Programming in C

Lecture Outline

Aims and Objectives
Passing Arguments to a C Program
Calling C programs with multiple, optional Arguments
Pointers to Functions
Complicated Declarations

• Passing optional, multiple arguments to a C program

• Pointers to Functions

• Recognising complex pointer declarations

• So far, our C programs have been written with main() not accepting a parameter list.

• By convention, main() can take two parameters

  int argc /* argument count */
  
  char *argv[] /* argument list */
  
  /* e.g., echo hey "you in" there 
  
  argc = 4
  
  argv[0] = echo
  
  argv[1] = hey
  
  argv[2] = you in
  
  argv[3] = there
  
  argv[argc] = NULL 
  */
  

• Example: 1.c

  #include <stdio.h>
  
  int main(int argc, char *argv[])
  {
  	while (--argc > 0)
  		printf((argc > 1) ? "%s " : "%s", *++argv);
  	printf("\n");
  	return 0;
  }
  

• You may have a C program that changes its behaviour based on optional arguments.

• For instance, you might write a program, wc, that counts words, characters, and/or lines from standard input.

  wc -l -c -w    /* or */         wc -wl 
  
  #include <stdio.h>
  
  main(int argc, char *argv[])
  {
  	int c, word = 0, line = 0, character = 0;
  	int return_val = 1;
  
  	while (--argc > 0 && (*++argv)[0] == '-') {
  		while(c = *++argv[0]) {
  		 switch (c) {
  		  case 'w':
  		   word = 1;
  		   break;
  		  case 'l':
  		   line = 1;
  		   break;
  		  case 'c':
  		   character = 1;
  		   break;
  		  default:
  		   printf("wc: Illegal option %c\n", c);
  		   argc = 0;
  		   break;
  		 }
  		}
  	}
  	
  	if (argc != 0)
  		printf("Usage: wc -l -c -w\n");
  	else {/* call function to count words, lines, characters 
  		count(*cs, *ws, *ls);*/
  		int cs = 1023, ws = 192, ls = 15;
  		printf((character == 1) ? "chars = %d\n" : "", cs);
  		printf((word == 1) ? "words = %d\n" : "", ws);
  		printf((line == 1) ? "lines = %d\n" : "", ls);
  	}
  }
  

• Let's say that you have a program which will call a function in one of two different ways, depending on its parameter list

• Let's say that we want to write a sorting algorithm that will sort its input either numerically or alphabetically

  #include <stdio.h>
  #include <string.h>
  
  #define MAXLINES 5000
  char *lineptr[MAXLINES];
  
  int readlines(char *lineptr[], int maxlines);
  int writelines(char *lineptr[], int nlines);
  
  void wsort(void *lineptr[], int left, int right, int (*comp)(void *, void *));
  int numcmp(char *, char *);
  
  main(int argc, char *argv[])
  {
  	int nlines;
  	int numeric = 0; /* 1 if numeric sort */
  
  	if (argc > 1 && strcmp(argv[1], "-n") == 0)
  		numeric = 1;
  
  	if ((nlines = readlines(lineptr, MAXLINES))>=0) {
  		 wsort((void **) lineptr, 0, nlines - 1,
  		  (int (*)(void *, void *))(numeric ? numcmp : strcmp));
  		 writelines(lineptr, nlines);
  		 return 0;
  	} else {
  		 printf("Input too big to sort\n");
  		 return 1;
    }
  }
  void wsort(void *v[], int left, int right, 
  	int (*comp)(void *, void *))
  {
  	int i, last;
  	void swap(void *v[], int, int);
  
  	if (left >= right)
  		return;
  	swap(v, left, (left + right) / 2);
  	last = left;
  	for (i = left + 1; i <= right; i++)
  		if ((*comp)(v[i], v[left]) < 0)
  			swap(v, ++left, i);
  	swap(v, left, last);
  	wsort(v, left, last-1, comp);
  	wsort(v, last+1, right, comp);
  }
  

• Pointers can be very hard to read, because you will have a mixture of *, () and []

• You cannot just read the declarations from left to right, because the position of () can be used to force precedence, as well as indicate that a token is the name of a function

  int *f(); /* function returning pointer to int */
  
  int (*f)(); /* pointer to function returning int */
  
  Some complicated declarations
  
  char **argv
  	argv: pointer to pointer to char
  
  int (*daytab)[13]
  	daytab: pointer to array[13] of int
  
  int *daytab[13]
  	daytab: array[13] of pointer to int
  
  void *comp()
  	comp: function returning pointer to void
  
  void (*comp)()
  	comp: pointer to function returning void
  
  char (*(*x())[])()
  	x: function returning pointer to array[] of pointers to function returning char
  
  char (*(*x[3])())[5]
  	x: array[3] of pointers to functions returning pointer to array[5] of char
  
  

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