Variables, IO, and Operators

Variable Names

• variables user defined identifiers

• variables are case sensitive, numVar and numvar are two different

  variables

• All variables must be declared before they are used

Standard IO

• C provides a set of functions in the header file stdio.h to read inputs and write outputs.
• C treats all devices as files:

Formatting Outputs - printf()

int printf (char *format , arg1 , arg2 , ...)

• printf prints a list of arbitrary arguments (arg1, arg2, ...) on the standard output under control of the format
• returns the number of characters printed
• format contains two types of objects:
  • ordinary characters: printed to the output stream
  • specifiers: causes conversion and printing of the next successive argument

Format Spceifiers

• %d (%i) int signed decimal
• %u int unsigned decimal
• %o int unsigned octal value
• %x (%X) int unsigned hex value
• %f float or double
• %e (%E) float or double exponential format
• %s array of char (string)
• %p pointer address stored in pointer
• %ld long signed decimal
• %hd short signed decimal

Reading inputs - scanf()

int scanf (char *format , arg1 , arg2 , ...)

• scanf reads characters from the keyboard, interprets them according to the specifiers in format, and stores the results through the remaining arguments
• each argument after format is a pointer (&c) indicates the memory address where the input should be store

Relational and Logical Operators

• Relational Operators: ==, !=, >, <, >=, <=
• Logical Operators: &&, ||, !

Bitwise Operators

• Bitwise Operators: &, |, ^, ⇠, <<, >>
• bit manipulation operators, only used with integer datatypes

Assignment Operators

• Assignment Operators: =, +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
• Most binary operators op have a corresponding assignment operator op=

Misc Operators

• Misc Operators: sizeof(), & (address of a variable), * (pointer to a variable), ?: (if?then:otherwise)

Pointer and Addresses (* and &)

​ The unary operator * is the indirection or dereferencing operator; when applied to a pointer, it accesses the object the pointer points to

int x =1, y =2;
int *ip; /* ip is a pointer to int */ ip =&x; /* ip now points to x */
y = *ip; /* y is now 1 */
*ip = 0; /* x is now 0 */

Type Conversion

• In arithmetic and bitwise operation the narrower operand will be converted to the wider one before applying the operation
  • char -->short --> int --> unsigned int --> long --> unsigned long --> long long --> float --> double --> long double
• expressions like assignment might result in losing information like assigning longer integer to shorter or a float to an integer (may draw a warning)

Function

• every C program has at least one function: main()
• you can devide you program into sepatate functions, each function perform a specfic task
• The C standard library provides numerous built-in functions: printf() and scanf()
• The function declaration consists of:
  • the function’s name
  • return type
  • list of parameters
• A function definition provides the actual body of the function

Static Variables

• the compiler to keeps the static variables in existence during the life-time of the program
• local variables static allows them to maintain their values between function calls

Memory Layout of C Programs

• A typical memory representation of C program consists of following sections
  • Text segment
  • Initialized data segment
  • Uninitialized data segment
  • Stack
  • Heap

Text Segment

• text segment, also known as a code segment or text
• contains executable instructions
• may be placed below the heap or stack in order to prevent heaps and stack overflows from overwriting it
• shareable: single copy needs to be in memory for frequently executed programs
• readonly: to prevent a program from accidentally modifying its instructions

Initialized Data Segment

• simply the Data Segment: is a portion of virtual address space of a program,
• contains the global variables and static variables that are initialized by the programmer
• is not read-only since the values of variables can be altered at runtime

Uninitialized Data Segment

• often called the "bss" segment, "block started by symbol"
• Data in this segment is initialized by the kernel to arithmetic 0 before the program starts executing
• contains all global variables and static variables that are initialized to zero or do not have explicit initialization in source code, for instance:

Stack

• A "stack pointer" tracks the top of the stack
• The set of values pushed for one function call is termed a "stack frame"; A stack frame consists at minimum of a return address
• Each time a function is called, the address of where to return to and certain information about the caller’s environment, such as some of the machine registers, are saved on the stack
• The newly called function then allocates room on the stack for its variables

Heap

• Heap is the segment where dynamic memory allocation usually takes place
• The Heap area is managed by malloc, realloc, and free
• The Heap area is shared by all shared libraries and dynamically loaded modules in a process

Constants

• There are two simple ways in C to define constants:
• Using #define preprocessor
• Using const keyword