Segmentation Fault with Malloc

Question

I needed to implement and specific ADT, strqueue, for my CS class today and so I wrote up two functions: create_StrQueue(), and add_to_back(StrQueue sq, const char* str). Unfortunately, when I call create_StrQueue in add_to_back I get a seg fault, and I am unable to figure out why exactly. Here is the code that I wrote for these two functions:

[edit] I should probably malloc tempWord in add_to_back.

#include <stdlib.h>

// A strqueue is an ADT consisting of words
struct strqueue{
  StrQueue back;    // last StQueue in queue
  StrQueue next;    // next StrQueue in queue

  char* word;       // stored string
  int length;       // length of entire queue
};

typedef struct strqueue* StrQueue;

StrQueue create_StrQueue(void){

  StrQueue retq = malloc(sizeof(struct strqueue));  // get memory for a new strqueue
  retq->word = malloc(sizeof(char*)); 
  retq->word = NULL;
  retq->back = retq;       // set back pointer to itself
  retq->next = NULL;       // nothing after this strqueue yet

  return retq;
}

void add_to_back(StrQueue sq, const char* str){

  char* tempWord;
  sq->length++;

  for(int i=0; str[i]; ++i) tempWord[i]=str[i];  // copy string for the new strqueue

  if(sq->word==NULL) sq->word = tempWord;  // input strqueue was empty

  // input StrQueue was not empty, so add a new StrQueue to the back
  StrQueue new = create_StrQueue(); // results in seg fault
  new->word = tempWord;
  sq-back->next = new;  // swaping pointers around to add malloced StrQueue to the back
  sq->back = next;
}

I'm at a loss, and so I'm hoping someone can clarify what exactly is going on, because when I run main like so;

int main(void){

char* str1 = "Hello";

StrQueue sq = create_StrQueue(); // does not cause seg fault
add_to_back(sq, str1);
}

calling create_StrQueue() for the first time works just fine.

Answer 1

char* in the struct is a pointer to a character array. retq->word = malloc(sizeof(char*)); is not the right way to allocate a string; what this actually does is it assigns a tiny array to word , essentially useless, and then you overwrite what you just allocated by assigning NULL to word , leaking the memory. All memory allocated by malloc must be manually released later on with free . You are dealing with a pointer. Assigning data to it has no magic involved in C, you are simply replacing the value of the pointer itself.

In add_to_back , you need to allocate space to tempWord before copying data into it:

tempWord = malloc( strlen(str)+1 );

You add 1 to accommodate the null terminator in the string. Use strcpy to copy into tempWord instead of writing your own string copying method there, your method does not add a null terminator.

An even better solution would be to have create_StrQueue accept a const char* parameter, and do the string allocation and copying in there.

You should also avoid using the word new since that looks a bit confusing to a c++ programmer. :)