Guide to DECthreads


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Additionally, DECthreads provides nonportable priority range constants, as follows:
Policy Low High
SCHED_FIFO PRI_FIFO_MIN PRI_FIFO_MAX
SCHED_RR PRI_RR_MIN PRI_RR_MAX
SCHED_OTHER PRI_OTHER_MIN PRI_OTHER_MAX
SCHED_FG_NP PRI_FG_MIN_NP PRI_FG_MAX_NP
SCHED_BG_NP PRI_BG_MIN_NP PRI_BG_MAX_NP

The default priority varies by DECthreads platform. On DIGITAL UNIX, the default is 19 (that is, the POSIX priority of a normal timeshare process). On other platforms, the default priority is the midpoint between PRI_FG_MIN_NP and PRI_FG_MAX_NP. (Section 2.3.6 describes how to specify priorities between the minimum and maximum values.) Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified by param is invalid.
[ENOTSUP] An attempt was made to set the attribute to an unsupported value.

Associated Routines


pthread_attr_setschedpolicy

Changes the scheduling policy attribute of the specified thread attributes object.

Syntax

pthread_attr_setschedpolicy(
attr ,
policy );

Argument Data Type Access
attr opaque pthread_attr_t write
policy integer read
C Binding #include <pthread.h>

int
pthread_attr_setschedpolicy (
pthread_attr_t *attr,
int policy);


ARGUMENTS

attr

Thread attributes object to be modified.

policy

New value for the scheduling policy attribute. Valid values are as follows:

SCHED_OTHER is the default value. See Section 2.3.2.2 for a description of the scheduling policies.


DESCRIPTION

This routine sets the scheduling policy of a thread that is created using the attributes object specified by the attr argument. The default value of the scheduling attribute is SCHED_OTHER.

By default, a created thread inherits the policy of the thread calling pthread_create(). To specify a policy using this routine, scheduling inheritance must be disabled at the time the thread is created. Before calling pthread_create(), call pthread_attr_setinheritsched() and specify the value PTHREAD_EXPLICIT_SCHED for the inherit argument.

Never attempt to use scheduling as a mechanism for synchronization. (Refer to Chapter 1 and Chapter 2.)

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified by policy is invalid.

Associated Routines


pthread_attr_setscope

Sets the contention scope attribute of the specified thread attributes object.

Syntax

pthread_attr_setscope(
attr ,
scope );

Argument Data Type Access
attr opaque pthread_attr_t write
scope int read
C Binding #include <pthread.h>

int
pthread_attr_setscope (
pthread_attr_t *attr,
int scope);


ARGUMENTS

attr

Address of the thread attributes object whose contentions scope attribute is to be modified.

scope

New value for the contention scope attribute of the thread attributes object specified by attr.

DESCRIPTION

This routine uses the value specified in the scope argument to set the contention scope attribute of the thread attributes object specified in the attr argument. The specified attributes object must already be initialized at the time this routine is called.

When creating a thread, use a thread attributes object to specify nondefault values for thread attributes. The contention scope attribute specifies the set of threads with which a thread must compete for processing resources. The contention scope attribute specifies whether the new thread competes for processing resources only with other threads in its own process, called process contention scope, or with all threads on the system, called system contention scope.


Note

On DIGITAL UNIX, DECthreads supports both process contention scope and system contention scope threads. On OpenVMS, DECthreads supports only process contention scope threads. On Windows NT, DECthreads supports only system contention scope threads.

DECthreads selects at most one thread to execute on each processor at any point in time. DECthreads resolves the contention based on each thread's scheduling attributes (for example, priority) and scheduling policy (for example, round-robin).

A thread created using a thread attributes object whose contention scope attribute is set to PTHREAD_SCOPE_PROCESS contends for processing resources with other threads within its own process that also were created with PTHREAD_SCOPE_PROCESS. It is unspecified how such threads are scheduled relative to threads in other processes or threads in the same process that were created with PTHREAD_SCOPE_SYSTEM contention scope.

A thread created using a thread attributes object whose contention scope attribute is set to PTHREAD_SCOPE_SYSTEM contends for processing resources with other threads in any process that also were created with PTHREAD_SCOPE_SYSTEM.

Note that the value of the contention scope attribute of a particular thread attributes object does not necessarily correspond to the actual scheduling contention scope of any existing thread in your multithreaded program.

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[ENOSYS] This routine is not supported by the implementation.
[EINVAL] The value of the attribute being set is not valid.
[ENOTSUP] An attempt was made to set the attribute to an unsupported value.

Associated Routines


pthread_attr_setstackaddr

Changes the stack address attribute of the specified thread attributes object.

Syntax

pthread_attr_setstackaddr(
attr ,
stackaddr );

Argument Data Type Access
attr opaque pthread_attr_t write
stackaddr void read
C Binding #include <pthread.h>

int
pthread_attr_setstackaddr (
pthread_attr_t *attr,
void *stackaddr);


ARGUMENTS

attr

Address of the thread attributes object whose stack address attribute is to be modified.

stackaddr

New value for the stack address attribute of the thread attributes object specified by attr.

DESCRIPTION

This routine uses the value specified in the stackaddr argument to set the stack address attribute of the thread attributes object specified in the attr argument.

When creating a thread, use a thread attributes object to specify nondefault values for thread attributes. The stack address attribute of a thread attributes object points to the origin of the stack for a new thread.

The default value for the stack address attribute of an initialized thread attributes object is NULL.


Note

Correct use of this routine depends upon details of the target platform's stack architecture. Thus, this routine cannot be used in a portable manner.

The size of the stack must be at least PTHREAD_STACK_MIN bytes (see the pthread.h header file). However, because DECthreads must use a portion of this stack memory to begin thread execution and to maintain thread state, your program's "user thread code" cannot rely on using all of the stack memory allocated.


For your program to calculate a value for the stackaddr attribute, note that:

Also note that:

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.

Associated Routines


pthread_attr_setstacksize

Changes the stacksize attribute in the specified thread attributes object.

Syntax

pthread_attr_setstacksize(
attr ,
stacksize );

Argument Data Type Access
attr opaque pthread_attr_t write
stacksize size_t read
C Binding #include <pthread.h>

int
pthread_attr_setstacksize (
pthread_attr_t *attr,
size_t stacksize);


ARGUMENTS

attr

Threads attributes object to be modified.

stacksize

New value for the stacksize attribute of the thread attributes object specified by the attr argument. The stacksize argument must be greater than or equal to PTHREAD_STACK_MIN. PTHREAD_STACK_MIN specifies the minimum size (in bytes) of stack needed for a thread.

DESCRIPTION

This routine sets the stacksize attribute in the thread attributes object specified by the attr argument. Use this routine to adjust the size of the writable area of the stack for a new thread.

The size of a thread's stack is fixed at the time of thread creation. Only the initial thread can dynamically extend its stack.

Many compilers do not check for stack overflow. Ensure that the new thread's stack is sufficient for the resources required by routines that are called from the thread.

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The value specified by attr is invalid, or the value specified by stacksize is less than PTHREAD_STACK_MIN or exceeds a DECthreads-imposed limit.

Associated Routines


pthread_cancel

Allows a thread to request a thread to terminate execution.

Syntax

pthread_cancel(
thread );

Argument Data Type Access
thread opaque pthread_t read
C Binding #include <pthread.h>

int
pthread_cancel (
pthread_t thread);


ARGUMENTS

thread

Thread that receives a cancelation request.

DESCRIPTION

This routine sends a cancelation request to the specified target thread. A cancelation request is a mechanism by which a calling thread requests the target thread to terminate as quickly as possible. Issuing a cancelation request does not guarantee that the target thread will receive or handle the request.

When the cancelation request is acted on, all active cleanup handler routines for the target thread are called. When the last cleanup handler returns, the thread-specific data destructor routines are called for each thread-specific data key with a destructor and for which the target thread has a non-NULL value. Finally, the target thread is terminated.

Note that cancelation of the target thread runs asynchronously with respect to the calling thread's returning from pthread_cancel(). The target thread's cancelability state and type determine when or if the cancelation takes place, as follows:

  1. The target thread can delay cancelation during critical operations by setting its cancelability state to PTHREAD_CANCEL_DISABLE.
  2. Because of communication delays, the calling thread can only rely on the fact that a cancelation request will eventually become pending in the target thread (provided that the target thread does not terminate beforehand).
  3. The calling thread has no guarantee that a pending cancelation request will be delivered because delivery is controlled by the target thread.

When a cancelation request is delivered to a thread, termination processing is similar to that for pthread_exit(). For more information about thread termination, see the Thread Termination section of pthread_create().

This routine is preferred in implementing an Ada abort statement and any other language- or software-defined construct for requesting thread cancelation.

The results of this routine are unpredictable, if the value specified in thread refers to a thread that does not currently exist.

Return Values If an error condition occurs, this routine returns an integer indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The specified thread is invalid.
[ESRCH] The thread argument does not specify an existing thread.

Associated Routines


pthread_cleanup_pop

(Macro) Removes the cleanup handler routine from the calling thread's cleanup handler stack and optionally executes it.

Syntax

pthread_cleanup_pop(
execute );

Argument Data Type Access
execute integer read
C Binding #include <pthread.h>

void
pthread_cleanup_pop(
int execute);


ARGUMENTS

execute

Integer that specifies whether the cleanup handler routine specified in the matching call to pthread_cleanup_push() is executed. A nonzero value causes the cleanup handler routine to be executed.

DESCRIPTION

This routine removes the cleanup handler routine established by the matching call to pthread_cleanup_push() from the calling thread's cleanup handler stack, then executes it if the value specified in this routine's execute argument is nonzero.

A cleanup handler routine can be used to clean up from a block of code whether exited by normal completion, cancelation, or the raising (or reraising) of an exception. The routine is popped from the calling thread's cleanup handler stack and is executed with the arg argument when any of the following actions occur:

This routine and pthread_cleanup_push() are implemented as macros and must appear as statements and in pairs within the same lexical scope. You can think of the pthread_cleanup_push() macro as expanding to a string whose first character is a left brace ({) and pthread_cleanup_pop() as expanding to a string containing the corresponding right brace (}).

Return Values None

Associated Routines


pthread_cleanup_push

(Macro) Establishes a cleanup handler routine to be executed when the thread exits or is canceled.

Syntax

pthread_cleanup_push(
routine,
arg );

Argument Data Type Access
routine procedure read
arg user_arg read
C Binding #include <phtread.h>

void
pthread_cleanup_push(
void (*routine)(void *),
void *arg);


ARGUMENTS

routine

Routine executed as the cleanup handler.

arg

Argument pass to the cleanup handler routine.

DESCRIPTION

This routine pushes the specified routine onto the calling thread's cleanup handler stack. The cleanup handler routine is popped from the stack and executed with the arg argument when any of the following actions occur:

This routine and pthread_cleanup_pop() are implemented as macros and must appear as statements and in pairs within the same lexical scope. You can think of the pthread_cleanup_push() macro as expanding to a string whose first character is a left brace ({) and pthread_cleanup_pop() as expanding to a string containing the corresponding right brace (}).

Return Values None

Associated Routines


pthread_condattr_destroy

Destroys a condition variable attributes object.

Syntax

pthread_condattr_destroy(
attr );

Argument Data Type Access
attr opaque pthread_condattr_t write
C Binding #include <pthread.h>

int
pthread_condattr_destroy (
pthread_condattr_t *attr);


ARGUMENTS

attr

Condition variable attributes object to be destroyed.

DESCRIPTION

This routine destroys the specified condition variable attributes object---that is, the object becomes uninitialized.

Condition variables that were created using this attributes object are not affected by the deletion of the condition variable attributes object.

After calling this routine, the results of using attr in a call to any routine (other than pthread_condattr_init()) are unpredictable.

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[EINVAL] The attributes object specified by attr is invalid.

Associated Routines


pthread_condattr_init

Initializes a condition variable attributes object.

Syntax

pthread_condattr_init(
attr );

Argument Data Type Access
attr opaque pthread_condattr_t write
C Binding #include <pthread.h>

int
pthread_condattr_init (
pthread_condattr_t *attr);


ARGUMENTS

attr

Address of the condition variable attributes object to be initialized.

DESCRIPTION

This routine initializes the condition variable attributes object specified by the attr argument with a set of default attribute values.

When an attributes object is used to create a condition variable, the values of the individual attributes determine the characteristics of the new condition variable. Attributes objects act as additional arguments to condition variable creation. Changing individual attributes in an attributes object does not affect any condition variables that were previously created using that attributes object.

You can use the same condition variable attributes object in successive calls to pthread_condattr_init(), from any thread. If multiple threads can change attributes in a shared attributes object, your program must use a mutex to protect the integrity of that attributes object.

Results are undefined if this routine is called and the attr argument specifies a condition variable attributes object that is already initialized.

Currently, no attributes affecting condition variables are defined. You cannot change any attributes in the condition variable attributes object.

The pthread_condattr_init() and pthread_condattr_destroy() routines are provided for future expandability of the DECthreads pthread interface and to conform with the POSIX.1c standard. These routines serve no useful function, because there are no pthread_condattr_set*() type routines available at this time.

Return Values If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:
Return Description
0 Successful completion.
[ENOMEM] Insufficient memory exists to initialize the condition variable attributes object.

Associated Routines


pthread_cond_broadcast

Wakes all threads that are waiting on the specified condition variable.

Syntax

pthread_cond_broadcast(
cond );

Argument Data Type Access
cond opaque pthread_cond_t modify
C Binding #include <pthread.h>


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