Seg Fault in Knowledge Tree

Question

I am implementing a knowledge tree in c that can read from a file. I am getting a seg fault in my newStr function. I'm not able to test the rest of my code with this problem. I don't have much experience with c. Any help would be greatly appreciated.

my .c file #include #include #include"animal.h" #include #include

/*returns a new node for the given value*/
struct Node * newNode (char *newValue) 
{
struct Node * tree;
tree = (struct Node*)malloc(sizeof(struct Node));
tree -> value = newStr(newValue);
return tree;
}


/* returns a new string with value passed as an argument*/
char * newStr (char * charBuffer)
{
int i;
int length = strlen(charBuffer);
char newStr;
if(charBuffer[0] == 'A' || charBuffer[0] == 'Q'){
    for(i=1; i<length; i++)
        newStr += charBuffer[i]; 
}
return (newStr + "\0");
}

/*Read from a File and create a tree*/
struct Node * readATree(FILE * f)
{
  char c;
  char buffer[100];
  struct Node * newTree;
  c = fgetc(f);
  if (c == 'A'){
     fgets(buffer, 100, f);
     newTree = newNode(buffer);
     newTree -> left = NULL;
     newTree -> right = NULL;
    }
  else{
     fgets(buffer, 100, f);
     newTree = newNode(newStr(buffer));
     newTree->left = readATree(f);
     newTree->right = (struct Node *) readAtree(f);
     }
  return newTree;

}

/*Write Tree to a File*/
void writeAFile(struct Node* tree, FILE * f)
{
    char buffer[100];
    strcpy(buffer, tree->value);
    if(tree != 0){
        if(tree->left == NULL && tree->right == NULL){
            fputc((char)"A", f);
            fputs(buffer,f);
        } else{
            fputc((char)"Q",f);
            fputs(buffer,f);
            writeAFile(tree->left, f);
            writeAFile(tree->right,f);
        }
    }
}

/*The play should start from here*/
int main (){
    struct Node* node;
    struct Node* root;
    char ans[100];
    char q[100];
    FILE * f;
    f = fopen("animal.txt", "r+");
    if(f != NULL)
        readATree(f);
    else{
        node = newNode("Does it meow?");
    node->right = NULL;
    node->right->right=NULL;
    node->left->left=NULL;
    node->left=newNode("Cat");
    root = node;
}
while(node->left != NULL && node->right != NULL){
    printf(node->value);
    scanf(ans);
    if(ans[0] == (char)"Y" || ans[0] == (char)"y")
        node = node->left;
    else if(ans[0] == (char)"N" || ans[0] == (char)"n")
        node = node->right;
    else
        printf("That is not a valid input.\n");
}
if(ans[0] == (char)"Y" || ans[0] == (char)"y")
    printf("I win!");
else if(ans[0] == (char)"N" || ans[0] == (char)"n"){
    printf("What is your animal");
    scanf(ans);
    printf("Please enter a yes or no question that is true about %s?\n", ans);
    scanf(q);
    node->right = newNode(q);
    node->right->left = newNode(ans);
    node->right->right = NULL;
}
writeAFile(root,f);
fclose(f);
return 0;
}

.h file #include

struct Node {
char *value;
struct Node * left;
struct Node * right;
};

struct Node * newNode (char *newValue) ;
char * newStr (char * charBuffer);
struct Node * readATree(FILE * f);
void writeAFile(struct Node* tree, FILE * f);

Answer 1

"+" operator as string concatenation does not work in c.

If you actually want to copy the a string use strdup(). This function allocates memory and copies the string into it.

Don't forget to free the allocated memory when done using it.

Answer 2

char * newStr (char * charBuffer)
{
  int i;
  int length = strlen(charBuffer);
  char newStr;
  if(charBuffer[0] == 'A' || charBuffer[0] == 'Q'){
    for(i=1; i<length; i++)
        newStr += charBuffer[i]; 
  }
  return (newStr + "\0");
}

Well, there's a few interesting things here... To get down to brass tacks, you're trying to copy the contents of a character pointer into another and this function isn't going to do that. All you're really doing is summing the value of each char in charBuffer into newStr because a char is really just an 8-bit integer and then you return that integer as a pointer through an implicit cast so it is now being treated as a memory address.

You should look to use strdup(), as has been noted, since this is exactly what the function is supposed to do. No need to reinvent the wheel. :)

Answer 3

There might be several more, but here's some points on what's wrong:

1. Your newStr function is just very, very wrong. At a guess you'd want something like:

   char * newStr (char * charBuffer)
   {
     char *newStr;
     if(charBuffer[0] == 'A' || charBuffer[0] == 'Q') {
       newStr = strdup(&charBuffer[1]);
     } else {
       newStr = strdup("");
     }
     if(newStr == NULL) {
         //handle error
     }
     return newStr;
   }
   

2. You can't cast a string to a char like you do here:

    if(ans[0] == (char)"Y" || ans[0] == (char)"y")
   

   Do instead(same for similar code elsewhere too)

    if(ans[0] =='Y' || ans[0] == 'y')
   

3. Same as above when you call putc, don't do

    fputc((char)"A", f);
   

   Do

    fputc('A', f);
   

4. scanf needs a format string, don't do:

   scanf(ans);
   

   Do e.g. (or just use fgets again)

   if(scanf("%99s",ans) != 1) {
      //handle error
    }