1CMSC 212 – S05 (lect 16)Announcementsz Program #3– Due Tuesday 8:00 PMz Reading– Chapter 17 (Today)• skip 17.5.4– Chapter 13 (Tuesday)2CMSC 212 – S05 (lect 16)Creating your own librariesz Libraries– collections of object (.o) files that work together– can be linked into other programs• linking can happen prior to execution• linking can be done during program executionz Static libraries– linked into the program as part of the link step– require space in each executable that uses themz Dynamic (shared) libraries– linked into the program at program startup– require only one copy for the entire system– Have version numbers associated with them• controls which version work with which applications3CMSC 212 – S05 (lect 16)Creating Static Librariesz UNIX Command ar– LIBRARY=myLib.a– OBJS=file1.o file2.o file3.o– ar cru $(LIBRARY) $(OBJS)z Using a Static Library– gcc -o myProgram main.o myLib.a4CMSC 212 – S05 (lect 16)Creating Dynamic (shared) Librariesz Use special gcc flags– -nostdlib -shared -fPIC -Wl,-soname,mylib.so.1• -nostdlib - no standard C library needed• -shared - generate a shared library• -fPIC - generate position independent code• -Wl,-soname,mylib.so.1– name the shared object mylib.so.1z Example Makefile RulesLIBFLAGS = -nostdlib -shared -fPIC -Wl,-soname,[email protected]: avl.c table.h$(CC) $(LIBFLAGS) $(CFLAGS) avl.c -o libavl.soln -f -s libavl.so libavl.so.15CMSC 212 – S05 (lect 16)Special Features of Shared Librariesz void _init()– Special function in shared libraries– invoked automatically when the library is loaded• works with pre-linked (-llibName)• works when using dlopen– allows library to run initialization code• prepare internal data structures• invoke functions to register actions with main program6CMSC 212 – S05 (lect 16)Runtime Linkingz Can Load a library at runtime– Allows skins, plugins, etc to workz C functions to support this:– void * dlopen(const char *pathname, int mode)• pathname is the name of a shared library• mode controls operation (RTLD_NOW)– void *dlsym(void *handle, const char *name);• lookup a function by name in the passed shared library• Return a pointer to that function (or NULL if not found)– int dlclose(void *handle);• returns 0 on sucess7CMSC 212 – S05 (lect 16)Dlopen Exampletypedef void (*displayFuncPtr)(char *file);typedef struct {displayFuncPtr displayFunc; char *mediaType;} renderTypePlugin;renderTypePlugin *loadPlugin(char *library, char *mediaType) {void *dlPtr; renderTypePlugin *plugin;dlPtr = dlopen(library, RTLD_NOW);if (!dlPtr) return NULL;/* … allocate plugin and check return …*/plugin->displayFunc = (displayFuncPtr ) dlsym(dlPtr, "draw");if (!plugin->displayFunc) /* error, couldn't find function */plugin->mediaType = strdup(mediaType);dlcose(dlPtr);return plugin;}8CMSC 212 – S05 (lect 16)Implementation Hidingz Goals– Export a public API– Hide details of implementation• allows implementation to evolve• better able to reason about a large system– don't need to know how it works, just that it works• protects IP in implementation– Get the right abstractions• This is the hardest part!z Techniques– No code in header files– Careful pointer definitions• opaque pointers9CMSC 212 – S05 (lect 16)Opaque Pointer Typesz Generic Pointers– make API return void * pointers– useful for building containers• such as maps, sets, etc.– lacks type checking–Example:• void *createTable();• addItem(void *table, void *data);z Pointers to structs (struct defintions are hidden)– Public header files define API and pointer to abstract types• API users only see public header files– Private header files define actual structures• Include public header files• Visible to modules implementing the API10CMSC 212 – S05 (lect 16)Information Hiding Examplez Project #3 revisitedz Change ast.h (to be the public header)struct _node;typedef struct _node ASTnode;z Create astP.h (private header)struct _node {nodeType type;struct _node *left;struct _node *right;int value;operatorType operator;char *name;};11CMSC 212 – S05 (lect 16)Containers vs. APIsz Containers– designed to hold a collection of the same or similar objects– provide common access to stored itemsz API– encapsulates a useful bit of functionality– Example: AST's in project #3– May use containers to store information12CMSC 212 – S05 (lect 16)Common Container Abstractionsz Stack– Last in first out semantics– Push and pop are operationsz Queue– First in first out semantics– Enqueue and dequeue are only operations normallyz Table (really a variation on a map)– Storage of keyword,values– Supports insert, lookup, delete13CMSC 212 – S05 (lect 16)Table Containerz Can have many underlying implementations– Arrays– Linked list– Hash table (open or chained)– Trees (many different trees possible)z Each implementation has performance attributes– How much space is required?– How efficient is insert vs. lookup vs. deletez No single right implementation for purposes– Project #4 explores two alternative implementations14CMSC 212 – S05 (lect 16)Designing a Good APIz Why it matters– Can always change a bad implementation– Changing an API is impacts all usersz Elements of a good design– Can be easily used for many different purposes– Focus on key ideas, not how they work• Lookup/Store data NOT insert into tree– Logical units of work with each function call– Simple (and common) things should be easy• add convenience functions if needed– functions that can be expressed using others in APIz Elements of a bad design– Exposes aspects on implementation– Abstractions don't match common use pattern15CMSC 212 – S05 (lect 16)A Bad API Exampletypedef struct {float real;float imaginary;} complexNumber;complexNumber *createComplex();float getRealPart(complexNumber *num);float getImaginaryPart(complexNumber *num);void setNumber(complexNumber *num, float real, float imaginary);z Hides nothingz Provides little (to no) value added beyond struct16CMSC 212 – S05 (lect 16)More Elements of good API Designz Consistent Return Values across functions– 0 is always success– negative is always failurez Consistent parameters– use the same names for the same things in different functions– how copy semantics are handled• are strings copied when passed to