Sunday, 23 January 2011

The C Programming Language (K&R) 01x16—Char Arrays & Functions—Section 1.10

Section 1.10 of “The C Programming Language” by Brian Kernighan and Dennis Ritchie aka K&R looks at the scope of variables in C. There is a difference between a variable declared inside a function and outside. The first is confined to the function in which it is declared. It exists only when the function is called and dies when the program leaves the function. Static variables are discussed elsewhere and provide an except. The other is global in scope and created when the program begins. It can be accessed by any function.

The sample code from 01x13 is modified to use global variables.

The variables are declared as they were in the functions but are placed at the top of the source file (i.e., outside of any function).

The functions contain the same variable “declarations” except the keyword extern is added in front.

I haven’t seen it used a great deal for a couple of reasons. As long as the program is running, these global variables take up memory even if they won’t be used again. A key objective should be to use a little RAM as possible. Second, global variables can be changed in ways that make debugging difficult. It can lead to unexpected results. I avoid using them, but they do serve a purpose at times.

Sample Code.

I am using Visual C++ 2010 and created the sample code as a console application.

// Function prototype.
int getline();
void copy();

// The standard library includes the system function.
#include <cstdlib>

// Standard I/O library.
#include <cstdio>

#define MAXLINE 1000 /* maximum input line length */

int max; /* maximum length seen so far */
char line[MAXLINE]; /* current input line */
char longest[MAXLINE]; /* longest line saved here */

int main()
     int len; /* current line length */
     extern int max;
     extern char longest[];

     max = 0;

     while ((len = getline()) > 0)
           if (len > max) {
           max = len;

     if (max > 0) /* there was a line */
           printf("%s", longest);

     // Keep console window open.

     // Return some value.
     return 0;

} // end main

/* getline: read a line into s, return length */
int getline()
     int c, i;
     extern char line[];

     for (i=0; i < MAXLINE-1 && (c=getchar())!=EOF && c!='\n'; ++i)
           line[i] = c;

     if (c == '\n') {
           line[i] = c;

     line[i] = '\0';

     return i;

/* copy: copy 'from' into 'to'; assume to is big enough */
void copy()
     int i;
     i = 0;
     extern char line[], longest[];

     while ((longest[i] = line[i]) != '\0')


The longest line is…

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