Linux| |pthread_cancel function && cancel point

Understanding of pthread_cancel function and cancellation point

1. What does the pthread_cancel function do

The pyhread_cancel function just sends a request to the thread. The request is to terminate the thread.

• So for this request, it is just a suggestion for threads

• The thread may not terminate immediately, and will continue to run until it reaches the cancellation point.

 

2. Cancel the understanding

• The cancellation point is a place where the thread checks whether it is cancelled

How does the cancellation point appear?

• For cancellation points, there will be cancellation points for certain functions

• For example: sleep, wait, waitpid, waitid, send and other functions

• Figure: These functions can be used as cancellation points

 

3.pthread_setcancelstate

• This function can set the cancel state, there are two states PTHREAD_CANCEL_ENABLE (cancelable state) and PTHREAD_CANCEL_DISABLE (uncancelable state).

#include <pthread.h>
​
pthread_setcancelstate(int state, int* oldstate);
​
parameter:
    state: Change the current state to state
    oldstate: put the original state of the thread into the space pointed to by oldtype
return value:
    Return 0 on success, error code on failure

These two steps are an atomic operation.

• For the pthread_cancel function, the default is PTHREAD_CANCEL_ENABLE to cancel the state . , When the state is set to PTHREAD_CANCEL_DISABLE , the call to pthread_cancel will not kill the thread . Rather, the cancellation request for this thread is still in a suspended state (i.e. pending ) until the thread is canceled state to PTHREAD_CANCEL_ENABLE , the thread will process all of the pending requests on the next cancellation point .

 

4. pthread_setcanceltype

Set the type of cancellation.

#include <thread.h>
​
int pthread_testcanceltype(int type, int* oldtype);
​
parameter:
    type: Set the type of cancellation to type
    oldtype: put the cancellation type of the thread in the space pointed to by oldtype
return value:
    Return 0 on success, error code on failure

• Our default cancellation type is postponed cancellation. That is, it will run to the cancellation point and then cancel

• There are PTHREAD_CANCEL_DEFERRED (deferred cancellation) for the cancellation type that can be set, which is the default cancellation type.

• PTHRAED_CANCEL_ASYNCHRONOUS (asynchronous cancellation), after adopting asynchronous cancellation, the thread can be cancelled at any time, and it cannot be cancelled without getting the cancellation point.

 

5. pthread_testcancel

• For a thread that does not call the above-mentioned function that can generate a cancellation point, then the thread has no cancellation point and cannot be cancelled, and the cancellation request will always be suspended and will not be processed by the thread.

• So if you want to cancel a thread, you can add a cancellation point to the thread yourself

• The pthread_testcancel function can add a cancellation point to the program itself.

#include <thread.h>
​
void pthread_testcancel(void);

Call the pthread_testcancel function, if a cancellation request is in a pending state, and the cancellation is not set to invalid, then the thread will be cancelled immediately. However, if the cancellation is set to invalid, then the pthread_testcancel function call has no effect .