C - deleting node in doubly linked list -

when try delete every other element in list using deleteinst method method nothing linked list , there no errors. i'm not sure why it's not working i've seen same deleteinst method used in different program. maybe has pointers. if run deleteinst(track.prev, &head); without while loop list still remains unchanged.

please let me know if have idea or need more info. thank time.

int main() { node *head; node *track; head = readnodelist(stdin);  track = lastnode(head); //goes last node.  while(track != null){      int i=0;             //delete every other node     if(i%2 == 1){         deleteinst(track, &head);     } i++; track = track->prev;     } }  void deleteinst(instruction *victim, instruction **head){    if(*head == null || victim == null)     return;    if(*head == victim)     *head = victim->next;    if(victim->next != null)     victim->next->prev = victim->prev;    if(victim->prev != null)     victim->prev->next = victim->next;         free(victim);   return; } 

one obvious glaring problem: don't want this:

 deleteinst(track, &head);  track = track->prev; 

the instant free node, lose right access members. save first recover after delete:

 node *save_prev = track->prev;  deleteinst(track, &head);  track = save_prev; 

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another thing i'd check list structure correct, (only during debug):

static void checklist (node *curr) {     int count = 0;      // precon: head->prev must null.      if (curr != null) {         if (curr->prev != null) {             puts ("linked list structure error a!");             exit (1);         }     }      // check nodes.      while (curr != null) {         // precon: curr->prev->next must curr.          if (curr->prev != null) {             if (curr->prev->next != curr) {                 puts ("linked list structure error b!");                 exit (1);             }         }          // precon: curr->next->prev must curr.          if (curr->next != null) {             if (curr->next->prev != curr) {                 puts ("linked list structure error c!");                 exit (1);             }         }          // move next , keep count.         curr = curr->next;         count++;     }      // okay, output success message size.      printf ("linked list structure okay, size = %d\n", count); } 

calling checklist (head) validate linked list meets validity preconditions, on off-chance may have buggy code elsewhere, such when creating list in readnodelist().

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beyond that, suggest single stepping code in ide or debugger after readnodelist() see it's doing. and, if don't have ide/debugger, pepper source code lots of lines like:

printf ("debug %d: track = %p\n", __line__, track); 

and examine output of debug statements analyse flow through program.

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now, if debugging exercise, may surprise find out deleteinst never appears called, because i seems set 0.

and fault of little problem lies here:

while (track != null) {      int = 0; // <<<<<<<      //delete every other node      if (i%2 == 1) {         deleteinst (track, &head);     }     i++;     track = track->prev; } 

yes, that's right, setting i 0 every single time through loop, hence i % 2 never equal 1. need initialise i before loop (and undefined behaviour of accessing freed memory removed well):

int = 0; while (track != null) {      node *save_prev = track->prev;      //delete every other node      if (i%2 == 1)         deleteinst (track, &head);     i++;     track = save_prev; }