我试图使用pthreads和信号量在C中实现睡眠理发问题的解决方案，只有部分要求是每个动作必须在发生时打印出来，例如：
我一直在解决这个问题，只是为了获得各种死锁或无序打印报表。
问题的classical solution在这里不会完全适用，因为这会导致每次都打印语句（例如＆＃34; Barber已经睡着了＃34;其次是＆＃34;客户已经唤醒理发师＆＃34;）或者由于上下文切换（可能是也可能不是真正的问题）而无法打印语句。
我目前的解决方案是伪代码，它是这样的：
int chairs = N # available chairs
bool is_sleeping = false
bool is_cutting = false
bool finished = false # All customers taken care of (Changed in main)
# Semaphores for read/write access
semaphore rw_chairs = 1
semaphore rw_sleeping = 1
semaphore rw_cutting = 1
semaphore barber_ready = 0 # Barber ready to cut hair
semaphore sleeping = 0 # Barber is sleeping
semaphore cutting = 0 # Barber cutting a customer's hair
def Barber():
while not finished:
wait(rw_chairs)
wait(rw_sleeping)
# If no chairs are being used
if chairs == N:
is_sleeping = true
print("Barber is sleeping!")
signal(rw_chairs) # Allow others to read/write
signal(rw_sleeping)
wait(sleeping) # Wait for customer to wake him up
signal(barber_ready) # Allow customers into the chair
print("Barber woke up!")
else:
signal(rw_signal)
chairs += 1
signal(barber_ready)
signal(rw_chairs)
# If the barber isn't done for the day, help the customer
if not finished:
print("Helping a customer")
# Wait a random amount of time
print("Finished helping a customer")
signal(cutting) # Let the customer leave after the hair cut
else:
print("Barber is done for the day!")
def Customer():
bool helped = false
while not helped:
wait(rw_chairs)
wait(rw_cutting)
if chairs == N and not is_cutting: # If the barber is free
wait(rw_sleeping)
if is_sleeping:
signal(sleeping) # Wake the barber up
is_sleeping = false
print("Customer has woken the barber up")
signal(rw_sleeping)
is_cutting = true
signal(rw_chairs)
signal(rw_cutting)
wait(barber_ready) # Wait for the barber to be ready
wait(cutting) # Wait for him to finish cutting hair
print("Customer has had his hair cut")
helped = true
else if chairs > 0:
chairs -= 1
print("Customer is waiting in a chair")
signal(rw_chairs)
signal(rw_cutting)
wait(barber_ready)
wait(cutting)
print("Customer has had his hair cut")
helped = true
else:
signal(rw_chairs)
signal(rw_office)
print("All chairs taken, will return later")
# Wait a random amount of time
print("Customer is leaving the barbershop")
如果没有死锁，使用3把椅子，我得到以下输出：
Barber has fallen asleep
Customer is waiting in a chair
Customer is waiting in a chair
Customer is waiting in a chair
All chairs used, will return later
All chairs used, will return later
... (repeat the above line infinitely)
我清楚地知道，理发师并没有正确地让顾客进入 - 但即便如此，我觉得我的结构完全错了。我认为我接近这个问题是错误的，并且可能会使问题过于复杂。
如果这里的任何人对如何构建工作解决方案有任何建议，或者有关重构当前解决方案的建议，那么我们将不胜感激。或者，如果我在正确的轨道上，可能是朝着正确的方向推进。
谢谢！
答案 0 :(得分：0)
我设法让它发挥作用。
看到我以前在这里没有看过这个问题，我会回答我自己的问题，以防其他人遇到这样寻求帮助，就像我一样。
我原来的方法并没有完全关闭，但是为了让它正常工作，需要进行一些小的调整。我没有使用6个信号量，而是使用5：2控制读/写访问，3个控制排队/休眠。
伪代码如下：
# Constants
SLEEPING = 0
AWAKE = 1
BUSY = 2
# Read/Write semaphores
Semaphore rw_chairs = 1 # Access to chairs
Semaphore rw_status = 1 # Access to barber status
# Access semaphores
Semaphore waiting = 0 # Line for waiting customers
Semaphore cutting = 0 # Semaphore to hold customer while getting hair cut
Semaphore sleeping = 0 # Semaphore for barber to sleep
int chairs = n # Available chairs to sit in
int status = AWAKE # Barber Status (could also be a boolean for sleeping)
bool finished = false # Is the program finished
def Barber():
while not finished:
wait(rw_chairs)
wait(rw_status)
# If there is nobody waiting and the barber isn't asleep
if chairs == n and status != SLEEPING:
barber = SLEEPING;
print("Barber went to sleep!")
signal(rw_status)
signal(rw_chairs)
wait(sleeping) # Go to sleep
print("Barber woke up!")
signal(waiting) # Let the next customer in
else:
signal(waiting) # Let the next customer in
chairs += 1 # Free the chair they sat in
barber = BUSY # Set the barber to busy
signal(rw_status)
signal(rw_chairs)
# Once it gets here, the barber is awake and must help someone
if not finished:
print("Barber is cutting a customer's hair")
# Cut the customer's hair
print("Barber finished cutting the customer's hair")
signal(cutting) # Release the customer so they can leave
# barber's finally done
print("Barber finished work for the day");
def Customer():
bool helped = false
print("Customer arrived at the Barber Shop!")
while not helped:
wait(rw_chairs)
wait(rw_status)
if chairs == n and status == SLEEPING:
status = AWAKE
print("Customer woke up the barber")
signal(rw_status)
signal(sleeping) # Wake up the barber
wait(waiting) # Be the next customer to get helped (happens immediately)
signal(rw_chairs)
helped = true; // This customer is being helped
else:
signal(rw_status)
if chairs > 0:
chairs -= 1 # Claim a chair
signal(rw_chairs)
print("Customer sitting in a chair")
wait(waiting) # Wait for the barber
helped = true; // This customer is being helped
else:
print("Barber Shop is busy, will return later")
signal(rw_chairs)
# Wait a while before returning
# If the customer is being helped
if helped:
print("Customer is getting a hair cut")
wait(cutting)
print("Customer is finished getting a haircut")
主要是，一旦所有客户线程都已加入，它就会设置为true，并唤醒（可能）睡觉的Barber来完成程序。
通过这种方法，理发师和顾客首先阅读椅子，然后是状态，并以相反的顺序释放它（防止循环等待的任何死锁）。