...
Deadlock requires all four conditions, so to prevent deadlock, prevent preventing deadlock requires preventing any one of the four conditions. This guideline recommends locking One simple solution is to lock the mutexes in a predefined order to prevent , which prevents circular wait.
Noncompliant Code Example
The following code has behavior that depends on the runtime environment and the platform's scheduler. However, with proper timing, the main() function will deadlock when running thr1 and thr2, where thr1 tries to lock ba2's mutex, while thr2 tries to lock ba1's mutex in the deposit() function, and the program will not progresshang.
| Code Block | ||||
|---|---|---|---|---|
| ||||
#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;
} deposit_thr_args;
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) {
deposit_thr_args *args = (deposit_thr_args *)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_suceess != 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;
int result;
deposit_thr_args *arg1;
deposit_thr_args *arg2;
bank_account *ba1;
bank_account *ba2;
create_bank_account(&ba1, 1000);
create_bank_account(&ba2, 1000);
arg1 = (deposit_thr_args *)malloc(sizeof(deposit_thr_args));
if (arg1 == NULL) {
/* Handle error */
}
arg2 = (deposit_thr_args *)malloc(sizeof(deposit_thr_args));
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;
}
|
...