes as above(with same main.cc ). Its not printing anything
> and waiting infinitely. I cannot understand how to interpret this
> problem, Is it problem with Library?
> 
>     The run file of my library is similar to the /libc/ run file. 

merely compiling the library for Genode is merely the first (and
probably the easiest) step of porting a C runtime like Bionic to Genode.
The main part of the work is the creation of a libc "back end" that acts
as the glue between the C runtime and the underlying Genode system. To
give you an idea of the role of this glue, here are two examples:

* The back end has to provide the memory for the C runtime's malloc and
  free functions. On Linux, the back end of the C runtime performs a
  mmap system call to obtain chunks of anonymous memory to be used at
  malloc/free backing store. On Genode, your back end would have to use
  Genode's RAM session and RM session interfaces instead.

* For input and output (I/O), the back end has to translate I/O function
  calls (like read, write, stat, etc) to Genode's various session
  interfaces (terminal session, file-system session, log session, etc).
  This is far from trivial because, unlike a POSIX system, Genode does
  not have mechanisms that perfectly correspond to the C runtime
  function calls. For example, on Genode, there are no file descriptors.
  So the back end of the C runtime needs to emulate the semantics of
  file descriptors by using Genode's mechanisms.

To implement a back end for the Bionic C runtime, you may take cues from
the back end of our existing libc, which you can find at [1]. But in
order to go ahead, you will ultimately need to gain a good understanding
of the internal structure of the Bionic library.

I would start with stdout (as you just tried). You first need to
determnine the back-end function called by the Bionic library when
issuing the printf function, e.g., by following the call graph within
the Bionic library. On a Linux system, the back end of the Bionic libc
will eventually issue a "write" system call. Your custom back end would
instead issue a call to Genode::Log_session::write.

[1]
https://github.com/genodelabs/genode/tree/master/repos/libports/src/lib/libc

Good luck!
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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