Anonymous functions in C （Cで匿名関数）

0_README.md

Using GNU extensions.

Refferences:

• https://groups.google.com/g/comp.lang.c/c/O2EF7PxfOn4?pli=1
• https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html
• https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html

1_simple.c

#include <stdio.h>

int main(int argc, char *argv[]) {
  int int42 = ({ int _(const char *s) {
      printf("lambda: %s\n", s);
      return 42;
  } _; })("foo");

  const int (*fn)(const char *) = ({ int _(const char *s) {
      printf("lambda: %s\n", s);
      return 42;
  } _; });

  int42 = fn("foo");
}

libevent.c

#include <err.h>
#include <stdlib.h>
#include <stddef.h>
#include <unistd.h>

#include <event2/event.h>

int main(int argc, char *argv[]) {
  struct event_base *base = event_base_new();
  if (base == NULL)
    err(EXIT_FAILURE, "event_base_new");
  struct event *ev = event_new(
    base,
    STDIN_FILENO,
    EV_READ,
    ({ void _(evutil_socket_t fd, short event, void *arg) {
      printf("callbacked! %d %hd %p\n", fd, event, arg);
    } _; }),
    NULL);
  if (ev == NULL)
    err(EXIT_FAILURE, "event_new");
  struct timeval timeout = {1, 0};
  (void)event_add(ev, &timeout);
  (void)event_base_dispatch(base);
  event_free(ev);
  event_base_free(base);
}

libevent_invalid.c

#include <err.h>
#include <stdlib.h>
#include <stddef.h>
#include <unistd.h>

#include <event2/event.h>

struct event *event_new_with_anonymous_INVALID(struct event_base *base, evutil_socket_t fd, short events, void *arg) {
  struct event *ev = event_new(
    base,
    fd,
    events,
    // This argument is passed as a stack variable (at least on x86_64 Linux);
    // i.e., will be discarded after returning this function (event_new_with_anonymous_INVALID()).
    ({ void _(evutil_socket_t fd, short event, void *arg) {
      printf("callbacked! %d %hd %p\n", fd, event, arg);
    } _; }),
    arg);
  if (ev == NULL)
    err(EXIT_FAILURE, "event_new");

  return ev; // now the pointer to the anonymous function discarded!
}

int main(int argc, char *argv[]) {
  struct event_base *base = event_base_new();
  if (base == NULL)
    err(EXIT_FAILURE, "event_base_new");

  const struct event *ev = event_new_with_anonymous_INVALID(base, STDIN_FILENO, EV_READ, NULL);
  // The function pointer which ev has is invalid!

  struct timeval timeout = {1, 0};
  (void)event_add(ev, &timeout);

  (void)event_base_dispatch(base); // SEGV when trying to call anonymous function

  event_free(ev);
  event_base_free(base);
}