The C Programming Language

1. The C Programming Language Eric Vidal CS 280

2. What is C? • Originally: System programming language for the UNIX operating system • Today: One of the most dominant development languages for general purpose applications, especially for low-level software

3. Sample C Programs • Hello, world! #include<stdio.h> intmain(intargc, char *argv[], char *envp[]) { printf("Hello, world!\n"); return 0; }

4. Quicksort #include<stdio.h> voidquicksort(int *array, intstart, intend) { intlast_start = start, last_end = end, swap; /* we do not need to sort zero/one item arrays */ if (start >= end) return; /* move items greater than pivot to the end */ /* and keep items less than pivot at the start */ start++; while (start < end) { if (array[start] > array[last_start]) { swap = array[start]; array[start] = array[end]; array[end--] = swap; } else start++; } /* move pivot to the center of the array */ if (array[start] > array[last_start]) start--; swap = array[last_start]; array[last_start] = array[start]; array[start] = swap; /* recursively sort array before and after pivot */ quicksort(array, last_start, start - 1); quicksort(array, start + 1, last_end); } intmain(intargc, char *argv[], char *envp[]) { intfoo[7] = { 4, 1, 6, 10, 9, 7, 3 }; inti; quicksort(foo, 0, 6); for (i = 0; i < 7; i++) printf("%d\n", foo[i]); return 0; } Sample C Programs

5. Real-World C Applications • Most modern operating systems • Kernel – Linux kernel, NT kernel, etc. • Command line processors – bash, csh, cmd.exe, etc. • Native windowing system – X Window System, Windows Shell • Other utilities – grep, make, etc. • Most modern compilers • GNU Compiler Collection – gcc, g++, etc. • Visual Studio’s base compilers – cl.exe, rc.exe, etc. • Most modern PC and console games 

6. Evolution of C • CPL – Cambridge Programming Language or Combined Programming Language (1963) • Invented by Christopher Strachey, et al. • Heavily influenced by Algol 60 • Too complex to implement on existing computers • BCPL – Basic CPL (1966) • Invented by Martin Richards • Programming language of choice for the Multics systems in Bell Laboratories

7. Evolution of C • B Programming Language (1969) • Invented by Ken Thompson • Implemented on a DEC PDP-7 with 8K 18-bit words of memory • Intended as the system implementation language for UNIX • Revision of an earlier language, “Bon” • Not named after the B in BCPL, but heavily influenced by it

8. Evolution of C • BCPL versus B versus C • BCPL and B are both typeless; C has types • Pointers and arrays are both integer indexes to the memory array In BCPL: In B: In C: letv = vec 10 autov[10]; intv[10]; v!i = 42 v[i] = 42; v[i] = 42; • In B (unlike in BCPL and C), output is not native code, but threaded code which operates on a stack machine • BCPL (unlike B and C) allows nested procedures, and links between separately compiled modules must be explicitly stated

9. Evolution of C • NB – New B (1971) • Invented by Dennis Ritchie • On the DEC PDP-11 with 12K 16-bit words of memory, character data is not accessed in words but in bytes • Required two distinct data types: int and char • Floating point arithmetic will also require a new float data type • NB compiles to native code

10. Evolution of C • NB versus C • Previously, arrays and pointers are the same, but Ritchie wanted to implement structures: structdirentry { intinumber; charname[14]; }; • Pointer to an array != actual array in the structure • Inconvenient for reading from disk

11. Evolution of C • C Programming Language (1972) • Also invented by Ritchie, proper successor to B • In C, arrays are still similar to pointers • Pointer to an array is created only when the array name is mentioned in an expression • Generalization of pointers: inti, *pi, **ppi; intfi(), *fpi(), (*pfi)(); int *api[10], (*pai)[10];

12. Evolution of C • K&R C – first C standard (1978) • Appeared in The C Programming Language by Brian Kernighan and Dennis Ritchie • && and || operators • Previously & and | means both logical and bitwise AND and OR • Unfortunately, precedence was never fixed (to preserve compatibility with B): if (a & mask == b) /* logical error */ • Preprocessor #include<header.h> #defineMACRO • unsigned, long, union, enum • Type casts

13. Evolution of C • ANSI C or C89 – first official C standard (1989) • Produced by the ANSI X3J11 working group • Required the types of formal arguments in the type signature of the function: In K&R C: In ANSI C: doublesin(); doublesin(double); • const, volatile • Ratified a Standard C Library • Should be implemented by all ANSI C-compliant vendors

14. Evolution of C • ISO/IEC 9899 C90 – second official C standard (1990) • Exactly the same as C89, with changes to numbering to reflect ISO practice • ISO/IEC 9899 C99 – current official C standard (1999) • Produced by the ISO/IEC/JTC/SC22/WG14 working group • Added invariant ISO646 and multibyte support extensions • Clarified the existing standard via technical corrigenda: • ISO/IEC 9899 TCOR1 (1995) • ISO/IEC 9899 TCOR2 (1996) • Available at: http://std.dkuug.dk/JTC1/SC22/WG14/

15. Disadvantages of C • Ambiguity in variable use • Type safety (integers versus pointers) • Fence post errors (arrays versus pointers) • Indirection problems int *fp(); int (*pf)(); int *(*pfp)(); • Treats strings as null-terminated character arrays • Finding the length of the string is O(n) • Generally not well-suited for string processing

16. Disadvantages of C • Array problems • Dynamically changing the size of an array is clumsy • Functions expecting multiple pointers to arrays cannot be optimized fully for SIMD or vector machines; arrays may overlap • Modularization problems • Only two levels of naming: external (global) and internal (function) • Developers must create their own modularization conventions

17. Disadvantages of C • Memory problems • C itself only provides two types of storage: automatic (in the stack segment) and static (in the data segment) • Dynamically allocated storage in the heap (malloc() and free()) is tedious and error-prone • Automatic garbage collection is difficult

18. Advantages of C • Low-level functionality • Bitwise operators • Pointers • Type looseness • High-level syntax • Abstract enough to describe underlying algorithms • Result: A language that can do practically everything, is portable to other systems, and runs as fast as assembly

19. Advantages of C • Small and simple • Easy to parse; compilers occupy very little memory • Ties with UNIX • Language not designed in isolation, but in a real environment with emphasis on practicality • Meets the needs of programmers, but does not supply too much • Compatibility with old programs – C has been remarkably stable through the years (unlike FORTRAN or Pascal)

20. Compilers for C • First C compiler, cc, is developed by Dennis Ritchie • First portable C compiler, pcc, is developed by Steve Johnson (1978) • Became the reference implementation for K&R C • Currently a ton of compilers available • Mostly because compiler theory classes use a subset of C for their final project 

21. Compilers for C • Generally available C compilers (also works for C++): • Borland C++Builder • http://www.borland.com/cbuilder/ • Microsoft Visual C++ .NET • http://msdn.microsoft.com/visualc/ • The GNU Compiler Collection • For Linux: http://gcc.gnu.org/ • For Windows: http://www.mingw.org/

22. References • Ritchie, Dennis M. The Development of the C Language. April 1993. Internet on-line. Available from <http://cm.bell-labs.com/cm/cs/who/dmr/chist.html> [31 July 2003] • International Standards Organization. 2003. ISO/IECJTC1/SC22/WG14 – C. Denmark: International Standards Organization, 2003 [cited 31 July 2003]. Available from World Wide Web: (http://std.dkuug.dk/JTC1/SC22/WG14/)