Ubuntu Manpage: pthread_setcancelstate, pthread_setcanceltype

NAME

       pthread_setcancelstate, pthread_setcanceltype - set cancelability state and type

SYNOPSIS

       #include <pthread.h>

       int pthread_setcancelstate(int state, int *oldstate);
       int pthread_setcanceltype(int type, int *oldtype);

       Compile and link with -pthread.

DESCRIPTION

The pthread_setcancelstate() sets the cancelability state of the calling thread to the
value given in state. The previous cancelability state of the thread is returned in the
buffer pointed to by oldstate. The state argument must have one of the following values:

PTHREAD_CANCEL_ENABLE
The thread is cancelable. This is the default cancelability state in all new
threads, including the initial thread. The thread's cancelability type determines
when a cancelable thread will respond to a cancellation request.

PTHREAD_CANCEL_DISABLE
The thread is not cancelable. If a cancellation request is received, it is blocked
until cancelability is enabled.

The pthread_setcanceltype() sets the cancelability type of the calling thread to the value
given in type. The previous cancelability type of the thread is returned in the buffer
pointed to by oldtype. The type argument must have one of the following values:

PTHREAD_CANCEL_DEFERRED
A cancellation request is deferred until the thread next calls a function that is a
cancellation point (see pthreads(7)). This is the default cancelability type in
all new threads, including the initial thread.

PTHREAD_CANCEL_ASYNCHRONOUS
The thread can be canceled at any time. (Typically, it will be canceled
immediately upon receiving a cancellation request, but the system doesn't guarantee
this.)

The set-and-get operation performed by each of these functions is atomic with respect to
other threads in the process calling the same function.

RETURN VALUE

       On success, these functions return 0; on error, they return a nonzero error number.

ERRORS

       The pthread_setcancelstate() can fail with the following error:

       EINVAL Invalid value for state.

       The pthread_setcanceltype() can fail with the following error:

       EINVAL Invalid value for type.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────┬─────────────────────┬─────────┐
       │Interface                 │ Attribute           │ Value   │
       ├──────────────────────────┼─────────────────────┼─────────┤
       │pthread_setcancelstate(), │ Thread safety       │ MT-Safe │
       │pthread_setcanceltype()   │                     │         │
       ├──────────────────────────┼─────────────────────┼─────────┤
       │pthread_setcancelstate(), │ Async-cancel-safety │ AC-Safe │
       │pthread_setcanceltype()   │                     │         │
       └──────────────────────────┴─────────────────────┴─────────┘

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.

NOTES

For details of what happens when a thread is canceled, see pthread_cancel(3).

Briefly disabling cancelability is useful if a thread performs some critical action that
must not be interrupted by a cancellation request. Beware of disabling cancelability for
long periods, or around operations that may block for long periods, since that will render
the thread unresponsive to cancellation requests.

Asynchronous cancelability
Setting the cancelability type to PTHREAD_CANCEL_ASYNCHRONOUS is rarely useful. Since the
thread could be canceled at any time, it cannot safely reserve resources (e.g., allocating
memory with malloc(3)), acquire mutexes, semaphores, or locks, and so on. Reserving
resources is unsafe because the application has no way of knowing what the state of these
resources is when the thread is canceled; that is, did cancellation occur before the
resources were reserved, while they were reserved, or after they were released?
Furthermore, some internal data structures (e.g., the linked list of free blocks managed
by the malloc(3) family of functions) may be left in an inconsistent state if cancellation
occurs in the middle of the function call. Consequently, clean-up handlers cease to be
useful.

Functions that can be safely asynchronously canceled are called async-cancel-safe
functions. POSIX.1-2001 and POSIX.1-2008 require only that pthread_cancel(3),
pthread_setcancelstate(), and pthread_setcanceltype() be async-cancel-safe. In general,
other library functions can't be safely called from an asynchronously cancelable thread.

One of the few circumstances in which asynchronous cancelability is useful is for
cancellation of a thread that is in a pure compute-bound loop.

Portability notes
The Linux threading implementations permit the oldstate argument of
pthread_setcancelstate() to be NULL, in which case the information about the previous
cancelability state is not returned to the caller. Many other implementations also permit
a NULL oldstat argument, but POSIX.1 does not specify this point, so portable applications
should always specify a non-NULL value in oldstate. A precisely analogous set of
statements applies for the oldtype argument of pthread_setcanceltype().

EXAMPLE

       See pthread_cancel(3).

COLOPHON

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