sync example note1
coconutnut
 2021-11-13 09:53:44      

Cync example for C concurrent programing

Threads

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#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <assert.h>

#define RUNS (4096 * 256)
#define THREADS 4

static int counter = 0;

void *count(void* null) {
  printf("New thread created\n");
  
  // do something
}

int main() {
  // declare an array of 4 threads
  pthread_t handlers[THREADS];

  for (intptr_t i = 0; i < THREADS; i++) {
    int res = pthread_create(&handlers[i], NULL, count, NULL);
    assert(!res);
  }

  for (int i = 0; i < THREADS; i++) {
    int res = pthread_join(handlers[i], NULL);
    assert(!res);
  }

  if (counter != RUNS * THREADS) {
    printf("Didn't count so well. :/, found %d\n", counter);
  } else {
    printf("Counted up to %d.\n", counter);
  }
}

Lock

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#include "lock.h"

void* count(void* null) {
  for (int r = 0; r < RUNS; r++) {
    lock_acquire(&lock);
    counter++;
    lock_release(&lock);
  }

  return NULL;
}

Better version: use lock_wait(&lock) and lock_wake_up(&lock) to prevent bucy wait, but rely on a notification.

Atomic variable

Atomic variables (~atomic registers) do not enforce atomic operation blocks.

Need to use atomic operation.

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static atomic_int counter = 0;

void* count(void* null) {
  for (int r = 0; r < RUNS; r++) {
    atomic_fetch_add(&counter, 1);
  }

  return NULL;
}

Another example

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int expected = 0;
if (atomic_compare_exchange_strong(&leader[r], &expected, tid)) {
	atomic_fetch_add(&nb_leaders[r], 1); // used to check correctness
}

Debug

当有include .h时，报错

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clang: error: linker command failed with exit code 1

可以在Terminal

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gcc election2.c lock.c -o ekection2.o
./election2.o