Odd behaviour of user-defined min and max functions

Question

I'm writing a tree-like, and as part of that, I need it to have two functions, both of which return a pointer: one to make leaf nodes, and another to make internal nodes. My header file goes as follows:

#ifndef NODE_H
#define NODE_H
#include<stdint.h>

typedef uint32_t Number;

typedef struct Node
{
Number val, height;
struct Node *left, *right, *parent;
} Node;

//makes leaves
Node* make_node (Number);

//makes internal nodes
Node* make_internal (Node*, Node*);

//you'll see why we need these
inline Number num_min(Number, Number);

inline Number num_max(Number, Number);

#endif

The implementation is as follows:

#include "node.h"
#include <stdlib.h>

inline Number num_min(Number a, Number b) {
  return (a < b) ? a : b;
}

inline Number num_max(Number a, Number b) {
  return (a > b) ? a : b;
}

//makes a new leaf node
Node* make_node (Number n) {
  Node* u = calloc(1, sizeof(Node));
  u->val = n;
  u->height = 0;
  return u;
}

//makes a new internal node
Node* make_internal (Node* u, Node* v) {
  Node* uv = calloc(1, sizeof(Node));
  uv->left = u;
  uv->right = v;
  uv->val = num_min(u->val, v->val);
  uv->height = num_max(u->height, v->height) +1;
  u->parent = uv;
  v->parent = uv;
}

Now, make_node works fine (as tested using assert statements), but make_internal doesn't. For example, executing this code causes a coredump:

Node* u = make_node(10);
Node* v = make_node(20);
Node* uv = make_internal(u, v);
assert(uv->val == 10); //this assert fails

When I check the value of uv->val (using printf) at runtime, it turns out to 20 instead of 10! I'm completely confused (and new to C), and would appreciate all possible help.

You should turn the warning level to eleven i.e. -Wall -Wextra -Wpedantic for GCC. — diapir
– diapir, Commented Jun 23, 2014 at 11:44
This is undefined behavior caused by a missing return statement. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Jun 23, 2014 at 11:44
And an inline function declaration without definition in a header file is pointless. — mafso
– mafso, Commented Jun 23, 2014 at 12:35

adjan · Accepted Answer · 2014-06-23 11:40:31Z

5

make_internal is not returning uv. It's result can be anything.

answered Jun 23, 2014 at 11:40

adjan

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Jack · Accepted Answer · 2014-06-23 11:42:14Z

3

Try to compile with -Wreturn-type to realize that make_internal doesn't return anything although it is declared to return a Node*:

prism:Dev jack$ gcc -Wreturn-type a.c
a.c: In function ‘make_internal’:
a.c:49: warning: control reaches end of non-void function

answered Jun 23, 2014 at 11:42

Jack

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