CODEWITHSUNDEEP
cmultithreadingposix
Vitto
Vitto

Reputation: 399

When are thread function local variables allocated with Posix?

I know it's a very specific question and it's not very interesting for a high level programmer, but I would like to know when exactly are allocated the local variables of a thread function, in other words after

pthread_create(&thread, &function, ...)

is executed, can I say that they exists in memory or not (considering that the scheduler could have not executed the thread yet)? I tried to search in the posix library code but it's not easy to understand, I arrive at the clone function, written in assembly, but than I cannot find che code of the system call service routine sys_clone to understand what exactly it does. I see in the clone code the invocation of the thread function, but I think this should happen only in the created thread (which could have never been executed by the scheduler when pthread_create is terminated) and not in the creator.

Upvotes: 1

Views: 86

Answers (1)

John Bollinger
John Bollinger

Reputation: 180211

in other words after

pthread_create(&thread, &function, ...)

is executed, can I say that they exists in memory or not (considering that the scheduler could have not executed the thread yet)?

POSIX does not give you any reason for confidence that the local variables of the initial call to function function() in the created thread will have been allocated by the time pthread_create() returns. They might or might not have been, and indeed, the answer might not even be well defined inasmuch as different threads do not necessarily have a consistent view of machine state.

There is no special significance to the local variables of a thread's start function relative to the local variables of any other function called in that thread. Moreover, although pthread_create() will not return successfully until the new thread has been created, that's a separate question from whether the start function has even been entered, much less whether its local variables have been allocated.

Upvotes: 3

Related Questions