c++ Threads inside for loop print wrong values

Question

I\'m trying to understand Multi-threading in c++, but I\'am stuck in this problem: if I launch threads in a for loop they print wrong values. This is the code:

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Answer 1

Two problems:

1. You have no control over when the thread runs, which means the value of the variable i in the lambda might not be what you expect.

2. The variable i is local for the loop and the loop only. If the loop finishes before one or more thread runs, those threads will have an invalid reference to a variable whose lifetime have ended.

You can solve both these problems very simply by capturing the variable i by value instead of by reference. That means each thread will have a copy of the value, and that copy will be made uniquely for each thread.

Answer 2

Another thing :
Do not wait until to have always an ordered sequence: 0, 1, 2, 3, ... because the multithreading execution mode has a specificity: indeterminism.

Indeterminism means that the execution of the same program, under the same conditions, gives a different result.

This is due to the fact that the OS schedules threads differently from one execution to another depending on several parameters: CPU load, priority of other processes, possible system interruptions, ...

Your example contains only 5 threads, so it's simple, try to increase the number of threads, and for example put a sleep in the processing function, you will see that the result can be different from one execution to another .

Answer 3

The [&] syntax is causing i to be captured by reference. So quite often therefore i will be further advanced when the thread runs than you might expect. More seriously, the behaviour of your code is undefined if i goes out of scope before a thread runs.

Capturing i by value - i.e. std::thread([i](){ print_id(i); }) is the fix.