Introducing functions

• In Mathematics, we apply a function, such as the trigonometric function cos, to one or more values.
  The function performs an evaluation, and returns a result.

• In many programming languages, including C, we call or invoke a function.
  We evaluate zero or more expressions, the result of each expression is copied to a memory location where the function can receive them as arguments, the function's statements are executed (often involving the arguments), and a result is returned (unless the function is stuck in an infinite-loop or exits the process!)

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    // check the number of arguments
    if(argc != 2) {
        ....
        exit(EXIT_FAILURE);
    }
    else {
        ....
        exit(EXIT_SUCCESS);
    }
    return 0;
}

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    int result;

    // check the number of arguments
    if(argc != 2) {
        ....
        result = EXIT_FAILURE;
    }
    else {
        ....
        result = EXIT_SUCCESS;
    }
    return result;
}

Why do we require functions?

1. Functions allow us to group together statements that have a strongly related purpose - statements, in combination, performing a single task.
   We prefer to keep such statements together, providing them with a name (as for variables), so that we may refer to the statements, and call them, collectively.
   This provides both convenience and readability.

2. We often have sequences of statements that appear several times throughout larger programs.
   The repeated sequences may be identical, or very similar (differing only in a very few statements). We group together these similar statement sequences, into a named function, so that we may call the function more than once and have it perform similarly for each call.
   • Historically, we'd identify and group similar statements into functions to minimize the total memory required to hold the (repeated) statements.
   • Today, we use functions not just to save memory, but to enhance the robustness and readability of our code (both good Software Engineering techniques).

3. From the Systems Programming perspective, the operating system kernel employs functions as well-defined entry points from user-written code into the kernel.
   Such functions are named system calls.

4. Functions provide a convenient mechanism to package and distribute code. We can distribute code that may be called by other people's code, without providing them with a complete program.
   We frequently use libraries for this purpose.

5. And, in sufficiently advanced operating systems, multiple running processes can share a single instance of a function, such as printf() - provided that the function's code cannot be modified by any process, and the function makes references to each process's distinct data and the parameters passed to the function.
   Libraries of such functions are often termed shared libraries.

Where do we find functions?

1. We've already begun writing our own functions, in the same file as main(), to simplify our code and to make it easier to read.

2. Soon, we'll write our own functions in other, multiple files, and call them from our main file.

3. Collections of related functions are termed libraries of functions.
   The most prominent example, that we've already seen, is C's standard library - a collection of frequently required functions that must be provided by a standards' conforming C compiler.
   In our programming, so far, we've already called library functions such as:
   printf(),  atoi(),  and  exit().

4. Similarly, there are many task-specific 3rd-party libraries. They are not required to come with your C compiler, but may be downloaded or purchased - from Lecture 1 -

   Application domain	(a sample of) 3rd-party libraries
   operating system services (files, directories, processes, inter-process communication)	OS-specific libraries, e.g. glibc, System32, Cocoa
   web-based programming	libcgi, libxml, libcurl
   data structures and algorithms	the generic data structures library (GDSL)
   GUI and graphics development	OpenGL, GTK, Qt, wxWidgets, UIKit, Win32, Tcl/Tk
   image processing (GIFs, JPGs, etc)	GD, libjpeg, libpng
   networking	Berkeley sockets, AT&T's TLI
   security, cryptography	openssl, libmp
   scientific computing	NAG, Blas3, GNU scientific library (gsl)
   concurrency, parallel and GPU programming	OpenMP, CUDA, OpenCL, openLinda (thread support is defined in C11, but not in C99)

The role of function main()

• receive and check the program's command-line arguments,

• report errors detected with command-line arguments, and then call exit(EXIT_FAILURE),

• call functions from main(), typically passing information requested and provided by the command-line arguments, and

• finally call exit(EXIT_SUCCESS) if all went well.

• All error messages printed to the stderr stream.

• All 'normal' output printed to the stdout stream (if not to a requested file).

The datatype of a function

1. functions whose role is to just perform a task, and to then return control to the statement (pedantically, the expression) that called it.
   Such functions often have side-effects, such as performing some output, or modifying a global variable so that other statements may access that modified value.
   These functions don't return a specific value to the caller, are termed void functions, and we casually say that they "return void".

2. functions whose role is to calculate a value, and to return that value for use in the expressions that called them. The single value returned will have a type, such as int, char, bool, or float. These functions may also have side-effects.

#include <stdio.h>
#include <stdlib.h>

void output(char ch, int n)        
{
    for(int i=1 ; i<=n ; i=i+1) {
        printf("%c", ch);
    }
}

int main(int argc, char *argv[])
{
    output(' ', 19);
    output('*',  1);
    output('\n', 1);

    return 0;
}

#include <stdio.h>
#include <stdlib.h>

extern double sqrt(double x);

float square(float x)
{
    return x * x;
}

int main(int argc, char *argv[])
{
    if(argc > 2) {
      float a, b, sum;

      a   = atof(argv[1]);
      b   = atof(argv[2]);

      sum = square(a) + square(b);
      printf("hypotenuse = %f\n",
             sqrt(sum) );
    }
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

float square(float x)
{
    return x * x;
}

int main(int argc, char *argv[])
{
    if(argc > 2) {
      float a, b, sum;

      a   = atof(argv[1]);
      b   = atof(argv[2]);

      sum = square(a) + square(b);
      printf("hypotenuse = %f\n",
             sqrt(sum) );
    }
    return 0;
}

In the 3rd example, we're being more correct, by #includ-ing the <math.h> header file - instructing the C compiler find the correct prototype for sqrt() on this system.

In the 2nd and 3rd cases we must compile the examples with:  cc [EXTRAOPTIONS] -o program program.c -lm 

to instruct the linker to search the math library for any missing code (we require the sqrt() function).

Passing parameters to functions

• a sequence of expressions are separated by commas, as in:
  a = average3( 12 * 45, 238, x - 981 );

• each of these expressions has a datatype. In the above example, each of the expressions in an int.
• when the function is called, the expressions are evaluated, and the value of each expression is assigned to the parameters of the function:

  float average3( int x, int y, int z ) { return (x + y + z) / 3.0; }

• during the execution of the function, the parameters are local variables of the function.

They have been initialized with the calling values (x = 12 * 45 ...), and the variables exist while the function is executing.
They "disappear" when the function returns.
• Quite often, functions require no parameters to execute correctly. We declare such functions with:

  void backup_files( void ) { ..... }

  and we just call the functions without any parameters: backup_files();

Very common mistakes with parameter passing

• The order of evaluation of parameters is not defined in C. For example, in the code:

  int square( int a ) { printf("calculating the square of %i\n", a); return a * a; } void sum( int x, int y ) { printf("sum = %i\n", x + y ); } .... .... sum( square(3), square(4) );

  are we hoping the output to be:

      calculating the square of 3     // the output on PowerPC Macs
      calculating the square of 4
      sum = 25
  
  or
  
      calculating the square of 4     // the output on Intel Macs
      calculating the square of 3
      sum = 25
  

  Do not assume that function parameters are evaluated left-to-right. The compiler will probably choose the order of evaluation which produces the most efficient code, and this will vary on different processor architectures.
  (A common mistake is to place auto-incrementing of variables in parameters.)

Very common mistakes with parameter passing, continued

• Another common mistake is to assume that function arguments and parameters must have the same names to work correctly.
  Some novice programmers think that the matching of names is how the arguments are evaluated, and how arguments are bound to parameters.
  For example, consider the code:

  int sum3( int a, int b, int c ) { return a + b + c; } .... int a, b, c; a = 1; b = 4; c = 9; printf("%i\n", sum3(c, a, b) );

  Here, the arguments are not "shuffled" until the names match.
  It is not the case that arguments must have the same names as the parameters they are bound to.
  Similarly, the names of variables passed as arguments are not used to "match" arguments to parameters.
  If you ever get confused by this, remember that arithmetic expressions, such as 2*3 + 1, do not have names, and yet they are still valid arguments to functions.

Very common mistakes with parameter passing, continued

• While not an example of an error, you will sometimes see code such as:

          return x;
  

  and sometimes:

          return(x);
  

  While both are correct, the parentheses in the 2nd example are unnecessary.
  return is not a function call, it is a statement, and so does not need parentheses around the returned value.
  However - at any point when writing code, use extra parentheses if they enhance the readability of your code.

The static keyword

The static keyword in C11 plays two very distinct roles:

• When appearing before a global variable or (global) function definition, static signifies that the variable or function is only visible from within that file. If a C11 program is written in multiple source-code files, code within the 'other' files cannot 'see' the static variable or function. This enables global variables and functions to be hidden from 'other' files, and their names to re-used by 'other' files.

• When appearing before a local variable within a function, static signifies that the variable retains its value between calls to the function. A typical use is to count the number of times a function has been called (provided that the variable has been initialised!)

  #include <stdio.h> #include <stdlib.h> // myfunction IS ONLY VISIBLE WITHIN THIS FILE, AND IS CALLED BY main static void myfunction(void) { static int count = 1; // retains its value between function calls int local = 0; // is re-initialised on each function call printf("count=%i local=%i\n", count, local); ++count; ++local; } // main IS NOT DECLARED AS static BECAUSE THE OPERATING SYSTEM MUST BE ABLE TO CALL IT int main(int argc, char *argv[]) { for(int i=0 ; i < 5 ; ++i) { myfunction(); } exit(EXIT_SUCCESS); }

  When compiled and executed will produce:

  count=1  local=0
  count=2  local=0
  count=3  local=0
  count=4  local=0
  count=5  local=0
  

  
  

Functions receiving a variable number of arguments

#include <stdio.h>

  int   i = 238;
  float x = 1.6;

  printf("i is %i, x is %f\n", i, x);
  ....
  printf("this function call only has a single argument\n");  
  ....
  printf("x is %f, i is %i, and x is still %f\n", x, i, x);  

prompt> cc -o try try.c
try.c:9:20: warning: format specifies type 'int' but the argument has type 'char *'
      [-Wformat]
    printf("%i\n", "hello");
            ~~     ^~~~~~~
            %s
1 warning generated.