Linked list.

list.c

#include <assert.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>

struct node {
  int elem;
  struct node* next;
};

void node_push(struct node** node, int elem) {
  assert(node);
  if(*node) {
    struct node* new_node = malloc(sizeof(struct node));
    new_node->elem = elem;
    new_node->next = *node;
    *node = new_node;
  } else {
    *node = malloc(sizeof(struct node));
    (*node)->elem = elem;
    (*node)->next = NULL;
  }
}

struct list {
  struct node* head;
};

struct list list_new(void) {
  struct list list;
  list.head = NULL;
  return list;
}

void list_drop(struct list* list) {
  assert(list);
  struct node* node = list->head;
  while(node) {
    struct node* next = node->next;
    free(node);
    node = next;
  }
  list->head = NULL;
}

void list_push(struct list* list, int elem) {
  assert(list);
  node_push(&list->head, elem);
}

// All functions receive pointers to keep the interface consistent.
bool list_empty(struct list* list) {
  assert(list);
  return !list->head;
}

// Unsafe: may segfault if list is empty.
int list_pop(struct list* list) {
  assert(list);
  int elem = list->head->elem;
  struct node* node = list->head;
  list->head = list->head->next;
  free(node);
  return elem;
}

// Unsafe: may segfault if list is empty.
int list_peek(struct list* list) {
  assert(list);
  return list->head->elem;
}

void list_append(struct list* list, int elem) {
  assert(list);
  if(list->head) {
    struct node* node = list->head;
    while(node->next) {
      node = node->next;
    }
    node->next = malloc(sizeof(node));
    node->next->elem = elem;
    node->next->next = NULL;
  } else {
    struct node* node = malloc(sizeof(node));
    node->elem = elem;
    node->next = NULL;
    list->head = node;
  }
}

void list_show(struct list* list) {
  assert(list);
  if(list->head) {
    struct node* node = list->head;
    while(node) {
      printf("%d ", node->elem);
      node = node->next;
    }
  }
  printf("\n");
}

int main(void) {
  {
    struct list list = list_new();

    assert(list_empty(&list) == true);

    list_append(&list, 3);
    list_append(&list, 2);
    list_append(&list, 1);
    list_show(&list);

    assert(list_empty(&list) == false);

    list_push(&list, 2);
    list_push(&list, 1);
    list_show(&list);

    assert(list_pop(&list) == 1);
    assert(list_pop(&list) == 2);
    list_show(&list);

    assert(list_peek(&list) == 3);

    list_drop(&list);
  }

  {
    struct list list = list_new();

    list_push(&list, 3);
    list_push(&list, 2);
    list_push(&list, 1);
    list_show(&list);

    list_drop(&list);
  }
  return 0;
}