c generic heapsort | 易学教程

问题

Okay so I need to create a 'generic' heapsort in c and this is what I have so far (I might be missing some closing brackets in code but they just got lost when I moved my code here)

void srtheap(void *, size_t, size_t, int (*)(const void *, const void *));
void heapify(void *, size_t, size_t, size_t, int (*)(const void *, const void *)); 

void srtheap(void *base, size_t nelem, size_t size, int (*compar)(const void *, const void *)) {
  void *p1, *p2;
  void *last = base + (size*(nelem-1));
  for (size_t curpos = nelem-1; curpos>0; curpos-2){
    p1 = base + ((curpos-1)/2)*size;
    if(compar(last, (last-size)) >= 0){ 
      if(compar(last, p1) > 0){
        swap(last, p1, size);
        heapify(base, nelem, curpos, size, compar); 
      }
    }
    else { //LEFT>RIGHT
      if(compar(last-size, p1) > 0){
         swap(last-size, p1, size);
         heapify(base, nelem, curpos-1, size, compar);
      }
           //otherwise, parent is greater than LEFT & RIGHT,
           //or parent has swapped with child, iteration done, repeat through loop
    }      //end else, children have been compared to parent
           //end check for two children, only left child if this loop is skipped
    last = last-(2*size);
  }





/*
  **Now heapify and sort array
  */
  while(nelem > 0){
    last = base + (size*(nelem-1)); 
    swap(base, last, size);
    nelem=nelem-1;
    heapify(base, nelem, 0, size, compar); //pass in array, #elements, starting pos, compare
  }

}

void heapify(void *root, size_t numel, size_t pos, size_t sz, int (*compar)(const void *, const void *)){
  void *rc, *lc, *p1;
  while(pos < numel){
    rc = root+((pos+1)*2)*sz; //right child
    lc = root+(((pos+1)*2)-1)*sz; //left child
    p1 = root+(pos*sz); //parent
    if((pos+1)*2 < numel){ //check if current element has RIGHT
      if (compar(rc, lc)>=0){
    if(compar(rc, p1)>0) {
      swap(rc, p1, sz);
      pos=(pos+1)*2; //move to RIGHT, heapify
        }
    else {
      pos = numel; //PARENT>LEFT&RIGHT, array is heapified for now 
        }
      } //end RIGHT>LEFT
      else { //LEFT>RIGHT
    if(compar(lc, p1) >0 ) {
      swap(lc, rc, sz);
      pos=((pos+1)*2)-1; // move to LEFT, heapify
    }
        else {
      pos = numel; //PARENT>LEFT&RIGHT, array is heapified for now
        } //end inner if, else
      }//end LEFT,RIGHT comparison
    }//end check for RIGHT
    else if (((pos+1)*2)-1 < numel){ //else, check if element has LEFT
      if(compar(lc, p1)>0){
    swap(lc, p1, sz);
    pos=((pos+1)*2)-1; //move to LEFT, continue heapify
      }
      else {
    pos = numel; //PARENT>LEFT, array is heapified for now
      }
    }//end check for LEFT
    else { //current element has no children, array is heapified for now
      pos = numel;
    }
  }
}

In addition I have a main file that includes a compare function. Essentially, the base address of the array, number of elements, size of each element, and the compare function are passed into my heapsort functions.

When I run the program I am getting a segmentation fault which I assume means I am trying to access memory that is not allocated to me. So I guess I'm wondering does anybody see any problems where I am accessing illegal memory addresses or could point me to a debugger that I can use to figure it out?

Thanks!

回答1:

Debuggers are often useful in diagnosing sources of memory errors. Let us know what happens when you run your code from a debugger.

回答2:

for (size_t curpos = nelem-1; curpos>0; curpos-2){

curpos-2 doesn't have any effect. Did you mean -- or -=2?

Also, strictly speaking, you can't do pointer arithmetic on void * pointers and even if your compiler allows it, don't rely on it. Instead, cast it to a char * so that you are guaranteed that ptr + x will only add x bytes and not some multiple of x.

You might also find it useful to create a macro to index into the element array. That would make your code a lot more readable:

#define ELEM(base, i) ((char *)(base) + (i)*size)

回答3:

You can use gdb to help debug this. First, compile your program with -g to enable debugging symbols. Then run: gdb heapsort where heapsort is the name of the program. Then type run, hit enter, and wait until it crashes. Useful commands include bt for backtrace and p to print the values in variables.

来源：https://stackoverflow.com/questions/4259413/c-generic-heapsort

标签

heapsort