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Building a target library

Building a target library. An example of Newlib. Role of a library. After the creation of the cross compiler the creation of a library is the next major task There are a number of choices If the system has pre-compiled or source libraries that can be used

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Building a target library

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  1. Building a target library An example of Newlib

  2. Role of a library • After the creation of the cross compiler the creation of a library is the next major task • There are a number of choices • If the system has pre-compiled or source libraries that can be used • May not be possible will all compilers or languages • Use the standard GNU libc • Port another library

  3. Where does the library fit in the system? • There is a 3 level hierarchy of compiler – library – and operating system • The link between C and the library should be straight forward • The OS – library link can be more problematic • It is possible to do without a library Compiler Operating System Library Implements calls Adds functionality Does the calls Calls functions getchar,

  4. Which library? • Selection of the library is important • The features and functions of the board may dictate the library • i.e. if the board has lots of special, non-standard functions your library will have to implement these • Should use conformant library • i.e. ANSI, IEEE • Beware of over conformance

  5. GNUlibc vs Newlib vs Minos • There are often a number of options • The original libraries with the board • In our case the Minos libraries • GNU has its own library GNU libc • There are a number of other libraries around • For example Newlib developed by Cygnus for cross development • Another library, slib, has been developed for the uCLinux, embedded linux system

  6. Which library? GNU libc Newlib Minos lib + + - -

  7. Using Newlib • Newlib is a good choice for embedded systems work • It provides ANSI and K & R standard C libraries • It has a good range of functionality • It is reasonably small • It is intended for porting – lots of ports available • Integrated with GNU

  8. Issues around porting a library • Does the code need to be reentrant? • This is only really important in a multiprocessing environment • In general it is better to build a reentrant library • The library may have to assume that there is no/limited OS support for reentrancy • Newlib provides library support for reentrancy

  9. Reentrancy • If the Operating System does not provide support for process state variables, i.e. error numbers, file descriptors, then either the library code will be non-reentrant or the library will have to support such variables Process 1 Process 2 Read, write, etc Volatile State data Volatile State data Library Code

  10. System Call Interface • _open_r • _close_r • _lseek_r • _read_r • _write_r • _fork_r • _wait_r • _stat_r • _fstat_r • _link_r • _unlink_r • _sbrk_r

  11. A system call stub

  12. A system call implementation

  13. A system call implementation

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