8/21/18 C Programming Basics GCC 8.2.0 & GDB 8.1.1 • nike.cs.uga.edu • brew macOS � gcc-8 – brew install gcc, or – brew upgrade gcc Also need Xcode CLT – xcode-select – install – Brew info gcc • brew install gdb • https://sourceware.org/gdb/wiki/BuildingOnDarwin 1
8/21/18 Set up: LIBRARY : PATH • On nike.cs.uga.edu • LD_LIBRARY_PATH – ~maria/usr/GDB-8.1.1/lib – ~maria/usr/GCC-8.2.0/lib64 – ~maria/usr/GCC-8.2.0/lib • PATH – ~maria/usr/GCC-8.2.0/bin/gcc .login (tcsh) or .profile (bash/ksh) ls /home/profs/maria/usr/GCC-8.2.0/ ls /home/profs/maria/usr/GDB-8.1.1/ setenv GCC_HOME /home/profs/maria/usr/GCC-8.2.0/ csh/tcsh setenv GDB_HOME /usr/profs/maria/usr/GDB-8.1.1/ setenv PATH ${GCC_HOME}/bin/:${PATH} setenv PATH ${GDB_HOME}/bin/:${PATH} setenv LD_LIBRARY_PATH ${GCC_HOME}/lib:${LD_LIBRARY_PATH} setenv LD_LIBRARY_PATH ${GCC_HOME}/lib64:${LD_LIBRARY_PATH} setenv LD_LIBRARY_PATH ${GDB_HOME}/lib:${LD_LIBRARY_PATH} export GCC_HOME=/home/profs/maria/usr/GCC-8.2.0/ bash/ksh export GDB_HOME=/usr/profs/maria/usr/GDB-8.1.1/ PATH= ${GCC_HOME}/bin/:${PATH} PATH =${GDB_HOME}/bin/:${PATH} export=LD_LIBRARY_PATH=${GCC_HOME}/lib:${LD_LIBRARY_PATH} export=LD_LIBRARY_PATH=${GCC_HOME}/lib64:${LD_LIBRARY_PATH} export=LD_LIBRARY_PATH=${GDB_HOME}/lib:${LD_LIBRARY_PATH} 2
8/21/18 • C Basics • Debugging Tools • C Standard Library – getopt – stdio.h – stdlib.h – string.h C Basics • For Some: – Overlap of C++ System Programming. • Pointers, Arrays, structs, Casting • Memory Management • Function Pointers / Generic Types • Strings 3
Carnegie Mellon 8/21/18 Variable Declarations & Qualifiers Global Variables: ■ Defined outside functions, seen by all files ■ Use “extern” keyword to use a Scope of Variables ■ global variable defined in another file Const Variables: ■ • Global Variables: For variables that won’t change ■ – Declared outside functions Data stored in read-only data section • Seen by all files ■ • Use extern to use a global variables Static Variables: ■ declared in another file • const Variables For locals, keeps value between invocations ■ – Data stored in read-only segment – USE SPARINGLY ■ (variable does not change) Note: static has a different meaning when • static Variables ■ referring to functions (not visible outside of – In functions – value is persistent between invocations (not volatile). object file) – Static Functions – not visible outside object file (not same semantic). https://www.geeksforgeeks.org/scope-rules-in-c/ https://www.geeksforgeeks.org/const-qualifier-in-c/ Casting • Converts variables to different types: – Implicit : (automatic by compiler) • Type promotion (upgraded to ‘data type). – trigged by multiple data types in an expression. • Type “demotion” (can happen, look out for this): – Loss of sign, and overflow. – Example: long long � float https://www.geeksforgeeks.org/type-conversion-c/ 4
8/21/18 ctutorial/global[hmv].c – Explicit - “type casting” • (type) expression Cautions: – Casting down: • Truncates – Casting up pointers ctutorial/pointers.c Pointers • Stores address of a value in memory • Access the value by de-referencing *aptr. – Used when reading or writing a value to a given address • Be aware: – De-referencing NULL causes undefined behavior. E.g., segmentation error. 5
8/21/18 ctutorial/pointers.c Carnegie Mellon Pointers Pointers • Pointers to type A ■ Pointer to type A references a block references a block of of sizeof(A) bytes sizeof(A) bytes. ■ Get the address of a value in • Get the address of a memory with the ‘ & ’ operator ■ Pointers can be aliased , or pointed value in memory with to same address & • Pointers can be aliased, or pointed to the same address. generate_scheduler.c • pointers.c – Demo of pointer arithmetic – Type affect the arithmetic • Adding x to a pointer A – A + x � (A+x* sizeof(TYPE_OF_PTR_A)) 6
8/21/18 passing_args.c Call by value vs. Call by reference • Call-by-value: – Changes made to arguments passed to a function aren’t reflected in the calling function • Call-by-reference: – Changes made to arguments passed to a function are reflected in the calling function • C is call by value by default, so anything past is by value, you can use a pointer to simulate call by reference – Parameter is an address and the address still does not change but now the content what it points to can. Arrays and Strings • Arrays : fixed-size collection of elements of the same type. – Can allocate on the stack or on the heap: int A[10]; //A on the stack //A on the heap . int * A = calloc(10, sizeof(int)); • Strings : Null-character (‘\0’) terminated character arrays – Null-character tells us where the string ends – All standard C library functions on strings assume null-termination 7
8/21/18 structs.c Struct • Collection of values under on name in a single block of memory – Access fields using ‘.’ – Use a pointer access via ‘ � ’ typedef.c typedef • Typedefs • Function pointers. https://www.newty.de/fpt/fpt.html https://www.geeksforgeeks.org/function-pointer-in-c/ 8
8/21/18 Carnegie Mellon C program Memory Layout C Program Memory Layout • Heap & Stack Stack vs. Heap Data vs. Data • Stack: – Local function variables, Function arguments. – Deallocated after variable leaves scope • Do not return pointer to a stack allocated variable • Do not reference the address of a variable outside its scope • Heap: – Dynamically allocated (malloc/calloc) • Data Section: – Global, constants. • Example: // a is a pointer on the stack to a // memory block on the heap. int * a = malloc( sizeof(int) ) 9
8/21/18 Heap: Malloc, Free, Calloc void* malloc (size_t size): allocate block of memory of size bytes does not initialize memory void* calloc (size_t num, size_t size): allocate block of memory for array of num elements each size bytes long initializes memory to zero void free(void* ptr): frees memory block, previously allocated by malloc, calloc, realloc, pointed by ptr use exactly once for each pointer you allocate size argument: should be computed using the sizeof operator sizeof: takes a type and gives you its size e.g., sizeof(int), mem_mgt_valgrind.sh Memory Management Rules • Client should free memory allocated by client code • Library should free memory allocated by Library code • Number of mallocs = Number of frees – # of mallocs > # of frees // memory leak – # of mallocs < # frees // double free. • Only malloc when necessary – Persistent, variable size data structure = necessary. 10
8/21/18 Valgrind • Find common errors detect memory leaks • Common Errors – Illegal read/write errors – Use of unitialized values – Illegal frees – Overlapping source/destination addresses • Questions: – Enough memory? – Did you accidentally free? – Did something multiple times (but did not mean to) – Use something you just freed. Valgrind http://valgrind.org/docs/manual/quick-start.html 11
8/21/18 Other Topics • Head files and Header Guards • Macros • C-Libraries. Header Guards • Avoids the double inclusion problem. #ifndef MARIA_H #define MARIA_H #endif 12
8/21/18 Macros • Replace a name with a macro definition – No function call overhead – Find and replace mechanics of a text editor. • Typical Uses: – Defining constants (INT_MAX, ARRAY_SIZE) – Simple operations: MAX(a,b) – Require, Ensure • Caution: – Use parenthesis around expressions, to avoid substitution problems – Do not pass expression with side effect as arguments to macros. #define INT_MAX 0x7FFFFFFFF #define MAX(A, B) ((A) > (B) ? (A) : (B)) #define REQUIRES(COND) assert(COND) #define WORD_SIZE 4 #define NEXT_WORD(a) ((char*)(a) + WORD_SIZE) string.h • Be Aware of ‘ \0 ’ string termination. • dest need to be large enough to hold src • src contains n bytes (check). • src and dest do not overlap 13
8/21/18 Common Array Methods void memcpy( void *dest, void *src, size_t n ) void strcpy( void *dest, void *src ) char *strncat(char *dest, char *src, size_t n) char *strcat(char *dest, char *src) char *strdup( const char *s1) char *strncat (char *dest, char *src, size_t n) char *strcat (char *dest, char *src) int strncmp (char *str1, char *str2, size_t n) int strcmp(char *str1, char *str2) char *strstr (char *str1, char *str2) char *strtok (char *str, char *delimiters) size_t strlen (const char *str) void *memset (void *ptr, int val, size_t n) stdlib.h • Malloc, calloc, free • atoi • System Calls – exit(), abort() • Searching, Sorting – bsearch() – qsort() 14
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