CON35-C. Avoid deadlock by locking in a predefined order

#include <stdlib.h>
#include <threads.h>
 
typedef struct {
  int balance;
  mtx_t balance_mutex;
} bank_account;

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

void create_bank_account(bank_account **ba,
                         int initial_amount) {
  bank_account *nba = (bank_account *)malloc(
    sizeof(bank_account)
  );
  if (nba == NULL) {
    /* Handle error */
  }

  nba->balance = initial_amount;
  if (thrd_success
      != mtx_init(&nba->balance_mutex, mtx_plain)) {
    /* Handle error */
  }

  *ba = nba;
}

int deposit(void *ptr) {
  transaction *args = (transaction *)ptr;

  if (thrd_success != mtx_lock(&args->from->balance_mutex)) {
    /* Handle error */
  }

  /* Not enough balance to transfer */
  if (args->from->balance < args->amount) {
    if (thrd_success
        != mtx_unlock(&args->from->balance_mutex)) {
      /* Handle error */
    }
    return -1; /* Indicate error */
  }
  if (thrd_success != mtx_lock(&args->to->balance_mutex)) {
    /* Handle error */
  }

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

  if (thrd_success
      != mtx_unlock(&args->from->balance_mutex)) {
    /* Handle error */
  }

  if (thrd_success
      != mtx_unlock(&args->to->balance_mutex)) {
    /* Handle error */
  }

  free(ptr);
  return 0;
}

int main(void) {
  thrd_t thr1, thr2;
  transaction *arg1;
  transaction *arg2;
  bank_account *ba1;
  bank_account *ba2;

  create_bank_account(&ba1, 1000);
  create_bank_account(&ba2, 1000);

  arg1 = (transaction *)malloc(sizeof(transaction));
  if (arg1 == NULL) {
    /* Handle error */
  }
  arg2 = (transaction *)malloc(sizeof(transaction));
  if (arg2 == NULL) {
    /* Handle error */
  }
  arg1->from = ba1;
  arg1->to = ba2;
  arg1->amount = 100;

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

  /* Perform the deposits */
  if (thrd_success
     != thrd_create(&thr1, deposit, (void *)arg1)) {
    /* Handle error */
  }
  if (thrd_success
      != thrd_create(&thr2, deposit, (void *)arg2)) {
    /* Handle error */
  }
  return 0;
} 

#include <stdlib.h>
#include <threads.h>
 
typedef struct {
  int balance;
  mtx_t balance_mutex;
 
  /* Should not change after initialization */
  unsigned int id;
} bank_account;

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

unsigned int global_id = 1;

void create_bank_account(bank_account **ba,
                         int initial_amount) {
  bank_account *nba = (bank_account *)malloc(
    sizeof(bank_account)
  );
  if (nba == NULL) {
    /* Handle error */
  }

  nba->balance = initial_amount;
  if (thrd_success
      != mtx_init(&nba->balance_mutex, mtx_plain)) {
    /* Handle error */
  }

  nba->id = global_id++;
  *ba = nba;
}

int deposit(void *ptr) {
  transaction *args = (transaction *)ptr;
  int result = -1;
  mtx_t *first;
  mtx_t *second;

  if (args->from->id == args->to->id) {
    return -1; /* Indicate error */
  }

  /* Ensure proper ordering for locking */
  if (args->from->id < args->to->id) {
    first = &args->from->balance_mutex;
    second = &args->to->balance_mutex;
  } else {
    first = &args->to->balance_mutex;
    second = &args->from->balance_mutex;
  }
  if (thrd_success != mtx_lock(first)) {
    /* Handle error */
  }
  if (thrd_success != mtx_lock(second)) {
    /* Handle error */
  }

  /* Not enough balance to transfer */
  if (args->from->balance >= args->amount) {
    args->from->balance -= args->amount;
    args->to->balance += args->amount;
    result = 0;
  }

  if (thrd_success != mtx_unlock(second)) {
    /* Handle error */
  }
  if (thrd_success != mtx_unlock(first)) {
    /* Handle error */
  }
  free(ptr);
  return result;
}