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CS241

CS241. Introduction To C Lawrence Angrave. Crash course in C. Crash. char *p; p=0; *p=4 . ?. char * search(char*) { char result[100]; … do stuff return result; }. ?. static int debug=1; main(…) { }. ?. int counter() { static int c=1; printf(“The value of c is %x”,&c);

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CS241

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  1. CS241 • Introduction To C • Lawrence Angrave

  2. Crash course in C

  3. Crash • char *p; • p=0; • *p=4

  4. ? • char * search(char*) { • char result[100]; • … do stuff • return result; • }

  5. ? • static int debug=1; • main(…) • { • }

  6. ? • int counter() { • static int c=1; • printf(“The value of c is %x”,&c); • return c++; • }

  7. Linkage & Scope

  8. Linkage • Identifiers (variables & functions) visible to another c file? • Main.c = Compilation unit • Myutils.c = Another compilation unit • Static : No!

  9. util.c • static int notthisone; • int runsuperfast=0; • void die(char*) { • static int privatecounter; • fprintf(stderr,mesg); • exit(1); • }

  10. main.c • Refer to runsuperfast and die? • extern int runsuperfast; • void die(char *mesg);

  11. main.c • Refer to runsuperfast and die? • void die(char *); // declaration! • extern int runsuperfast;

  12. Declarations to library functions? • #include <stdio.h> • #include <stdlib.h> • #include “myutils.h”

  13. C Compiler

  14. gcc Gnu Compiler • Many options! • man gcc • Three for the price of one • Preprocessor • Compiler • Linker

  15. C Variants • C99 - A ‘nicer’ C • /* Multi-line comments */ • // line comment (Not allowed in C89) • Mix variable declarations & code inside functions

  16. Pointers

  17. Pointers • How to shoot yourself in the foot and not realize it

  18. int *p • Q. What is p? • How much storage is required for p? • Where is p in memory?

  19. int *p • What is the value of • p == &*p; • p[0] == *p

  20. char **argv • Explain this to your neighbor

  21. char ***argv • Explain this to your neighbor • When would you use it?

  22. Argument Arrays • An argument array is an array of pointers terminated by a NULL pointer.Each element of the array is of type char * and represents a string.

  23. Character strings • Basic support in Compiler for string constants and string pointers • Everything else is a library function • strcmp • strncmp • strlen • strcat

  24. strcmp • strcmp(“Hello”,”Hello”) returns what? • strcmp(“1”,”2”) returns what?

  25. Why is this bad? • char *p=“Hello”; • strcpy(p,” World”);

  26. Ans: Understand storage

  27. Identifies & Linkage Summary

  28. Identifiers • “Identifier” denotes an object: a function, a tag, member of a structure, etc. We mainly consider identifiers associated with functions and variables • Scope is a region of program text where identifier is used • Identifiers declared more than once may refer to the same object because of linkage • Each identifier has a linkage class of external, internal or none. • An identifier representing an object has a linkage class related to storage class (storage duration).

  29. Linkage classes • Linkage class determines whether variables can be accessed in files other than the one in which they are declared. • Internal linkage class means they can only be accessed in the file in which they are declared. • External linkage class means they can be accessed in other files. • Variables declared outside any function and function name identifiers have external linkage by default.They can be given internal linkage with the key word static. • Variables declared inside a function are only known inside that function and are said to have no linkage.

  30. Storage classes: static and automatic • Static storage class refers to variables that, once allocated, persist throughout the execution of a program. • Automatic storage class refers to variables which come into existence when the block in which they are declared is entered and are discarded when the defining block is exited. • Variables declared inside a function have automatic storage class unless they are declared to be static.These are usually allocated on the program stack. • Variables defined outside any functions have static storage class. • The word statichas two meanings is C.One is related to storage class and the other to linkage class.

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