How does one locate a pointer error? - c++

Question

I am attempting to create a program to create a Markov chain but I am having pointer problems. When I run the Program I get a segmentation fault.
#include <stdio.h>
#include <cstring>
#include <cstdlib>
struct word;
struct nextword
{
word* sourceword;
word* next = 0;
};
int wordcount;
struct word
{
char* wordstr;
struct word* next = 0;
nextword* followingword = 0;
int nextwordcount = 0;
};
int main()
{
word* firstword = 0;
char * buffer = 0;
long length;
FILE * f = fopen ("alice.txt", "rb");
if (f)
{
fseek (f, 0, SEEK_END);
length = ftell (f);
fseek (f, 0, SEEK_SET);
buffer = (char *)malloc (length);
if (buffer)
{
fread (buffer, 1, length, f);
}
fclose (f);
}
if (buffer)
{
char wordbuffer[500];
int fileindex = 0;
while(fileindex < length-1)
{
int wordindex = 0;
while(buffer[fileindex] != ' ')
{
wordbuffer[wordindex] = buffer[fileindex];
wordindex++;
fileindex++;
}
if(wordindex != 0)
{
wordbuffer[wordindex] = '\0';
word* newword = (word*)malloc(sizeof(word));
char* newwordstr = (char*)malloc((strlen(wordbuffer)+1)*sizeof(char));
strcpy(newword->wordstr, newwordstr);
if(!firstword)
{
firstword = newword;
}
else
{
word* testword = firstword;
while(!testword->next)
{
testword = (testword->next);
}
testword->next = newword;
printf(newword->wordstr);
}
}
return 0;
}
}
else
{
return 1;
}
}
I attempted to remove the file reading part and replace it with a hard coded string, but the problem remained.

Answer 1

You might want to read about STL and use a list. Or use a C list, see a couple of examples,
Adding node in front of linklist
How to pop element from tail in linked list?
Trying to make linkedlist in C
Several problems. Fixed some. compiles.
I have annotated the code with places where you need to fix bounds checking, and the big problem was likely the strcpy to the struct word->wordstr uninitialized char*,
#include <stdio.h>
#include <cstring>
#include <cstdlib>
struct word;
struct nextword
{
word* sourceword;
word* next = 0;
};
int wordcount;
struct word
{
char* wordstr; //what do you think this pointer points to?
struct word* next = 0;
nextword* followingword = 0;
int nextwordcount = 0;
};
int main()
{
FILE* fh = NULL;
word* firstword = 0;
char* buffer = 0;
char* fname = "alice.txt";
long length = 0; //you did not initialize length
if ( (fh = fopen ("alice.txt", "rb")) )
{
//why not use fstat to get file size?
//why not use mmap to read file?
fseek (fh, 0, SEEK_END);
length = ftell (fh); //ok, length set here
fseek (fh, 0, SEEK_SET);
if( (buffer = (char *)malloc (length)) )
{
fread (buffer, 1, length, fh);
}
fclose (fh);
}
else
{
printf("error: cannot open %s",fname);
exit(1);
}
printf("read %s, %ld\n",fname,length);
if (!buffer)
{
printf("error: cannot open %s",fname);
exit(1);
//use exit, to return from main() //return 1;
}
//already checked buffer
{
int fileindex = 0;
//put wordbuffer after fileindex, avoids stackoverflow overwrite
char wordbuffer[500]; //500 bytes on stack, initialize?
memset(wordbuffer,0,sizeof(wordbuffer));
while(fileindex < length-1)
{
int wordindex = 0;
//several errors in this line, check for null terminator,
//check for newline, tab, basically any whitespace
//while(buffer[fileindex] != ' ')
while( buffer[fileindex] && buffer[fileindex] != ' ' )
{
wordbuffer[wordindex] = buffer[fileindex];
wordindex++;
fileindex++;
//here is another error, do not overflow your stack based buffer
if( wordindex>sizeof(buffer)-1 ) break; //do not overflow buffer
}
wordbuffer[wordindex] = '\0'; //terminate wordbuffer
//since you chose wordindex signed, you want it > 0
if(wordindex > 0)
{
//use a constructor
word* newword = (word*)malloc(sizeof(word));
//use a constructor
//or just use strdup, since it is just a cstring
char* newwordstr = strdup(wordbuffer);
//no, just set pointer to the above allocated string
//strcpy(newword->wordstr, newwordstr);
newword->wordstr = newwordstr;
if(!firstword)
{
firstword = newword;
}
else
{
word* testword = firstword;
while(!testword->next)
{
testword = (testword->next);
}
testword->next = newword;
printf(newword->wordstr);
}
}
return 0;
}
}
exit(0); //done
}
This compiles and runs without error, you need to look up linked list handling. You should implement a linked list, and then add word elements to list.