19 files changed, 752 insertions, 0 deletions
diff --git a/2025-linuxdays/anon.c b/2025-linuxdays/anon.c
new file mode 100644
index 0000000..392534b
--- /dev/null
+++ b/2025-linuxdays/anon.c
@@ -0,0 +1,17 @@
+#include <inttypes.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  struct point {
+    union {
+      struct {
+        uint32_t x, y;
+      };
+      uint64_t u64;
+    };
+  };
+
+  struct point a = {.x = 0x56, .y = 0x38};
+  printf("%.16" PRIx64 "\n", a.u64);
+  return 0;
+}
diff --git a/2025-linuxdays/bitfield.c b/2025-linuxdays/bitfield.c
new file mode 100644
index 0000000..a854158
--- /dev/null
+++ b/2025-linuxdays/bitfield.c
@@ -0,0 +1,17 @@
+#include <inttypes.h>
+#include <stdint.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  union pixel {
+    struct c {
+      unsigned r : 5;
+      unsigned g : 6;
+      unsigned b : 5;
+    } c;
+    uint16_t val;
+  };
+
+  union pixel p = {.c.r = 0x1b, .c.g = 0x4, .c.b = 0xa};
+  printf("%.4" PRIx16 " == %" PRIx16 "\n", p.val, 0x1b | 0x4 << 5 | 0xa << 11);
+}
diff --git a/2025-linuxdays/bits.c b/2025-linuxdays/bits.c
new file mode 100644
index 0000000..f16b232
--- /dev/null
+++ b/2025-linuxdays/bits.c
@@ -0,0 +1,20 @@
+#include <assert.h>
+#include <stdbit.h>
+
+int main(int argc, char *argv[]) {
+  assert(stdc_leading_zeros(0xffU) == 24);
+  assert(stdc_leading_ones(0xff000000U) == 8);
+  assert(stdc_trailing_zeros(0xff000000U) == 24);
+  assert(stdc_trailing_ones(0xffU) == 8);
+  assert(stdc_first_leading_zero(0xff000000U) == 9);
+  assert(stdc_first_leading_one(0xffU) == 25);
+  assert(stdc_first_trailing_zero(0xffU) == 9);
+  assert(stdc_first_trailing_one(0xff000000U) == 25);
+  assert(stdc_count_zeros(0xffU) == 24);
+  assert(stdc_count_ones(0xffU) == 8);
+  assert(stdc_has_single_bit(1U << 13));
+  assert(stdc_bit_width(0xf0U) == 8);
+  assert(stdc_bit_floor(0xf1U) == 0x80);
+  assert(stdc_bit_ceil(0xf1U) == 0x100);
+  return 0;
+}
diff --git a/2025-linuxdays/cleanup.c b/2025-linuxdays/cleanup.c
new file mode 100644
index 0000000..078114f
--- /dev/null
+++ b/2025-linuxdays/cleanup.c
@@ -0,0 +1,16 @@
+#include <assert.h>
+#include <stdbit.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+static void _free(char **ptr) {
+  printf("Freeing %p=%p\n", ptr, *ptr);
+  free(*ptr);
+}
+
+int main(int argc, char *argv[]) {
+  [[gnu::cleanup(_free)]] char *buf = malloc(18);
+  [[gnu::cleanup(_free)]] char *null = NULL;
+  printf("Pointers: %p=%p %p=%p\n", &buf, buf, &null, null);
+  return 0;
+}
diff --git a/2025-linuxdays/constructor.c b/2025-linuxdays/constructor.c
new file mode 100644
index 0000000..08e2b6c
--- /dev/null
+++ b/2025-linuxdays/constructor.c
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+[[gnu::constructor]]
+static void _our_constructor(void) {
+  printf("Constructor\n");
+}
+[[gnu::destructor]]
+static void _our_destructor(void) {
+  printf("Destructor\n");
+}
+
+int main(int argc, char *argv[]) {
+  printf("main\n");
+  return 0;
+}
diff --git a/2025-linuxdays/funcptr.c b/2025-linuxdays/funcptr.c
new file mode 100644
index 0000000..e4b3a83
--- /dev/null
+++ b/2025-linuxdays/funcptr.c
@@ -0,0 +1,19 @@
+#include <stdio.h>
+
+typedef void (*cb_t)(void *, int);
+typedef cb_t *scb_t;
+struct cb {
+  cb_t cb;
+  const char *str;
+};
+void cb(void *self, int v) {
+  struct cb *s = self;
+  printf("%s: %d\n", s->str, v);
+}
+#define callcb(CB, ...) (*(CB))(CB, __VA_ARGS__)
+
+int main(int argc, char *argv[]) {
+  struct cb c = {.cb = cb, .str = "Hello"};
+  scb_t sc = &c.cb;
+  callcb(sc, 42);
+}
diff --git a/2025-linuxdays/generic.c b/2025-linuxdays/generic.c
new file mode 100644
index 0000000..dcb9a78
--- /dev/null
+++ b/2025-linuxdays/generic.c
@@ -0,0 +1,13 @@
+#include <inttypes.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#define traceval(X)                                                            \
+  fprintf(stderr, _Generic((X), char *: "%s\n", int: "%d\n", void *: "%p\n"), X)
+
+int main(int argc, char *argv[]) {
+  traceval("Hello");
+  traceval(42);
+  traceval(NULL);
+  return 0;
+}
diff --git a/2025-linuxdays/nonnull.c b/2025-linuxdays/nonnull.c
new file mode 100644
index 0000000..7e2c644
--- /dev/null
+++ b/2025-linuxdays/nonnull.c
@@ -0,0 +1,12 @@
+#include <stdio.h>
+
+[[gnu::nonnull]]
+void prnt(char *str) {
+  printf("%s\n", str);
+}
+
+int main(int argc, char *argv[]) {
+  prnt("Some text");
+  prnt(NULL);
+  return 0;
+}
diff --git a/2025-linuxdays/overflow.c b/2025-linuxdays/overflow.c
new file mode 100644
index 0000000..a21bf10
--- /dev/null
+++ b/2025-linuxdays/overflow.c
@@ -0,0 +1,11 @@
+#include <assert.h>
+#include <stdckdint.h>
+#include <stdint.h>
+
+int main(int argc, char *argv[]) {
+  int8_t add, sub, mul;
+  assert(ckd_add(&add, 50, 85));
+  assert(ckd_sub(&sub, 10, 150));
+  assert(ckd_mul(&mul, 68, 2));
+  return 0;
+}
diff --git a/2025-linuxdays/pres.pdf b/2025-linuxdays/pres.pdf
new file mode 100644
index 0000000..c1f98c8
--- /dev/null
+++ b/2025-linuxdays/pres.pdf
diff --git a/2025-linuxdays/pres.typ b/2025-linuxdays/pres.typ
new file mode 100644
index 0000000..f0c8730
--- /dev/null
+++ b/2025-linuxdays/pres.typ
@@ -0,0 +1,494 @@
+#import "@preview/polylux:0.4.0": *
+#import "@preview/metropolis-polylux:0.1.0" as metropolis
+
+#show: metropolis.setup
+#show: metropolis.setup.with(footer: [Pokročilé praktiky v C])
+
+#slide[
+  #set page(header: none, footer: none, margin: 3em)
+
+  #text(size: 1.3em)[*Pokročilé praktiky v C*]
+
+  LinuxDays 2025
+
+  #metropolis.divider
+
+  #set text(size: .8em, weight: "light")
+  Karel Kočí
+
+  4.10.2025
+
+  https://git.cynerd.cz/presentations/tree/2025-linuxdays
+]
+
+#slide[
+= Svět C
+
+Standardy: C89, C95, C99, *C11*, C17, *C23*
+
+Kompilátory: *GCC*, *Clang*, MSVC, a další
+
+Standardní knihovny: *GLibC*, *Musl*, BSD, Nuttx, CRT
+
+\+ POSIX
+
+```c
+gcc -std=c23 ...
+clang -std=c23 ...
+```
+]
+
+#metropolis.new-section[Před C11\
+  Tohle už asi používáte]
+
+#slide[
+= Bitfield
+```c
+  union pixel {
+    struct c {
+      unsigned r : 5;
+      unsigned g : 6;
+      unsigned b : 5;
+    } c;
+    uint16_t val;
+  };
+  union pixel p = {.c.r = 0x1b, .c.g = 0x4, .c.b = 0xa};
+  printf("%.4" PRIx16 " == %" PRIx16 "\n",
+   p.val, 0x1b | 0x4 << 5 | 0xa << 11);
+```
+
+```
+$ gcc -std=c23 bitfield.c && ./a.out
+509b == 509b
+```
+]
+
+#slide[
+= Struktura s veřejnou částí / Rozhraní
+```c
+struct pub { void (*print_name)(struct pub *); };
+typedef struct pub *pub_t;
+#define pub_print_name(PUB) (PUB)->print_name(PUB)
+
+struct priv { struct pub pub; const char *name; };
+void pname_priv(struct pub *pub) {
+  struct priv *priv = (struct priv *)pub;
+  printf("%s", priv->name);
+}
+int main(int argc, char *argv[]) {
+  struct priv priv = {.pub = {pname_priv}, .name = "FOO"};
+  pub_t p = &priv.pub;
+  pub_print_name(p);
+  putchar('\n');
+```
+]
+
+#slide[
+= Struktura s veřejnou částí / Rozhraní
+```c
+struct wrap { struct pub *subpub; struct pub pub; };
+void pname_wrap(struct pub *pub) {
+  struct wrap *wrap = (void *)pub - offsetof(struct wrap, pub);
+  printf("Wrap: ");
+  wrap->subpub->print_name(wrap->subpub);
+}
+int main(int argc, char *argv[]) {
+  ...
+  struct wrap wrap = {.pub = {pname_wrap}, .subpub = &priv.pub};
+  wrap.pub.print_name(&wrap.pub);
+  putchar('\n');
+}
+```
+]
+
+#slide[
+= Double-pointer na funkci / Generická funkce
+```c
+typedef void (*cb_t)(void *, int);
+typedef cb_t *scb_t;
+struct cb { cb_t cb; const char *str; };
+void cb(void *self, int v) {
+  struct cb *s = self;
+  printf("%s: %d\n", s->str, v);
+}
+#define callcb(CB, ...) (*(CB))(CB, __VA_ARGS__)
+  struct cb c = {.cb = cb, .str = "Hello"};
+  scb_t sc = &c.cb;
+  callcb(sc, 42);
+```
+
+```
+$ gcc -std=c23 funcptr.c && ./a.out
+Hello: 42
+```
+]
+
+#metropolis.new-section["Novinky" v C]
+
+#slide[
+= Anonymní struct a union (*C11*)
+```c
+  struct point {
+    union {
+      struct {
+        uint32_t x, y;
+      };
+      uint64_t u64;
+    };
+  };
+
+  struct point a = {.x = 0x56, .y = 0x38};
+  printf("%.16" PRIx64 "\n", a.u64);
+```
+]
+
+#slide[
+= Static assert (*C11*)
+
+- *C11*: `_Static_assert`
+- *C23*: `static_assert`
+
+```c
+  static_assert(sizeof(intmax_t) >= 16,
+    "Platform must support 64bit integers")
+```
+
+```
+$ gcc -std=c23 static_assert.c
+static_assert.c:4:1: error: static assertion failed: "Platform must support 128bit integers"
+    4 | static_assert(sizeof(intmax_t) >= 16, "Platform must support 128bit integers");
+      | ^~~~~~~~~~~~~
+```
+]
+
+#slide[
+= Atomic (*C11*)
+
+```c
+  #include <stdatomic.h>
+  _Atomic _Bool a;                          atomic_bool a;
+  _Atomic int val;                          atomic_int val;
+  _Atomic struct foo *foo;
+  _Bool aa = atomic_load(&a);               aa = a;
+  atomic_store(&a, false);                  a = false;
+
+  #if (ATOMIC_BOOL_LOCK_FREE, 2)
+  // atomic_bool je vždy atomický bez zámku
+  #elif (ATOMIC_BOOL_LOCK_FREE, 1)
+  // atomic_bool někdy musí použít zámek
+  #else
+  // atomic_bool vždy použije zámek
+  #endif
+```
+]
+
+#slide[
+= Atomické operace (*C11*)
+
+```c
+  _Bool was = atomic_exchange(&a, false);
+  if (!atomic_compare_exchange_strong(&a, &was, true))
+    printf("Somebody changed it in the meantime\n");
+
+  int iwas = atomic_fetch_add(&val, 1);
+  iwas = atomic_fetch_sub(&val, 1);
+  iwas = atomic_fetch_or(&val, 0xf);
+  iwas = atomic_fetch_xor(&val, 0x10);
+  iwas = atomic_fetch_and(&val, 0xff);
+```
+]
+
+#slide[
+= Atomický příznak (*C11*)
+
+```c
+  atomic_flag event = ATOMIC_FLAG_INIT;
+  _Bool was = atomic_flag_test_and_set(&event);
+  atomic_flag_clear(&event);
+```
+]
+
+#slide[
+= Vlákna a další (*C11*)
+
+- `<threads.h>`
+- `thrd_t`, `mtx_t`, `cnd_t`
+- `thread_local` (`_Thread_local`) a `tss_t`
+]
+
+#slide[
+= Detekce přetečení a podtečení (*C23*)
+
+```c
+  int8_t add, sub, mul;
+  assert(ckd_add(&add, 50, 85));
+  assert(ckd_sub(&sub, 10, 150));
+  assert(ckd_mul(&mul, 68, 2));
+```
+
+Pozor: `true` = nevalidní výsledek
+]
+
+#slide[
+= Bitové operace (*C23*)
+
+```c
+  #include <stdbit.h>
+  assert(stdc_leading_zeros(0xffU) == 24);
+  assert(stdc_leading_ones(0xff000000U) == 8);
+  assert(stdc_trailing_zeros(0xff000000U) == 24);
+  assert(stdc_trailing_ones(0xffU) == 8);
+  assert(stdc_first_leading_zero(0xff000000U) == 9);
+  assert(stdc_first_leading_one(0xffU) == 25);
+  assert(stdc_first_trailing_zero(0xffU) == 9);
+  assert(stdc_first_trailing_one(0xff000000U) == 25);
+  assert(stdc_count_zeros(0xffU) == 24);
+  assert(stdc_count_ones(0xffU) == 8);
+  assert(stdc_bit_width(0xf0U) == 8);
+  assert(stdc_bit_floor(0xf1U) == 0x80);
+  assert(stdc_bit_ceil(0xf1U) == 0x100);
+```
+]
+
+#slide[
+= \_Generic (*C11*)
+```c
+#define traceval(X) \
+  fprintf(stderr, _Generic((X), \
+    char *: "%s\n", int: "%d\n", void *: "%p\n"), X)
+
+  traceval("Hello");
+  traceval(42);
+  traceval(NULL);
+```
+
+```
+$ gcc -std=c23 generic.c && ./a.out
+Hello
+42
+(nil)
+```
+
+Pozor: Všechny varianty musí být kompilovatelné.\
+Každá varianta generuje errory i warningy!
+]
+
+#slide[
+= Attributy (*C23*)
+
+Bylo ```c __attribute__(...)``` a nyní ```c [[...]]```.
+]
+
+#metropolis.new-section[Nestandardní C\
+  C podle kompilátoru]
+
+#slide[
+= Warning - switch
+
+```c
+int val = fgetc(stdin);
+switch (val) {
+  case ' ': printf("Secret space.... ");
+  default: printf("Got: %c\n", val);
+}
+```
+```
+$ gcc -Wimplicit-fallthrough -Wall -std=c23 warn-switch.c && ./a.out
+warn-switch.c: In function ‘main’:
+warn-switch.c:7:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
+    7 |     printf("Secret space.. ");
+      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
+warn-switch.c:8:3: note: here
+    8 |   default:
+      |   ^~~~~~~
+```
+]
+
+#slide[
+= Warning - switch
+
+```c
+int val = fgetc(stdin);
+switch (val) {
+  case ' ':
+    printf("Secret space.... ");
+    [[gnu::fallthrough]];
+  default:
+    printf("Got: %c\n", val);
+}
+```
+]
+
+#slide[
+= Warningy
+
+- ```c [[gnu::fallthrough]]```
+- ```c [[gnu::nonstring]]```
+- ```c [[gnu::unused]]```
+]
+
+#slide[
+= nonnull
+```c
+[[gnu::nonnull]] // [[gnu::nonnull(1)]]
+void prnt(char *str) { printf("%s\n", str); }
+  prnt("Some text");
+  prnt(NULL);
+```
+
+```
+$ gcc -std=c23 nonnull.c && ./a.out
+nonnull.c: In function ‘main’:
+nonnull.c:10:3: warning: argument 1 null where non-null expected [-Wnonnull]
+   10 |   prnt(NULL);
+      |   ^~~~
+nonnull.c:4:6: note: in a call to function ‘prnt’ declared ‘nonnull’
+    4 | void prnt(char *str) {
+      |      ^~~~
+Some text
+(null)
+```
+]
+
+#slide[
+= Další kontroly
+
+- ```c [[clang::counted_by]]```, ```c [[gnu::counted_by]]```
+- ```c [[gnu::malloc]]```
+- ```c [[gnu::alloc_size]]```
+- ```c [[gnu::fd_arg]]```, ```c [[gnu::fd_arg_read]]```, ```c [[gnu:fd_arg_write]]```
+- ```c [[gnu:: format]]```, ```c [[gnu:: format_arg]]]```
+
+https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
+https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
+https://clang.llvm.org/docs/AttributeReference.html
+]
+
+#slide[
+= Automatické uvolnění z heapy
+```c
+static void _free(char **ptr) {
+  printf("Freeing %p=%p\n", ptr, *ptr);
+  free(*ptr);
+}
+  [[gnu::cleanup(_free)]] char *buf = malloc(18);
+  [[gnu::cleanup(_free)]] char *null = NULL;
+  printf("Pointers: %p=%p %p=%p\n", &buf, buf, &null, null);
+```
+
+```
+$ gcc -std=c23 cleanup.c && ./a.out
+Pointers: 0x7ffffffdef68=0x4052a0 0x7ffffffdef70=(nil)
+Freeing 0x7ffffffdef70=(nil)
+Freeing 0x7ffffffdef68=0x4052a0
+```
+]
+
+#slide[
+= Weak
+```c
+[[gnu::weak]]
+void weakfun(void) {
+  printf("Weak function\n");
+}
+  weakfun();
+
+void weakfun(void) { printf("Standard function\n"); }
+```
+
+```
+$ gcc -std=c23 weak.c && ./a.out
+Weak function
+$ gcc -std=c23 weak.c weakfun.c && ./a.out
+Standard function
+```
+]
+
+#slide[
+= Transparent union
+```c
+union [[gnu::transparent_union]] un {
+  void *ptr;
+  intptr_t i;
+};
+void pasi(union un v) { printf("%p\n", v.ptr); }
+  pasi((intptr_t)0x42);
+  pasi((void *)&pasi);
+```
+
+```
+$ gcc -std=c23 transunion.c && ./a.out
+0x42
+0x401150
+```
+]
+
+#slide[
+= Transparent union callback
+```c
+union [[gnu::transparent_union]] un {
+  char *ptr;
+  intptr_t i;
+};
+void call(void (*cb)(union un)) { cb("FOO"); }
+void cb(char *str) { printf("%s\n", str); }
+  call(cb);
+```
+
+```
+$ gcc -std=c23 transunionf.c && ./a.out
+FOO
+```
+]
+
+#slide[
+= Constructor a Desctructor
+```c
+[[gnu::constructor]]
+static void _our_constructor(void) { printf("Constructor\n"); }
+[[gnu::destructor]]
+static void _our_destructor(void) { printf("Destructor\n"); }
+int main(int argc, char *argv[]) {
+  printf("main\n");
+}
+```
+
+```
+$ gcc -std=c23 constructor.c && ./a.out
+Constructor
+main
+Destructor
+```
+]
+
+#slide[
+= Switch range (GCC, Clang, ...)
+```c
+  int val = getchar();
+  switch (val) {
+  case '0' ... '9':
+    printf("Number\n");
+  break;
+  default:
+    printf("Not a number\n");
+    break;
+  }
+```
+]
+
+#slide[
+  #show: metropolis.focus
+  Děkuji za pozornost
+
+  Karel Kočí
+
+  #metropolis.divider
+
+  #text(size: 0.7em)[
+    https://git.cynerd.cz
+
+    https://gitlab.com/cynerd
+
+    https://git.cynerd.cz/presentations/tree/2025-linuxdays
+  ]
+]
diff --git a/2025-linuxdays/priv.c b/2025-linuxdays/priv.c
new file mode 100644
index 0000000..ed3ad1a
--- /dev/null
+++ b/2025-linuxdays/priv.c
@@ -0,0 +1,38 @@
+#include <stddef.h>
+#include <stdio.h>
+
+struct pub {
+  void (*print_name)(struct pub *);
+};
+typedef struct pub *pub_t;
+#define pub_print_name(PUB) (PUB)->print_name(PUB)
+
+struct priv {
+  struct pub pub;
+  const char *name;
+};
+void pname_priv(struct pub *pub) {
+  struct priv *priv = (struct priv *)pub;
+  printf("%s", priv->name);
+}
+
+struct wrap {
+  struct pub *subpub;
+  struct pub pub;
+};
+void pname_wrap(struct pub *pub) {
+  struct wrap *wrap = (void *)pub - offsetof(struct wrap, pub);
+  printf("Wrap: ");
+  wrap->subpub->print_name(wrap->subpub);
+}
+
+int main(int argc, char *argv[]) {
+  struct priv priv = {.pub = {pname_priv}, .name = "FOO"};
+  pub_t p = &priv.pub;
+  pub_print_name(p);
+  putchar('\n');
+
+  struct wrap wrap = {.pub = {pname_wrap}, .subpub = &priv.pub};
+  wrap.pub.print_name(&wrap.pub);
+  putchar('\n');
+}
diff --git a/2025-linuxdays/static_assert.c b/2025-linuxdays/static_assert.c
new file mode 100644
index 0000000..23ffbc8
--- /dev/null
+++ b/2025-linuxdays/static_assert.c
@@ -0,0 +1,9 @@
+#include <stdint.h>
+#include <stdio.h>
+
+static_assert(sizeof(intmax_t) >= 16, "Platform must support 128bit integers");
+
+int main(int argc, char *argv[]) {
+  printf("In the main\n");
+  return 0;
+}
diff --git a/2025-linuxdays/swrange.c b/2025-linuxdays/swrange.c
new file mode 100644
index 0000000..1889ba4
--- /dev/null
+++ b/2025-linuxdays/swrange.c
@@ -0,0 +1,14 @@
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  int val = getchar();
+  switch (val) {
+  case '0' ... '9':
+    printf("Number\n");
+	break;
+  default:
+    printf("Not a number\n");
+    break;
+  }
+  return 0;
+}
diff --git a/2025-linuxdays/transunion.c b/2025-linuxdays/transunion.c
new file mode 100644
index 0000000..cde6059
--- /dev/null
+++ b/2025-linuxdays/transunion.c
@@ -0,0 +1,14 @@
+#include <stdint.h>
+#include <stdio.h>
+
+union [[gnu::transparent_union]] un {
+  void *ptr;
+  intptr_t i;
+};
+
+void pasi(union un v) { printf("%p\n", v.ptr); }
+
+int main(int argc, char *argv[]) {
+  pasi((intptr_t)0x42);
+  pasi((void *)&pasi);
+}
diff --git a/2025-linuxdays/transunionf.c b/2025-linuxdays/transunionf.c
new file mode 100644
index 0000000..e210194
--- /dev/null
+++ b/2025-linuxdays/transunionf.c
@@ -0,0 +1,15 @@
+#include <stdint.h>
+#include <stdio.h>
+
+union [[gnu::transparent_union]] un {
+  char *ptr;
+  intptr_t i;
+};
+
+void call(void (*cb)(union un)) { cb("FOO"); }
+void cb(char *str) { printf("%s\n", str); }
+
+int main(int argc, char *argv[]) {
+  call(cb);
+  return 0;
+}
diff --git a/2025-linuxdays/warn-switch.c b/2025-linuxdays/warn-switch.c
new file mode 100644
index 0000000..0de8859
--- /dev/null
+++ b/2025-linuxdays/warn-switch.c
@@ -0,0 +1,14 @@
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+  int val = getchar();
+  switch (val) {
+  case ' ':
+    printf("Secret space.. ");
+	[[gnu::fallthrough]];
+  default:
+    printf("Got: '%c'\n", val);
+    break;
+  }
+  return 0;
+}
diff --git a/2025-linuxdays/weak.c b/2025-linuxdays/weak.c
new file mode 100644
index 0000000..4dea695
--- /dev/null
+++ b/2025-linuxdays/weak.c
@@ -0,0 +1,11 @@
+#include <stdio.h>
+
+[[gnu::weak]]
+void weakfun(void) {
+  printf("Weak function\n");
+}
+
+int main(int argc, char *argv[]) {
+  weakfun();
+  return 0;
+}
diff --git a/2025-linuxdays/weakfun.c b/2025-linuxdays/weakfun.c
new file mode 100644
index 0000000..edde6dd
--- /dev/null
+++ b/2025-linuxdays/weakfun.c
@@ -0,0 +1,3 @@
+#include <stdio.h>
+
+void weakfun(void) { printf("Standard function\n"); }