создание потока (thread creation)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
pthread_create — thread creation
Синопсис (Synopsis)
#include <pthread.h>
int pthread_create(pthread_t *restrict thread,
const pthread_attr_t *restrict attr,
void *(*start_routine)(void*), void *restrict arg);
Описание (Description)
The pthread_create() function shall create a new thread, with
attributes specified by attr, within a process. If attr is NULL,
the default attributes shall be used. If the attributes specified
by attr are modified later, the thread's attributes shall not be
affected. Upon successful completion, pthread_create() shall
store the ID of the created thread in the location referenced by
thread.
The thread is created executing start_routine with arg as its
sole argument. If the start_routine returns, the effect shall be
as if there was an implicit call to pthread_exit() using the
return value of start_routine as the exit status. Note that the
thread in which main() was originally invoked differs from this.
When it returns from main(), the effect shall be as if there was
an implicit call to exit() using the return value of main() as
the exit status.
The signal state of the new thread shall be initialized as
follows:
* The signal mask shall be inherited from the creating thread.
* The set of signals pending for the new thread shall be empty.
The thread-local current locale and the alternate stack shall not
be inherited.
The floating-point environment shall be inherited from the
creating thread.
If pthread_create() fails, no new thread is created and the
contents of the location referenced by thread are undefined.
If _POSIX_THREAD_CPUTIME is defined, the new thread shall have a
CPU-time clock accessible, and the initial value of this clock
shall be set to zero.
The behavior is undefined if the value specified by the attr
argument to pthread_create() does not refer to an initialized
thread attributes object.
Возвращаемое значение (Return value)
If successful, the pthread_create() function shall return zero;
otherwise, an error number shall be returned to indicate the
error.
Ошибки (Error)
The pthread_create() function shall fail if:
EAGAIN The system lacked the necessary resources to create
another thread, or the system-imposed limit on the total
number of threads in a process {PTHREAD_THREADS_MAX} would
be exceeded.
EPERM The caller does not have appropriate privileges to set the
required scheduling parameters or scheduling policy.
The pthread_create() function shall not return an error code of
[EINTR].
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
There is no requirement on the implementation that the ID of the
created thread be available before the newly created thread
starts executing. The calling thread can obtain the ID of the
created thread through the thread argument of the
pthread_create() function, and the newly created thread can
obtain its ID by a call to pthread_self().
Обоснование (Rationale)
A suggested alternative to pthread_create() would be to define
two separate operations: create and start. Some applications
would find such behavior more natural. Ada, in particular,
separates the ``creation'' of a task from its ``activation''.
Splitting the operation was rejected by the standard developers
for many reasons:
* The number of calls required to start a thread would increase
from one to two and thus place an additional burden on
applications that do not require the additional
synchronization. The second call, however, could be avoided
by the additional complication of a start-up state attribute.
* An extra state would be introduced: ``created but not
started''. This would require the standard to specify the
behavior of the thread operations when the target has not yet
started executing.
* For those applications that require such behavior, it is
possible to simulate the two separate steps with the
facilities that are currently provided. The start_routine()
can synchronize by waiting on a condition variable that is
signaled by the start operation.
An Ada implementor can choose to create the thread at either of
two points in the Ada program: when the task object is created,
or when the task is activated (generally at a ``begin''). If the
first approach is adopted, the start_routine() needs to wait on a
condition variable to receive the order to begin ``activation''.
The second approach requires no such condition variable or extra
synchronization. In either approach, a separate Ada task control
block would need to be created when the task object is created to
hold rendezvous queues, and so on.
An extension of the preceding model would be to allow the state
of the thread to be modified between the create and start. This
would allow the thread attributes object to be eliminated. This
has been rejected because:
* All state in the thread attributes object has to be able to
be set for the thread. This would require the definition of
functions to modify thread attributes. There would be no
reduction in the number of function calls required to set up
the thread. In fact, for an application that creates all
threads using identical attributes, the number of function
calls required to set up the threads would be dramatically
increased. Use of a thread attributes object permits the
application to make one set of attribute setting function
calls. Otherwise, the set of attribute setting function
calls needs to be made for each thread creation.
* Depending on the implementation architecture, functions to
set thread state would require kernel calls, or for other
implementation reasons would not be able to be implemented as
macros, thereby increasing the cost of thread creation.
* The ability for applications to segregate threads by class
would be lost.
Another suggested alternative uses a model similar to that for
process creation, such as ``thread fork''. The fork semantics
would provide more flexibility and the ``create'' function can be
implemented simply by doing a thread fork followed immediately by
a call to the desired ``start routine'' for the thread. This
alternative has these problems:
* For many implementations, the entire stack of the calling
thread would need to be duplicated, since in many
architectures there is no way to determine the size of the
calling frame.
* Efficiency is reduced since at least some part of the stack
has to be copied, even though in most cases the thread never
needs the copied context, since it merely calls the desired
start routine.
If an implementation detects that the value specified by the attr
argument to pthread_create() does not refer to an initialized
thread attributes object, it is recommended that the function
should fail and report an [EINVAL] error.
Будущие направления (Future directions)
None.
Смотри также (See also)
fork(3p), pthread_exit(3p), pthread_join(3p)
The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory
Synchronization, pthread.h(0p)
