Slides created by: Professor Ian G. Harris Hello World #include main() { printf(“Hello, world.\n”); }  #include is a compiler directive to include (concatenate) another file  main is the function where execution starts  printf is a library function, in the stdio library

Slides created by: Professor Ian G. Harris C Syntax Basics  Comments: single line //, multiline /* … */  Semicolon used to terminate all statements  # indicates a compiler directive (include, define, etc.)  Brackets { … } group lines of code for execution  Blankspace between lines is not important in C Still need to separate tokens int a, b, c; a = b + c; int a,b,c;a=b+c; correct incorrect (inta)correct

Slides created by: Professor Ian G. Harris Variables  All variables’ types must be declared Memory space requirements known statically Type checking can be performed at compile-time  Numerical Types: int, float, double  Other types: char, void  Type qualifiers: short, long, unsigned, signed, const Specify alternate sizes, constrain use modes

Slides created by: Professor Ian G. Harris Types for PICC Compiler Type Size Arith type  Type modifiers unsigned and signed bit1integer char8integer short16integer int16integer short long24integer long32integer float24real double24 or 32real

Slides created by: Professor Ian G. Harris Local vs. Global  Scope of a variable is limited to the function where it is declared (malloc later)  Local variables “disappear” when its function returns  Global variables are declared outside of a function Globals do not disappear when a function returns int x; main () { int y; foo(); } foo() { int y;} Global scope Local to main Local to foo

Slides created by: Professor Ian G. Harris Volatile Variables  The value of a volatile variable may change at any time, not just at an explicit assignment  Compiler optimizations are not applied to volatile variables  When can variables change without an explicit assignment? 1. Memory-mapped peripheral registers 2. Global variables modified by an interrupt service routine 3. Global variables accessed by multiple tasks within a multi- threaded application

Slides created by: Professor Ian G. Harris Volatile Example. while (*periph != 1); // wait until data transfer. // is complete.  periph is the mapped address of the peripheral status info  *periph is assigned by peripheral directly  Compiled code will move memory contents to a register  Memory will only be moved once because *periph does not change

Slides created by: Professor Ian G. Harris Types of Statements  Assignment – =  Relational -, ==, !=  Control Flow – if, for, while, do, switch  Arithmetic Operations - +, -, *, /, %  Logical Operations - &&, ||, !  Bitwise Logical Operations - &, |, ^

Slides created by: Professor Ian G. Harris Decimal, Binary Hexadecimal  All numbers are internally represented in binary  Decimal is the default representation in C  Binary is often useful in embedded programming – Individual bits are important to see TRISA = 0b110011; – Notated with “0b” prefix  Hexadecimal is succinct and matches binary 4x1 – Notated with “0x” prefix Ex. 39 = 0b = 0x27

Slides created by: Professor Ian G. Harris Logical Operators  These operators accept binary input and produce binary output &&, ||, !  Non-binary inputs are cast to binary values int a=1, b=0; c=56; d=-1, res; res = a && b;// res = 0 res = a || b;// res = 1 res = c && d;// res = 1

Slides created by: Professor Ian G. Harris Bitwise Logical Operators  These operators accept multi-bit inputs and return multi-bit results &, |, ^, >  Operation is performed on each corresponding pair of bits char a = 0b , b = 0b ; res = a & b;// res = 0b res = a | b;// res = 0b res = a ^ b;// res = 0b res = a << 4; // res = 0b res = b >> 4; // res = 0b

Slides created by: Professor Ian G. Harris Bitlevel Manipulation  Often need to control individual bits in registers  Constants may be defined to refer to individual bits TRISA vs. TRISA0 PORTA vs. RA0  Sometimes more convenient to assign many bits at once PORTA = 0b000111; PORTA = 7; TRISA = 0b111000;

Slides created by: Professor Ian G. Harris Function Calls  Functions enable simple code reuse  Control moves to function, returns on completion  Functions return only 1 value main() { int x; x = foo( 3, 4); printf(“%i\n”, x); } int foo(int x, int y) { return (x+y*3); }

Slides created by: Professor Ian G. Harris Function Prototypes main() { int x; x = foo(); } void foo() { printf (“Hi!\n”); }  Function prototypes declare the return value type and the types of the arguments  Allows the compiler to do type checking  Default return type is int if no prototype is provided void foo();

Slides created by: Professor Ian G. Harris Function Call Overhead main() { int x; x = foo(2); printf(“%i\n”, x); } int foo(int x) { int y=3; return (x+y*3); }  Program counter value needs to be restored after call  Local variables are stored on the stack  Function calls place arguments and return address on the stack 20: 21: 22: 30: 31: 103: 3local var 102: 2argument 101: 21return addr 100: 2local var

Slides created by: Professor Ian G. Harris Header Files  Files included at the top of a code file  Traditionally named with.h suffix  Include information to be shared between files Function prototypes extern s of global variables Global #define s  Needed to refer to libraries

Slides created by: Professor Ian G. Harris Type Casts  Tell the compiler to treat a variable as a different type  Allows typing rules to be violated  May change the space and functional requirements int x=5, y=3; float z; z = x / y; z = (float) x / (float) y;  If x and y are integers then x/y = 1  If x and y are floats then x/y = 1.66  Need a floating point divider

Slides created by: Professor Ian G. Harris Strings  Strings are arrays of chars  A char is the ASCII value of the corresponding character  Double quotes used for strings, single quotes for chars  Strings need a '\0' terminator char achar, astr[13]; achar = 'a'; strncpy(astr, “Hello world!”, 13);

Slides created by: Professor Ian G. Harris Pointers  c is a char. Size of c is the size of a char  & is the reference operator  &c is pointer to c, its address in memory  Size of &c is the size of an address  *d is a char  * is the dereference operator  d is the pointer to *d char c; c = 'a'; char *d; *d = 'a';

Slides created by: Professor Ian G. Harris Pointer Arithmetic  Arithmetic can be performed on pointers  Useful to explore known blocks of memory like arrays  Arrays are just pointers to contiguous memory blocks int varr[10]; printf (“%i”, *varr++); printf (“%i”, *varr); // First elt of array // Second elt of array

Slides created by: Professor Ian G. Harris Call by Reference  A pointer can be passed as an argument to a function  The called function can directly manipulate the variable main () { int x=0; foo(&x); printf (“%i\n”, x); } foo (int *var) { *var = 10; }  Normally a called function cannot access the caller's scope

Slides created by: Professor Ian G. Harris Structures  A collection of variables under a single name  Each grouped variable has its own name  A record with several fields  Pre-object oriented method of organization typedef struct { char name[20]; int age; } person; person harris;  Size of structure is the sum of its component fields

Slides created by: Professor Ian G. Harris Accessing Structure Data person p1, *p2; printf (“%i”, p1.age); printf (“%i”, *p2.age); printf (“%i”, p2->age); . operator returns the value of the member  -> operator dereferences structure pointer first  Structures are often passed by reference, so - > is useful