I can't figure out how to use double pointers - c

Question

I'm trying to use dynamic memory allocation but I can't figure out pointers.
I got the first part down.
void addtext(char **wordarray)
{
char word[N];
char endword[N] = "end";
int i=0;
int words=0;
while (scanf("%19s", word), strcmp(word,endword))
{
words++;
wordarray = realloc(wordarray, words*sizeof(char *));
wordarray[words-1] = malloc (N*sizeof(char));
strcpy(wordarray[words-1], word);
}
for (i=0; i<words; i++)
printf("%s\n", wordarray[i]);
return ;
}
But I'm having trouble when I try to call the same array in a different function.
void savetext(char **wordarray)
{
FILE *savedtext;
int i=0;
savedtext = fopen("Saved Text.txt","wt");
while(wordarray[i][0]!= '\0')
{
fputs(wordarray[i++],savedtext);
fputs(" ",savedtext);
}
return ;
}
My main function looks something like this:
int main (void)
{
char **wordarray;
addtext(wordarray);
savetext(wordarray);
return 0;
}
The second part of the code is obviously wrong, but I'm not sure how to exactly how to call those functions. My previous program didn't use any memory allocation so I didn't bother with pointers.I'm really new to c so any help would be appreciated.

Answer 1

Oh boy. Well, you have two big problems.
First, you never allocated the first wordarray. At the very least malloc it once:
char **wordarray = malloc(1);
Or even better, use malloc instead of realloc the first time (and initialize wordarray with 0!):
wordarray = wordarray ? realloc(wordarray, words * sizeof(char *))
: malloc(words * sizeof(char *));
Second, your addtext function is receiving a copy of this array, and doing stuff with it. Whatever the stuff is, it won't be saved in your wordarray outside, in main. What you need to do is pass a pointer to the array in your function, and edit the main object through that:
void addtext(char ***wordarray)
{
// ...
}
And lastly, you have some very big performance problems, allocating buffers so often. Use a proper growing vector implementation, or if you insist on writing your own at the very least grow it by doubling the size, or even better count the words and allocate the correct size.
Also your end string is arbitrarily allocated of length N, whatever that is. You don't need that, you already know the length. In fact the string is already in the read-only section of your binary, simply get a pointer to it:
const char *endword = "end";

Answer 2

Perhaps refactor your program to make the string creation its own function, and for symmetry, return storage of the string as its own function.
const int STRING_SIZE = 80;
void createString(char ** strPtr, int stringSize);
void freeString(char * strPtr);
int main(int argc, char ** argv) {
char * strValue = NULL;
createString(&strValue, STRING_SIZE);
// ... do stuff ...
freeString(strValue);
}
//
// end of main
//
void createString(char ** strPtr, int stringSize) {
//
// uses pass-by-reference to return *strPtr with allocated storage
//
*strPtr = (char *) calloc(stringSize, sizeof(char));
}
void freeString(char * strPtr) {
if(strPtr == NULL) return;
free(strPtr);
strPtr = NULL;
}

Answer 3

For starters the program has undefined behavior at least because the pointer wordarray was not initialized and has an indeterminate value
char **wordarray;
and this indeterminate value is used in a call of the function realloc in the function addtext
wordarray = realloc(wordarray, words*sizeof(char *));
Moreover the pointer is passed to the function addtext by value. That is the function deals with a copy of the value of the pointer. So changing the copy does not influence on the value stored in the original pointer. The original pointer in main will stay unchanged.
You need to pass the pointer by reference through a pointer to it.
Another problem of the function is that the number of stored strings will not be known outside the function addtext. You need at least append the array with a null pointer that will be used as a sentinel value.
Also this condition in the while loop within the function savetext
while(wordarray[i][0]!= '\0')
does not make a sense because within the function addtext you stop entering strings when the user will enter the string "end".
while (scanf("%19s", word), strcmp(word,endword))
^^^^^^^^^^^^^^^^^^^^
So it is not necessary that the preceding entered string is an empty string.
Here is a demonstrative program that shows how for example the function addtext can be declared and defined.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define N 20
size_t addtext( char ***wordarray )
{
char word[N];
const char *sentinel = "end";
size_t n = 0;
int success = 1;
while ( success && scanf( "%19s", word ) == 1 && strcmp( word, sentinel ) != 0 )
{
char **tmp = realloc( *wordarray, ( n + 1 ) * sizeof( char * ) );
success = tmp != NULL;
if ( success )
{
++n;
*wordarray = tmp;
( * wordarray )[n-1] = malloc( strlen( word ) + 1 );
if ( ( *wordarray )[n-1] ) strcpy( ( *wordarray )[n-1], word );
}
}
return n;
}
int main(void)
{
char **wordarray = NULL;
size_t n = addtext( &wordarray );
for ( size_t i = 0; i < n; i++ )
{
if ( wordarray[i] != NULL ) puts( wordarray[i] );
}
for ( size_t i = 0; i < n; i++ )
{
free( wordarray[i] );
}
free( wordarray );
return 0;
}
If to enter the following sequence of strings
one
two
three
end
then the program output will be
one
two
three
Correspondingly the declaration of the function savetext should be changed. There is not sense in this case to pass the pointer wordarray to the function by reference because the pointer itself is not changed within the function. Also you need to pass the number of elements in the allocated array, So the function declaration can look at least like
void savetext( char **wordarray, size_t n );