Page History

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>

typedef struct {
  int balance;
  pthread_mutex_t balance_mutex; 
} bank_account;

typedef struct {
  bank_account *from;
  bank_account *to;
  int amount;
} deposit_thr_args;

/* return negative on error */
int create_bank_account(bank_account **ba, int initial_amount) {

  int ret;
  bank_account *nba = malloc(sizeof(bank_account));
  if (nba == NULL) {
    return -1;
  }

  nba->balance = initial_amount;
  ret = pthread_mutex_init(&nba->balance_mutex, NULL);
  if (ret)
    exit(ret);

  *ba = nba;

  return 0;
}


void *deposit(void *ptr) {
	
  deposit_thr_args *args = (deposit_thr_args *)ptr;

  pthread_mutex_lock(&(args->from->balance_mutex));

  /* not enough balance to transfer */
  if (args->from->balance < args->amount) {
    pthread_mutex_unlock(&(args->from->balance_mutex));
    return NULL;
  }

  pthread_mutex_lock(&(args->to->balance_mutex));
  args->from->balance -= args->amount;
  args->to->balance += args->amount;

  pthread_mutex_unlock(&(args->from->balance_mutex));
  pthread_mutex_unlock(&(args->to->balance_mutex));

  return NULL;
}

int main() {

  pthread_t thr1, thr2;
  int err;

  bank_account *ba1, *ba2;

  err = create_bank_account(&ba1, 1000);
  if (err < 0) 
    exit(err);

  err = create_bank_account(&ba2, 1000);
  if (err < 0) 
    exit(err);

  deposit_thr_args *arg1 = malloc(sizeof(deposit_thr_args));
  if (arg1 == NULL)
    exit(-1);

  deposit_thr_args *arg2 = malloc(sizeof(deposit_thr_args));
  if (arg2 == NULL)
    exit(-1);

  arg1->from = ba1;
  arg1->to = ba2;
  arg1->amount = 100;

  arg2->from = ba2;
  arg2->to = ba1;
  arg2->amount = 100;

  /* perform the deposit */
  err = pthread_create(&thr1, NULL, deposit, (void *)arg1);
  if (err)
    exit(err);

  err = pthread_create(&thr2, NULL, deposit, (void *)arg2);
  if (err)
    exit(err);

  pthread_exit(NULL);

  return 0;
}

typedef struct {
  int balance;
  pthread_mutex_t balance_mutex; 
  unsigned int id; /* read only and should never be changed */
} bank_account;

unsigned int global_id = 1;

/* return negative on error */
int create_bank_account(bank_account **ba, int initial_amount) {

  int ret;
  bank_account *nba = malloc(sizeof(bank_account));

  if (nba == NULL) {
    return -1;
  }

  nba->balance = initial_amount;
  ret = pthread_mutex_init(&nba->balance_mutex, NULL);
  if (ret)
    exit(ret);

  nba->id = global_id++;

  *ba = nba;

  return 0;
}


void *deposit(void *ptr) {
	
  deposit_thr_args *args = (deposit_thr_args *)ptr;

  if (args->from->id == args->to->id) 
		return NULL;

  /* ensure proper ordering for locking */
  if (args->from->id < args->to->id) {
    pthread_mutex_lock(&(args->from->balance_mutex));
    pthread_mutex_lock(&(args->to->balance_mutex));
  } else {
    pthread_mutex_lock(&(args->to->balance_mutex));
    pthread_mutex_lock(&(args->from->balance_mutex));
  }

  /* not enough balance to transfer */
  if (args->from->balance < args->amount) {
    pthread_mutex_unlock(&(args->from->balance_mutex));
    pthread_mutex_unlock(&(args->to->balance_mutex));
    return NULL;
  }

  args->from->balance -= args->amount;
  args->to->balance += args->amount;

  pthread_mutex_unlock(&(args->from->balance_mutex));
  pthread_mutex_unlock(&(args->to->balance_mutex));

  return NULL;
}