Functions Kernighan/Ritchie: Kelley/Pohl: Chapter 4 Chapter 5

222 Functions  The heart of effective problem solving is problem decomposition  In C, this "top-down" approach is implemented using functions  A program consists of one or more files  Each file consists of zero or more functions, exactly one of which is called main()

333 Functions  In C, a function declaration and a function definition are two different things  A function declaration tells the compiler that a function exists, and what are its parameter types and return type  A function definition is the actual C code that describes what the function does

444 Lecture Overview  Function definition  Function declaration and invocation  Developing a large program  Storage classes – auto and extern  Storage classes – register and static  Coding standards and obfuscated code

555 Function Definition  A function has a header and a body:  The first int defines the return type int factorial (int n) /* header */ { /* body starts here */ int i, product = 1; for (i = 2; i <= n; ++i) product *= i; return product; }

666 Function Definition  Both the return type and the parameter list are optional, and may also be void :  If a function definition does not specify the return type, then it is assumed to be int  However, this will cause the compiler to issue a warning void welcome (void) { printf ("Welcome to this program.\n"); }

777 Functions with No Parameters  We can write the previous header as:  These formats have different meanings: An empty parameter list means that the parameters are unknown, and tells the compiler not to perform any parameter checks A void parameter defines explicitly that the function has no parameters, and this will be enforced by the compiler void welcome()

888 Functions with No Parameters – Example 1 #include 2 3 int f (void) { 4 return 7; 5 } 6 7 int main() { 8 printf ("f(): %d\n", f (3)); 9 return 0; 10 } [prompt] > gcc -o vparam vparam.c vparam.c: In function `main': vparam.c:8: error: too many arguments to function `f'

999 Functions with No Parameters – Example 1 #include 2 3 int f() { 4 return 7; 5 } 6 7 int main() { 8 printf ("f(): %d\n", f (3)); 9 return 0; 10 } [prompt] > gcc -o vparam vparam.c [prompt] > vparam f(): 7

10 The return Statement  The return statement may or may not include an expression  The parentheses around the returned value or expression are optional  Examples: return; return a + 2; return (a * b);

11 The return Statement  Returning a value from a non-void function is the responsibility of the programmer:  This will compile (with a warning), but will print out unpredictable results (garbage) int f() {} int main() { printf ("%d\n", f()); return 0; }

12 Lecture Overview  Function definition  Function declaration and invocation  Developing a large program  Storage classes – auto and extern  Storage classes – register and static  Coding standards and obfuscated code

13 Function Declaration  There are several ways to generate function declarations: Explicitly – using a prototype Implicitly – through the function definition Even more implicitly, by function invocation – if the compiler encounters an unknown function, it assumes a default declaration: int f();

14 Function Prototypes  Functions can be explicitly declared before they are used:  function declarations of this type are called function prototypes  A prototype defines the parameter types and the return type of the function float maximum (float x, float y); void print_info();

15 Function Prototypes #include void print_num (int); int main() { print_num (5); return 0; } void print_num (int a) { printf ("The number is: %d\n", a); } The number is: 5

16 Implicit Function Declaration  A function does not need a prototype if it is only used after its definition #include void print_num (int a) { printf ("The number is: %d\n", a); } int main() { print_num (5); return 0; }

17 Implicit Function Declaration  If the compiler encounters a previously undefined and undeclared function:  The function is assumed to be defined somewhere else, with the prototype: int main() { print_num (5); return 0; } int print_num();

18 Function Invocation  Program execution always begins with the main() function Exactly one must exist in any program  Functions are invoked by writing their name and an appropriate list of arguments within parentheses  The returned value of a function may be assigned to a variable, or just ignored

19 Functions – Call by Value  In C, arguments to functions are always passed by value  When an expression is passed as an argument, it is first evaluated, and only the result is passed to the function  The variables passed as arguments are not changed in the calling environment

20 Functions – Call by Value int main() { int n = 3, sum, compute_sum (int); printf("%d\n", n); /* 3 is printed */ sum = compute_sum (n); printf("%d\n", n); /* 3 is printed */ printf("%d\n", sum); /* 6 is printed */ return 0; } int compute_sum (int n) { /* sum from 1 to n */ int sum = 0; for ( ; n > 0; n--) /* n is changed */ sum += n; return sum; }

21 Lecture Overview  Function definition  Function declaration and invocation  Developing a large program  Storage classes – auto and extern  Storage classes – register and static  Coding standards and obfuscated code

22 Developing a Large Program  Typically, a large program is written in a separate directory as a collection of source files ('.c ') and header files ('.h ')  Source files contain mostly function definitions  Header files contain function prototypes and various definitions (e.g. constants and types)  Source files include header files as required

23 Developing a Large Program – Example  Suppose that we have a '.c ' file containing various utilities used by the program:  The accompanying '.h ' file is: #include "util.h" int mult (int n) { return n * MULT_FACTOR; } #define MULT_FACTOR 2 int mult (int n);

24 Developing a Large Program – Example  Any '.c ' file that wishes to use these utilities will include the ' util.h ' file: #include #include "util.h" int main() { printf ("%d\n", mult (30)); return 0; } 60

25 Compiling Multiple Files  The previous program can be compiled using the following command line  The form of include used here tells the compiler to look for the file in the current directory first, and only then search for it in the system directories gcc -o use_util use_util.c util.c

26 Incremental Compilation  Dividing the program into files may speed up compilation after small changes  It is possible to separate the compilation stage from the linkage stage: Compilation turns source code into object code Linkage turns several object files into a single executable file

27 Incremental Compilation  Using the compiler's -c option, object code files can be generated from source files:  This will create a corresponding '.o ' file for each '.c ' file: gcc -c use_util.c util.c use_util.o util.o ls *.o

28 Incremental Compilation  Now, all of the object files can be linked together to create an executable:  If changes have been made only to a single file, '.c ' and '.o ' files can be mixed together in the link line: gcc -o use_util use_util.o util.o gcc -o use_util use_util.c util.o

29 Lecture Overview  Function definition  Function declaration and invocation  Developing a large program  Storage classes – auto and extern  Storage classes – register and static  Coding standards and obfuscated code

30 Storage Classes  Every variable and function in C has two attributes – its type and its storage class  The four storage classes are: auto extern register static

31 The Storage Class auto  Variables declared within function bodies are automatic by default  When a block is entered, the system allocates memory for the automatic variables defined in it  When the block is exited, the system releases the memory that was set aside for the automatic variables

32 The Storage Class extern  One method of transmitting information across blocks and functions is to use external variables  When a variable is declared outside a function, storage is permanently assigned to it, and its storage class is extern  An external variable is considered to be global to all functions declared after it

33 The Storage Class extern int global; void set (int n) { global = n; } int get() { return global; } int main() { set (100); printf ("%d\n", get()); return 0; }

34 The Storage Class extern  External variables can also be shared across different files  For example, consider the definition of the variable a in this file: int a = 5; void inc() { a++; }

35 The Storage Class extern  Now, we can use a in another file:  The extern keyword tells the compiler to look for the definition of a elsewhere extern int a; void inc(); int main() { inc(); printf ("%d\n", a); return 0; }

36 Defining and Declaring External Variables  We distinguish between the declaration of an external variable and its definition  The definition causes storage to be set aside (and also serves as a declaration)  There must be only one definition of an external variable among all of the files that make up the source program int a = 5;

37 Defining and Declaring External Variables  A declaration of the variable should appear in any of the files that use it  This declares for the rest of the file that a is an int, but does not actually create it  Initialization of an external variable goes only with the definition extern int a;

38 Using Header Files for External Declarations  Normally, the declarations in the previous example will be contained in a separate header file, and included by the caller  The first file, ' counter.c ', will contain: #include "counter.h" int a = 5; void inc() { a++; }

39 Using Header Files for External Declarations  The header file ' counter.h ', will be:  And the main file will look like this: extern int a; void inc(); #include "counter.h" int main() { inc(); printf ("%d\n", a); return 0; }

40 Functions  Functions cannot be nested within blocks, and therefore all functions in C have an extern storage class  As we have seen in the previous example, to use inc() defined in a different file, we had to provide a prototype, but the extern declaration was implicit

41 Lecture Overview  Function definition  Function declaration and invocation  Developing a large program  Storage classes – auto and extern  Storage classes – register and static  Coding standards and obfuscated code

42 The Storage Class register  The storage class register tells the compiler to store specific variables directly in the CPU's registers:  This is just a recommendation, and the compiler will ignore it if not enough physical variables are available register int i;

43 The Storage Class static  Static variables have two different uses: To allow a local variable to retain its previous value when the block is reentered To provide a privacy mechanism that limits the visibility of variables and functions  These should not be confused with the different meanings of static in other languages, such as Java or C++

44 Static Local Variables  Normally, when a variable is defined within a function, it is reallocated with every call, and discarded every time the function ends  A static variable is assigned a single memory location before the program starts, and thus retains its value between subsequent calls of the function

45 Static Local Variables #include int f() { static int count = 0; return (count++); } int main() { printf ("%d ", f()); printf ("%d \n", f()); return 0; } 0 1 2

46 Static External Variables  The second use of static is to restrict the scope of external variables  Non-static external variables are available throughout the program  Static external variables are only available to functions defined in the same file And out of these, only to those that were defined after the static declaration

47 Static External Variables #define INITIAL_SEED 17 #define MULTIPLIER #define INCREMENT #define MODULUS static unsigned seed = INITIAL_SEED; unsigned int random() { seed = (MULTIPLIER * seed + INCREMENT) % MODULUS; return seed; } double probability() { seed = (MULTIPLIER * seed + INCREMENT) % MODULUS; return ((double)seed / MODULUS); }

48 Static Functions  Just as for external variables, applying the static storage class to a function effectively makes that function private Should be applied to definition and to declaration  The function inc() is now available only to other functions in the same file static void inc() { a++; }

49 Lecture Overview  Function definition  Function declaration and invocation  Developing a large program  Storage classes – auto and extern  Storage classes – register and static  Coding standards and obfuscated code

50 C Coding Standards  C provides the programmer with the ability to write powerful and compact code  Its syntax imposes very few restrictions, and leaves much room for originality  C compilers will issue an error only when they have no other choice  Thus, it is easy to write obfuscated code

51 Obfuscated Code  This tattoo belongs to Thomas Scovell from Auckland, New Zealand  It is a C program that prints “hello, world!” int i;main(){for(;i["]<i;++i){--i;}"];read('-'-'-',i+++"hell\ o, world!\n",'/'/'/'));}read(j,i,p){write(j/p+p,i---j,i/i);}

52 Obfuscated Code  Can you guess what this program does? long h[4];t(){h[3]-=h[3]/3000;setitimer(0,h,0);}c,d,l,v[]={(int)t,0,2},w,s,I,K =0,i=276,j,k,q[276],Q[276],*n=q,*m,x=17,f[]={7,-13,-12,1,8,-11,-12,-1,9,-1,1, 12,3,-13,-12,-1,12,-1,11,1,15,-1,13,1,18,-1,1,2,0,-12,-1,11,1,-12,1,13,10,-12, 1,12,11,-12,-1,1,2,-12,-1,12,13,-12,12,13,14,-11,-1,1,4,-13,-12,12,16,-11,-12, 12,17,-13,1,-1,5,-12,12,11,6,-12,12,24};u(){for(i=11;++i<264;)if((k=q[i])-Q[i] ){Q[i]=k;if(i-++I||i%12<1)printf("\033[%d;%dH",(I=i)/12,i%12*2+28);printf( "\033[%dm "+(K-k?0:5),k);K=k;}Q[263]=c=getchar();}G(b){for(i=4;i--;)if(q[i?b+ n[i]:b])return 0;return 1;}g(b){for(i=4;i--;q[i?x+n[i]:x]=b);}main(C,V,a)char* *V,*a;{h[3]= /(l=C>1?atoi(V[1]):2);for(a=C>2?V[2]:"jkl pq";i;i--)*n++=i< 25||i%12<2?7:0;srand(getpid());system("stty cbreak -echo stop u");sigvec(14,v, 0);t();puts("\033[H\033[J");for(n=f+rand()%7*4;;g(7),u(),g(0)){if(c<0){if(G(x+ 12))x+=12;else{g(7);++w;for(j=0;j<252;j=12*(j/12+1))for(;q[++j];)if(j%12==10){ for(;j%12;q[j--]=0);u();for(;--j;q[j+12]=q[j]);u();}n=f+rand()%7*4;G(x=17)||(c =a[5]);}}if(c==*a)G(--x)||++x;if(c==a[1])n=f+4**(m=n),G(x)||(n=m);if(c==a[2])G (++x)||--x;if(c==a[3])for(;G(x+12);++w)x+=12;if(c==a[4]||c==a[5]){s=sigblock( 8192);printf("\033[H\033[J\033[0m%d\n",w);if(c==a[5])break;for(j=264;j--;Q[j]= 0);while(getchar()-a[4]);puts("\033[H\033[J\033[7m");sigsetmask(s);}}d=popen( "stty -cbreak echo stop \023;cat - HI|sort -rn|head -20>/tmp/$$;mv /tmp/$$ HI\ ;cat HI","w");fprintf(d,"%4d on level %1d by %s\n",w,l,getlogin());pclose(d);}