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arrays of pointers to pointers

Posted on 1997-05-07
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Last Modified: 2010-04-15
I am a Pascal programmer learning C. To help me I am converting
programs I have written to C. My editor uses an array of
pointers to strings to make it possible to edit text files
larger than 64K. I can do it in Pascal, but I am having
trouble figuring out how to make an array of pointers to
strings. Here's the code so far, it compiles, but will not
load files more than a two or three lines long.

//LED project conversion from Turbo Pascal
//May 7, 1997 Stephen E. Wright, PhD
//This version is a only text file viewer for large files
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>

const MaxCharPerLine=1024;       /*max string length*/
const MaxLines=16000;            /*max number of lines*/
const PageSize=25;               /*PC screen height in lines*/

FILE *inputf;                    /*file to read*/
char file_name[79];              /*name of file to read*/
char string[MaxCharPerLine];     /*define a string*/
char *s = string;                /*pointer to a string*/
typedef char** str;              /*pointer to a pointer to a char*/
typedef str tLines[MaxLines];    /*define an array of above*/
tLines *t;

int keystroke, arrowkey;         /*used to identify keys pressed*/
unsigned int i, j, k;            /*counters*/
unsigned int MaxLineNumber;      /*number of lines read */

enum boolean {FALSE, TRUE};      /*simulate Pascal "boolean"*/
boolean ESCpressed, ScrChanged;  /*flags*/

void LoadTheFile(void)
{
   /*open a text file for reading*/
   do {
      puts("File: ");
      gets(file_name);
      if ((inputf = fopen(file_name, "rt") ) == NULL )
       printf("Error in file name : '%s'\n", file_name);
   } while (inputf==NULL);

   /*get memory for array of pointers to strings*/
   t = (tLines*) malloc(MaxLines * sizeof(*s));
   for (i=0; i<MaxLines; i++, *t[i]=NULL);

   /*read file and copy lines to memory */
   j = 0;
   while (!feof(inputf))
   {
      fgets(s, MaxCharPerLine, inputf);
      *t[j] = (char**) malloc(strlen(s)+1);
      strcpy(**t[j], s);
      j++;
   }
   /* if (last line does not end with newline) */
   if (s[strlen(string)-1] != 10) j++;
   MaxLineNumber=j;
   fclose(inputf);
}

void EnterText(void)
{
   int refreshCounter;
   int Topline;

   Topline = 1;
   ScrChanged=TRUE;
   ESCpressed=FALSE;

   while (!ESCpressed)
   {
      if (ScrChanged) /*update display*/
      {
         clrscr();

         if (PageSize > MaxLineNumber)               /*just one page*/
            refreshCounter=MaxLineNumber;
         else
         if (MaxLineNumber-PageSize < Topline)       /*last page*/
            refreshCounter=MaxLineNumber;
         else refreshCounter=Topline+PageSize-1;     /*all other pages*/

         /*note that screen coords out of sync with array coords*/
         for ( j=Topline-1; j<refreshCounter-1; j++)
         {
            strcpy(s, **t[j]);
            printf("%s", s);
         }
      }

      keystroke=getch();                       /*get a keystroke*/
      if (keystroke == 0) arrowkey = getch();  /*extended key pressed*/

      switch (keystroke)
      {
         case 27: ESCpressed=TRUE; break; /*ESC pressed*/
         case 0:
            switch (arrowkey)
            {
               case 81:                  /*PgDn*/
                  ScrChanged=TRUE;
                  if (MaxLineNumber<PageSize)
                     Topline = 1;
                  else
                     if ( (Topline+(2*PageSize)-1) < MaxLineNumber )
                        Topline = Topline+PageSize-1;
                     else
                        Topline = MaxLineNumber-PageSize+1;
               break;

               case 73:                  /*PgUp*/
                  ScrChanged=TRUE;
                  if (Topline-PageSize < 1)
                     Topline=1;
                  else
                     Topline=Topline-PageSize+1;
               break;
            } /*switch arrowkey*/
         break; /*case 0: */
         default: break;  /*do nothing with any other key pressed*/
      } /*switch keystroke*/
   } /*while !EndofFile */
}

int main(void)
{
   LoadTheFile();
   EnterText();
   return 0;
}
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Question by:swright243
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1 Comment
 
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LucHoltkamp earned 600 total points
ID: 1250290
Youre code is a mess! While browsing I noticed several errors. I strongly advice you to buy a GOOD book. The best I've ever read is "The C Programming Language" by Kernighan and Ritchie
I will give some pointer examples here:
int a;     // an int
int *b;    // a pointer to int
int c[3];  // an array of int's
int **d;   // a pointer to a pointer of int;
int *e[2]; // an array of int pointers

b = c;   // Yes c is in fact an pointer to int.
a = *b;  // assign the thing wich points to to a
b = &a;  // assign the adress of a (a pointer) to b
b = (int*)malloc(3*sizeof(int)); // create a array of int dynamically
d = e;   // Yes e is in fact an pointer to a pointer to int
b = *d;  // Right... (*d) is a pointer to int (just as b)
b = *e;  // Again, in fact b is now the first element of the array e
b = e[0]; // Exacly the same as b = *e;
...ETCETERA...

Let me give you an example of how it should be done:
First: a character string is a array of chars, so it's type is: char *
Second: a array of strings is {a array of (strings)} is a {array (of arrays of char)} is {a array of (char *)} is {(char *)* }

So a array of strings should be:
char **arrayOfStrings;
Usage:
char **arrayOfStrings;
// first lets allocate space for the array of strings
arrayOfStrings = (char **)malloc(NOLINES * sizeof(char *));
// initialize array
for (int i = 0; i < NOLINES; i++) arrayOfString[i] = NULL;

// second lets read in every line
char buffer[256];
while (fgets(buffer, 255, in))
{
  // strdup allocates space for the string AND copies the buffer
  // into it.
  arrayOfStrings[i] = strdup(buffer);
}

// Do what you have to with the arrayOfStrings

// cleanup
for (int i = 0; i < NOLINES; i++)
  if (arrayOfStrings[i] != NULL) free(arrayOfStrings[i]);
free(arrayOfStrings);



Now lets look at the most SEVERE errors in you code
First the declaration nightmare:

typedef char** str; /*pointer to a pointer to a char*/
typedef str tLines[MaxLines]; /*define an array of above*/
tLines *t;

Ok so str is a char ** (an array of char* = array of strings)
What's tLines ?? it's an array of str, wich is an array of char ** so it's an char ***. And t is  a pointer to such a thing. So its in fact a char **** (could be: an array of an array of an array of array of chars (What a file browser THIS is gonna be, a superdimensional one! ;) )
The bottomline is: don't use typedefs on these simple structures, it makes things more complicated, especially if you're a beginner.

Second: your allocation:

/*get memory for array of pointers to strings*/
t = (tLines*) malloc(MaxLines * sizeof(*s));

In fact you have to use malloc like this:
Type *t = (Type*) malloc(N *sizeof(Type));

But you use sizeof(*s), and (*s) is not of type tLines but of char !! In fact you allocate MaxLines bytes, but you should allocate MaxLines*4 bytes (MaxLines pointers). This is certainly gonna crash.
Also is the tLines not a type for an array of strings, but I explained that before.

// AAAAAAAAARGHHHH what's this:
*t[j] = (char**) malloc(strlen(s)+1);
strcpy(**t[j], s); // <- Crashhhhh :(

first you allocate space strlen(s)+1 bytes. You cast this to char** So in fact you using your space for pointers to char*
(youre making an array of char*). But the type you cast to an the sizecalculation in malloc are out of sinc. It should always be:
(Type*)malloc(no * sizeof(Type)). I guess you fiddled with your program until the compiler stopped complaining ;)

Well I hope I helped you a little, but I strongly advice you to buy Kernighan an Ritchie (believe me it's the best) before trying to code any further.

Good Luck, Luc
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