Mutex locks and threads- conditional is not being correctly checked.

[proven niche]

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[elder hemlock]

WARNING: ThreadSanitizer: data race (pid=1)
  Read of size 4 at 0x0000004040c0 by thread T2:
    #0 train_func(void*) /app/example.cpp:25 (output.s+0x4012a1)
    #1 <null> <null> (libtsan.so.0+0x333cf)

  Previous write of size 4 at 0x0000004040c0 by thread T1 (mutexes: write M9):
    #0 train_func(void*) /app/example.cpp:55 (output.s+0x401525)
    #1 <null> <null> (libtsan.so.0+0x333cf)

  Location is global 'station_pop' of size 20 at 0x0000004040c0 (output.s+0x0000004040c0)

  Mutex M9 (0x000000404100) created at:
    #0 pthread_mutex_init <null> (libtsan.so.0+0x4d483)
    #1 main /app/example.cpp:126 (output.s+0x401c21)

  Thread T2 (tid=4, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x5fdc5)
    #1 main /app/example.cpp:134 (output.s+0x401c8c)

  Thread T1 (tid=3, running) created by main thread at:
    #0 pthread_create <null> (libtsan.so.0+0x5fdc5)
    #1 main /app/example.cpp:129 (output.s+0x401c4d)

SUMMARY: ThreadSanitizer: data race /app/example.cpp:25 in train_func(void*)

so basically you have some UB because station_pop[i] != 0 happens at the same time as station_pop[current_station] -= passengers_pickup;

it might not be the cause, but it's definitely an issue

current_station == 0

and

current_station++;

should also be a data race

the execution that you're getting there is only plausible with optimizations enabled, I think

but it makes total sense why it happens

Train 1 sees

int current_station = 0;

and can safely assume that no other thread was able to modify this, because that would be a data race and thus UB

so

if (current_station == 0) {

is always true

[halcyon ingot]

sorry for the late reply but thank u!! i added more locks around accesses to shared variables but i still seem to be getting deadlock.. im not sure what the issue is

this is my updated function

i only get up to this point:

Train 0 ENTERS Station 0
Station 0 has 500 passengers to pick up
Picking up passengers...
Train 0 picked up 100 passengers at Station 0 and now has 100/100 passengers
Station 0 has 400 passengers to pick up
Train 0 LEAVES Station 0

[elder hemlock]

you should just make them atomic with _Atomic

simple variables like counters don't need locks, they just need to be atomic to be modified thread-safely

and you can run the program in a debugger to see where each thread gets stuck

[proven niche]

@halcyon ingot Has your question been resolved? If so, run !solved :)

[halcyon ingot]

thank you!

[proven niche]

@halcyon ingot

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[halcyon ingot]

!solved