diff --git a/examples/hello.c b/examples/hello.c
index 1cb8d44..6daaf9f 100644
--- a/examples/hello.c
+++ b/examples/hello.c
@@ -1,11 +1,6 @@
 #include "not_stdio.h"
 
-#define B(V, W) A(V)
-#define D B
-#define C D("hello world", 123)
-#define A puts
-
 int main()
 {
-    C;
+    puts("hello world");
 }
diff --git a/examples/macro_usage.c b/examples/macro_usage.c
new file mode 100644
index 0000000..e72852f
--- /dev/null
+++ b/examples/macro_usage.c
@@ -0,0 +1,9 @@
+#define B(V, W) A(V)
+#define D B
+#define C D("hello world", 123)
+#define A puts
+
+int main()
+{
+    C;
+}
diff --git a/examples/not_stdio.h b/examples/not_stdio.h
index 5423560..3107203 100644
--- a/examples/not_stdio.h
+++ b/examples/not_stdio.h
@@ -1,2 +1,6 @@
+#ifndef NOT_STDIO_H
+#define NOT_STDIO_H
 
 int puts(const char* __s);
+
+#endif
diff --git a/main.c b/main.c
index bd75a84..c57a951 100644
--- a/main.c
+++ b/main.c
@@ -30,6 +30,9 @@ int main(int argc, char* argv[])
     Tok tok;
     while ((tok = preproc_next(preproc)).ty != Tok_Eof) {
         // printf("%s\t%s\n", tokty_str(tok.ty), tok.text);
+        if (line > tok.line) {
+            line = 0;
+        }
         while (line < tok.line) {
             printf("\n");
             line += 1;
diff --git a/preproc.c b/preproc.c
index 3673e61..e86ac86 100644
--- a/preproc.c
+++ b/preproc.c
@@ -68,6 +68,28 @@ static size_t* syms_get(Syms* s, const char* ident)
     return nullptr;
 }
 
+typedef struct {
+    size_t conds;
+    size_t depth;
+} CondStack;
+
+void condstack_push(CondStack* s, bool cond)
+{
+    if (cond) {
+        s->conds |= 1ul << s->depth;
+    } else {
+        s->conds &= ~(1ul << s->depth);
+    }
+    s->depth += 1;
+}
+
+bool condstack_pop(CondStack* s)
+{
+    assert(s->depth > 0);
+    s->depth -= 1;
+    return (s->conds >> s->depth) & 1;
+}
+
 typedef struct {
     char* filename;
     char* text;
@@ -94,6 +116,8 @@ struct Preproc {
     ARRAY_FIELD(Def, defs);
     Syms syms;
 
+    CondStack condstack;
+
     bool next_can_be_directive;
 };
 
@@ -102,14 +126,16 @@ void preproc_init(Preproc** p, char* filename, char* text)
     *p = malloc(sizeof(Preproc));
 
     **p = (Preproc) {
+        .tok_arena = { },
         ARRAY_FIELD_INIT(files),
+
         ARRAY_FIELD_INIT(tok_stack),
         .curr = { },
 
         ARRAY_FIELD_INIT(defs),
         .syms = { },
 
-        .tok_arena = { },
+        .condstack = { },
 
         .next_can_be_directive = true,
     };
@@ -409,6 +435,8 @@ static Tok do_undef(Preproc* p)
     return p->curr;
 }
 
+Tok preproc_next_with_newline(Preproc* p);
+
 static Tok do_include(Preproc* p)
 {
     File* curr_file = &p->files[p->files_size - 1];
@@ -470,60 +498,136 @@ static Tok do_include(Preproc* p)
     lexer_init(&file->lexer, &file->lsl_iter, &p->tok_arena);
     p->curr = lexer_next(&file->lexer);
 
+    p->next_can_be_directive = true;
+
+    return preproc_next_with_newline(p);
+}
+
+static Tok interpret_directive(Preproc* p);
+
+static Tok do_ifndef(Preproc* p)
+{
+    int original_line = p->curr.line;
+
+    step(p);
+    if (p->curr.ty != Tok_Ident) {
+        fprintf(
+            stderr, "error: expected identifier on line %d\n", p->curr.line);
+        exit(1);
+    }
+    Tok ident = p->curr;
+    step(p);
+
+    if (p->curr.ty != Tok_Eof && p->curr.ty != Tok_Eol) {
+        fprintf(stderr,
+            "error: unexpected extra tokens on line %d\n",
+            p->curr.line);
+        exit(1);
+    }
+
+    bool cond = syms_get(&p->syms, ident.text) == nullptr;
+
+    condstack_push(&p->condstack, cond);
+
+    if (!cond) {
+
+        while (p->curr.ty != Tok_Eof) {
+            while (p->curr.ty != Tok_Eof
+                && (!p->next_can_be_directive || p->curr.ty != '#'))
+                step(p);
+            if (p->curr.ty == Tok_Eof)
+                break;
+            step(p);
+            if (p->curr.ty != Tok_Ident)
+                continue;
+            if (strcmp(p->curr.text, "endif") == 0)
+                return interpret_directive(p);
+        }
+
+        if (p->curr.ty != Tok_Eof) {
+            fprintf(stderr,
+                "error: unclosed '#ifndef' on line %d\n",
+                original_line);
+            exit(1);
+        }
+    }
+
+    return p->curr;
+}
+
+static Tok do_endif(Preproc* p)
+{
+    step(p);
+
+    if (p->curr.ty != Tok_Eof && p->curr.ty != Tok_Eol) {
+        fprintf(stderr,
+            "error: unexpected extra tokens on line %d\n",
+            p->curr.line);
+        exit(1);
+    }
+
+    condstack_pop(&p->condstack);
+
     return p->curr;
 }
 
 static Tok interpret_directive(Preproc* p)
 {
-    assert(p->curr.ty == '#');
-    step(p);
-    Tok kw = p->curr;
+    if (p->curr.ty == Tok_Eol)
+        return p->curr;
 
-    if (strcmp(kw.text, "define") == 0) {
-        return do_define(p);
-    } else if (strcmp(kw.text, "undef") == 0) {
-        return do_undef(p);
-    } else if (strcmp(kw.text, "include") == 0) {
-        return do_include(p);
-    } else if (strcmp(kw.text, "if") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "ifdef") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "ifndef") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "else") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "elif") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "elifdef") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "elifndef") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "endif") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "line") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "embed") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "error") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "warning") == 0) {
-        assert(false && "not implemented");
-    } else if (strcmp(kw.text, "pragma") == 0) {
-        fprintf(stderr,
-            "error: '%s' is not a preprocessor instruction on line %d\n",
-            kw.text,
-            kw.line);
+    if (p->curr.ty != Tok_Ident) {
+        fprintf(
+            stderr, "error: expected identifier on line %d\n", p->curr.line);
         exit(1);
     }
 
-    assert(false && "unreachable");
+    Tok kw = p->curr;
+
+    struct {
+        const char* keyword;
+        Tok (*fn)(Preproc* p);
+    } directives[] = {
+        { "define", do_define },
+        { "undef", do_undef },
+        { "include", do_include },
+        { "ifndef", do_ifndef },
+        { "endif", do_endif },
+
+        { "if", nullptr },
+        { "ifdef", nullptr },
+        { "else", nullptr },
+        { "elif", nullptr },
+        { "elifdef", nullptr },
+        { "elifndef", nullptr },
+        { "line", nullptr },
+        { "embed", nullptr },
+        { "error", nullptr },
+        { "warning", nullptr },
+        { "pragma", nullptr },
+    };
+
+    for (size_t i = 0; i < sizeof(directives) / sizeof(directives[0]); ++i) {
+        if (strcmp(kw.text, directives[i].keyword) == 0) {
+            assert(directives[i].fn && "not implemented");
+
+            return directives[i].fn(p);
+        }
+    }
+
+    fprintf(stderr,
+        "error: '%s' is not a preprocessor instruction on line %d\n",
+        kw.text,
+        kw.line);
+    exit(1);
 }
 
 Tok preproc_next_with_newline(Preproc* p)
 {
-    if (p->next_can_be_directive && p->curr.ty == '#')
+    if (p->next_can_be_directive && p->curr.ty == '#') {
+        step(p);
         return interpret_directive(p);
+    }
     return expand_tok(p);
 }