Code

 Header...
#define NDIM 2
typedef struct particle {
  float x[NDIM];
  float mass;
  struct particle *next;
}Particle;

// Allocate a new particle and add it to the start of the list
 struct particle* newparticle(Particle *first){
   ... 

»  Particle *p;

»  p=xmalloc(sizeof *p);
»  p->next=first;

  // Now initialise exactly as before
»  printf("Please enter the mass and x and y values.\n");
»  scanf("%f %f %f", &p->mass, &p->x[0], &p->x[1]);

»  return p;
}

// !! This function demonstrates a memory leak
/*
 * Free Particle "togo" and return the new first Particle
 * "first" must be the existing first member of the list
 */
 struct particle* freeparticle(Particle *togo,Particle *first){
   ... 

»  Particle *parent;
  
»  if (togo==NULL) // Do nothing
    return first;
  else if (first==NULL) {
»    fprintf(stderr, "Trying to remove something from an empty list!\n");
»    return NULL;
  }

»  if (togo==first) // Is it the first one?
    first=first->next;
  else {// Find togo's predecessor:
»    for (parent=first; parent->next!=togo; parent=parent->next) {
»      if (parent->next==NULL) {
»        fprintf(stderr, "%p isn't in the list!\n", togo);
»        return first;
  }
}

»    parent->next=togo->next;
  }

  // free(togo);
»  return first;
}

 int main(){
   ... 

»  int i, n;
»  Particle *first=NULL;

»  printf("How many structures would you like?\n");
»  scanf("%d", &n);

»  for (i=0; i<n; ++i) 
    first=newparticle(first);

  // Demonstrate some cases of freeing a particle
»  Particle *killme=first->next->next;
»  first=freeparticle(killme, first);
»  killme=first;
»  first=freeparticle(killme, first);
»  killme=NULL;
»  first=freeparticle(killme, first);

»  for ( Particle *p=first;p!=NULL; p=p->next) 
    printf("Mass: %f position: (%g, %g)\n", p->mass, p->x[0], p->x[1]);

»  return 0;
}