Coredump when parsing chapters - c

Question

I am doing a homework assignment that reads in a book. First, a line is read in and a pointer made to point at that line. Then a paragraph function reads in lines and stores their address into a array of pointers. Now, I am on reading a chapter (a paragraph recognized by the next line being broke). It should call get_paragraph() and store the address of paragraphs until it comes to a new chapter.
A new chapter is the only time in the book where the first character in the line is not a space. I think this is were I am having problems in my code. All functions up to this point work. I hope I have provided enough information. The code compiles but core dumps when started.
I am a student and learning so please be kind. Thanks.
char*** get_chapter(FILE * infile){
int i=0;
char **chapter[10000];//an array of pointers
// Populate the array
while(chapter[i]=get_paragraph(infile)) { //get address store into array
if(!isspace(**chapter[0])){ //check to see if it is a new chapter<---problem line?
// save paragraph not used in chapter using static to put into next chapter
break;
}
i++;//increment array
}
//add the null
chapter[++i]='\0';//put a null at the end to signify end of array
//Malloc the pointer
char**(*chap) = malloc(i * sizeof(*chap));//malloc space
//Copy the array to the pointer
i=0;//reset address
while(chapter[i]){//while there are addresses in chapter
chap[i] = chapter[i++];//change addresses into chap
}
chap[i]='\0';//null to signify end of chapter
//Return the pointer
return(chap);//return pointer to array
}
For those who would rather see without comments:
char*** get_chapter(FILE * infile){
int i=0;
char **chapter[10000];
while(chapter[i]=get_paragraph(infile)) {
if(!isspace(**chapter[0])){
break;
}
i++;
}
chapter[++i]='\0';
char**(*chap) = malloc(i * sizeof(*chap));//malloc space
i=0;
while(chapter[i]){
chap[i] = chapter[i++];
}
chap[i]='\0';
return(chap);
}

Answer 1

Comments inline.
char*** get_chapter(FILE * infile) {
int i=0;
// This is a zero length array!
// (The comma operator returns its right-hand value.)
// Trying to modify any element here can cause havoc.
char **chapter[10,000];
while(chapter[i]=get_paragraph(infile)) {
// Do I read this right? I translate it as "if the first character of
// the first line of the first paragraph is not whitespace, we're done."
// Not the paragraph just read in -- the first paragraph. So this will exit
// immediately or else loop forever and walk off the end of the array
// of paragraphs. I think you mean **chapter[i] here.
if(!isspace(**chapter[0])){
break;
}
i++;
}
// Using pre-increment here means you leave one item in the array uninitialized
// which can also cause a fault later on. Use post-increment instead.
// Also '\0' here is the wrong sort of zero; I think you need NULL instead.
chapter[++i]='\0';
char**(*chap) = malloc(i * sizeof(*chap));
i=0;
while(chapter[i]) {
// This statement looks ambiguous to me. Referencing a variable twice and
// incrementing it in the same statement? You may end up with an off-by-one error.
chap[i] = chapter[i++];
}
// Wrong flavor of zero again.
chap[i]='\0';
return(chap);
}

Answer 2

Can I suggest that you use for loops instead of whiles? You need to stop if you run out of space, so you might as well use the appropriate construct.
I suspect you have a bug in this code:
while(chapter[i]=get_paragraph(infile)) {
if(!isspace(**chapter[0])){
break;
}
i++;
}
chapter[++i]='\0';
Firstly, shouldn't it be chapter[i] instead of chapter[0]? You want to know if the pointer at chapter[i] points to a space, not the first pointer in chapter. So this will probably loop indefinitely - hence the need for a for loop, so you don't just loop forever accidentally.
Secondly, you increment i at the end of the while block, and then again in the chapter[++i] assignment. i has already been incremented by the final loop execution before the while condition breaks, so it is already the correct position to use. ++i increments before yielding the value, so presumably you meant to have i++ here, so that it would increment after yielding the current value of i. Either way, it's confusing one of us as to what you mean, so maybe just put the increment on a separate line for clarity. The compiler will sort out any available optimisation.
Finally (and I might well be wrong here) why are you setting the value to '\0'? That's a null character, isn't it? But your array is of pointers. The null pointer would be 0, rather than '\0', I think. If I'm right, you might have still got away with it if '\0' yields the same set of zeroes as the null pointer.

Answer 3

Have you tried single stepping though it in gdb, and occasionally dumping the local variables to see the current state? It's a good way to learn. You may want to add a few extra intermediate variables that "info locals" will automatically dump as well (pointers to the current XXX, where XXX is various items in your hierarchy)
I assume a GNU environment:
% gcc -g homework.c -o hw
% gdb hw
(gdb) b 10
(gdb) r
(gdb) info locals
(gdb) n
(gdb) info locals
...
Replace "10" with a suitable line number near the beginning of the function.

Answer 4

Shouldn't it be:
if(!isspace(**chapter[i])){
Each chapter[i] is a pointer to a pointer to a char, this char is the first character in each chapter. So **chapter[i] represents the first character in chapter i. Using chapter[0] will only look at the first chapter.