C Module System

1. C Module System C and Data Structures Baojian Hua bjhua@ustc.edu.cn

2. Software Systems are Large • Practical software systems tend to be large and complex: • Linux kernel consists of ~1000K LOC • So the general principals in designing large (even small) software systems are: • dividing into manageable smaller ones • separating specification (interface) from code (implementation)

3. Module System • Module systems offer a systematic method to organize software • Different parts can be separately developed, compiled, tested, debugged • Different parts are finally linked together • This process evolves compiling, linking and libraries, etc.

4. Module System • Different styles of module systems in languages: • ML signature and structure • Java interface and class • C (C++) header files (.h) and C files (.c) • This slide shows how to manage C’s module system: • source programs • (separate) compiling, linking, loading • and tools

5. dec & stm dec & stm dec & stm dec & stm exp exp exp exp Typical C Program Organization Program … file1 filen function1 functionn function1 functionm … …

6. General Process // General process from source files (.c) to // executables (.exe): libraries 1.c 1.i 1.o 2.c 2.i 2.o a.exe … … … n.c n.i n.o preprocessing Compiling linking

7. Example Revisited double area (int r); double area (int r) { double pi = 3.14; return (pi*r*r); } int main() { double f; f = area(5); return 0; }

8. First Try // main.c int main () { double f; f = area (5); return 0; } // area.c double area (int r); #define PI 3.14 double area (int r) { double f = PI *r *r; return f; } Try this demo! Wrong result? Why? Or even worse? f = area(5, 6, 7, 8, 9);

9. Second Try // main.c double area (int r); int main () { double f; f = area (5); return 0; } // area.c double area (int r); #define PI 3.14 double area (int r) { double f = PI *r *r; return f; } Try this demo! Is this perfect? What about here is 10000 files contains “area” and “area.c” being changed?

10. Third Try // area.h double area (int r); // area.c #include “area.h” #define PI 3.14 double area (int r) { double f = PI *r *r; return f; } // main.c #include “area.h” int main () … Try this demo! Is this perfect!?

11. Pitfalls // area.h double area (int r); int i = 0; // area.c #include “area.h” #define PI 3.14 double area (int r) { double f = PI *r *r; return f; } // main.c #include “area.h” int main () … Try this demo!

12. Final Version // area.h #ifndef AREA_H #define AREA_H double area (int r); #endif // area.c #include “area.h” #define PI 3.14 double area (int r) { double f = PI *r *r; return f; } // main.c #include “area.h” …

13. Preprocessing • Take source files (.c .h), generate intermediate files • file inclusion • Macro substitution • comments removal • … • afterwards, no header file needed any more • So, what’s the role of “.h” files?

14. Example // area.h #ifndef AREA_H #define AREA_H double area (int r); #endif // area.c #include “area.h” #define PI 3.14 double area (int r) { double f = PI*r*r; return f; } // main.c #include “area.h” int main () { area (5); }

15. Example // area.h #ifndef AREA_H #define AREA_H double area (int r); #endif // area.c double area (int r); #define PI 3.14 double area (int r) { double f = PI*r*r; return f; } // main.c #include “area.h” int main () { area (5); }

16. Example // area.h #ifndef AREA_H #define AREA_H double area (int r); #endif // area.c double area (int r); #define PI 3.14 double area (int r) { double f = 3.14*r*r; return f; } // main.c #include “area.h” int main () { area (5); }

17. Example // area.h #ifndef AREA_H #define AREA_H double area (int r); #endif // area.c double area (int r); double area (int r) { double f = 3.14*r*r; return f; } // main.c double area(int r); int main () { area (5); }

18. Example // area.h #ifndef AREA_H #define AREA_H double area (int r); #endif // area.c double area (int r); double area (int r) { double f = 3.14*r*r; return f; } // main.c double area(int r); int main () { area (5); }

19. Compiling • Generate binary object files (.obj) • object files in assembly or binary • may involve several intermediate phases • analysis • optimizations • … • See demo …

20. Example // area.c double area (int r); double area (int r) { double f = 3.14*r*r; return f; } // area.o 0101001010100101 1010010100101001 0100101010101010 // main.c double area(int r); int main () { area (5); } // main.o 1101010100101001 1010100100101001 0001001001010010

21. Linking • Object files often called relocatable • they are incomplete • function names, extern variables, etc. • Linking the process of linking all object files together • resolve reference to external entities • See demo …

22. Linking // Object files are incomplete: // main.o area(…) printf(…)

23. Linking // Resolve external references: // area.o area: … // main.o call area call printf // printf.o printf: …

24. Example // area.c double area (int r); double area (int r) { double f = 3.14*r*r; return f; } // area.o 0101001010100101 1010010100101001 0100101010101010 // a.exe 010101101 …………… // main.c double area(int r); int main () { area (5); } // main.o 1101010100101001 1010100100101001 0001001001010010

25. Static vs Dynamic Linking • Static: all object files must be available and link together • the generated .exe files are complete • what’s the pros and cons? • Dynamic: some object files are absent • the generated .exe files are incomplete • then how these absent object files are referenced? • what’s the pros and cons?

26. What are Libraries? • Libraries just are pre-written pre-compiled object files • Normally offered by the compiler company • For user program linking purpose • Header files are available • Ex: stdio.h, stdlib.h, ctype.h, …, from C standard library • Source code available (ex: gcc), or unavailable (ex: vc) • Same linking technique, but …

27. How to Implement Libraries? • In order to familiarize you with libraries implementation techniques and others, we next study carefully an example • stdio.h • Our goal is to study the “printf” function int printf (const char *format, …);

28. General Strategy • User program call the library function printf () • printf () internally makes operating system call to do the real work • details vary on different OS • OS calls hardware driver • Offered by hardware company user program libraries: printf() OS routines hardware driver

29. Case Study: Linux vs Windows fwrite () user program fwrite () user program write () int 0x80 write () NtWriteFile () int 0x2e sys_write () Kernel IoWriteFile () Kernel

30. API & SDK • Application programming interface (API) • a set of routines that nicely wrap up operating system calls • libraries are built on top of them • standard C libraries, runtime, etc. • Why API? • Software Development Kit (SDK) • A collection of APIs • header, libraries, files, and tools • Varies on different Windows version

31. Runtime • Runtime is a special set of libraries • For program startup and exit • get system info • libraries preparation, etc. • Normally NOT for called by user program • Again, vary between different systems