path: root/2025-linuxdays/pres.typ

#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
  ]
]