Updated: 11 December 1998

Guide to DECthreads

Obtains the scheduling parameters for an attribute of the specified thread attributes object.

Syntax

pthread_attr_getschedparam(
attr ,
param );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
param	struct sched_param	write

C Binding

Arguments

attr

Thread attributes object of the scheduling policy attribute whose parameters are obtained.

param

Receives the values of scheduling parameters for the scheduling policy attribute of the attributes object specified by the attr argument. Refer to the description of the pthread_attr_setschedparam() routine for valid parameters and their values.

Description

This routine obtains the scheduling parameters associated with the scheduling policy attribute of the specified thread attributes object.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_setschedparam()
• pthread_create()

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

Syntax

pthread_attr_getschedpolicy(
attr ,
policy );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
policy	integer	write

C Binding

Arguments

attr

Thread attributes object whose scheduling policy attribute is obtained.

policy

Receives the value of the scheduling policy attribute. Refer to the description of the pthread_attr_setschedpolicy() routine for valid values.

Description

This routine obtains the value of the scheduling policy attribute of the specified thread attributes object. The scheduling policy attribute defines the scheduling policy for threads created using the attributes object.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_setschedpolicy()
• pthread_create()

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

Syntax

pthread_attr_getscope(
attr ,
scope );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
scope	int	write

C Binding

Arguments

attr

Address of the thread attributes object whose contention scope attribute is obtained.

scope

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

Description

This routine obtains the value of the contention scope attribute of the thread attributes object specified in the attr argument and stores it in the location specified by the scope argument. The specified attributes object must already be initialized at the time this routine is called.

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.

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

Associated Routines

• pthread_attr_init()
• pthread_attr_setscope()

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

Syntax

pthread_attr_getstackaddr(
attr ,
stackaddr );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
stackaddr	void	write

C Binding

Arguments

attr

Address of the thread attributes object whose stack address attribute is obtained.

stackaddr

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

Description

This routine obtains the value of the stack address attribute of the thread attributes object specified in the attr argument and stores it in the location specified by the stackaddr argument. The specified attributes object must already be initialized at the time this routine is called.

The stack address attribute of a thread attributes object points to the origin of the stack for a new thread.

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

Return Values

Associated Routines

• pthread_attr_getguardsize()
• pthread_attr_getstacksize()
• pthread_attr_init()
• pthread_attr_setguardsize()
• pthread_attr_setstackaddr()
• pthread_attr_setstacksize()
• pthread_create()

Obtains the stacksize attribute of the specified thread attributes object.

Syntax

pthread_attr_getstacksize(
attr ,
stacksize );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
stacksize	size_t	write

C Binding

Arguments

attr

Thread attributes object whose stacksize attribute is obtained.

stacksize

Receives the value for the stacksize attribute of the thread attributes object specified by the attr argument.

Description

This routine obtains the stacksize attribute of the thread attributes object specified in the attr argument.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_setstacksize()
• pthread_create()

Initializes a thread attributes object.

Syntax

pthread_attr_init(
attr );

Argument	Data Type	Access
attr	opaque pthread_attr_t	write

C Binding

Arguments

attr

Address of a thread attributes object to be initialized.

Description

This routine initializes the thread attributes object specified by the attr argument with a set of default attribute values. A thread attributes object is used to specify the attributes of one or more threads when they are created. The attributes object created by this routine is used only in calls to the pthread_create() routine.

The following routines change individual attributes of an initialized thread attributes object:

• pthread_attr_setdetachstate()
• pthread_attr_setguardsize()
• pthread_attr_setinheritsched()
• pthread_attr_setschedparam()
• pthread_attr_setschedpolicy()
• pthread_attr_setscope()
• pthread_attr_setstackaddr()
• pthread_attr_setstacksize()

The attributes of the thread attributes object are initialized to default values. The default value of each attribute is discussed in the reference description for each routine listed above.

When a thread attributes object is used to create a thread, the object's attribute values determine the characteristics of the new thread. Thus, attributes objects act as additional arguments to thread creation. Changing the attributes of a thread attributes object does not affect any threads that were previously created using that attributes object.

You can use the same thread attributes object in successive calls to pthread_create(), from any thread. (However, you cannot use the same value of the stack address attribute to create multiple threads that might run concurrently; threads cannot share a stack.) If more than one thread might change the attributes in a shared attributes object, your program must use a mutex to protect the integrity of the attributes object's contents.

When you set the scheduling policy or scheduling parameters, or both, in a thread attributes object, you must disable scheduling inheritance if you want the scheduling attributes you set to be used at thread creation. To disable scheduling inheritance, before creating the new thread use the pthread_attr_setinheritsched() routine to specify the value PTHREAD_EXPLICIT_SCHED for the inherit argument.

Return Values

Associated Routines

• pthread_attr_destroy()
• pthread_attr_setdetachstate()
• pthread_attr_setguardsize()
• pthread_attr_setinheritsched()
• pthread_attr_setschedparam()
• pthread_attr_setschedpolicy()
• pthread_attr_setscope()
• pthread_attr_setstackaddr()
• pthread_attr_setstacksize()
• pthread_create()

Changes the detachstate attribute in the specified thread attributes object.

Syntax

pthread_attr_setdetachstate(
attr ,
detachstate );

Argument	Data Type	Access
attr	opaque pthread_attr_t	write
detachstate	integer	read

C Binding

Arguments

attr

Thread attributes object to be modified.

detachstate

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

PTHREAD_CREATE_JOINABLE	This is the default value. Threads are created in "undetached" state.
PTHREAD_CREATE_DETACHED	The created thread is detached immediately, before it begins running.

Description

This routine changes the detachstate attribute in the thread attributes object specified by the attr argument. The detachstate attribute specifies whether the thread created using the specified thread attributes object is created in a detached state or not. A value of PTHREAD_CREATE_JOINABLE indicates the thread is not detached, and a value of THREAD_CREATE_DETACHED indicates the thread is detached. PTHREAD_CREATE_JOINABLE is the default value.

Your program cannot use the thread handle (the value of type pthread_t returned by the pthread_create() routine) of a detached thread because the thread might terminate asynchronously, and a detached thread ID is not valid after termination. In particular, it is an error to attempt to detach or join with a detached thread.

When a thread that has not been detached completes execution, DECthreads retains the state of that thread to allow another thread to join with it. If the thread is detached before it completes execution, DECthreads is free to immediately reclaim the thread's storage and resources. Failing to detach threads that have completed execution can result in wasting resources, so threads should be detached as soon as the program is done with them. If there is no need to use the thread's handle after creation, create the thread initially detached.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_getdetachstate()
• pthread_create()
• pthread_join()

Changes the guardsize attribute of the specified thread attributes object.

Syntax

pthread_attr_setguardsize(
attr ,
guardsize );

Argument	Data Type	Access
attr	opaque pthread_attr_t	write
guardsize	size_t	read

C Binding

Arguments

attr

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

guardsize

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

Description

This routine uses the value specified in the guardsize argument to set the guardsize 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 guardsize attribute of a thread attributes object specifies the minimum size (in bytes) of the guard area for the stack of a new thread.

A guard area can help a multithreaded program detect overflow of a thread's stack. A guard area is a region of no-access memory that DECthreads allocates at the overflow end of the thread's stack. When any thread attempts to access a memory location within this region, a memory addressing violation occurs.

A new thread can be created with a default guardsize attribute value. This value is platform dependent, but will always be at least one "hardware protection unit" (that is, at least one page). For more information, see this guide's platform-specific appendixes.

After this routine is called, due to platform-specific factors DECthreads might reserve a larger guard area for the new thread than was specified in the guardsize argument. See this guide's platform-specific appendixes for more information.

DECthreads allows your program to specify the size of a thread stack's guard area for two reasons:

• When a thread allocates large data structures on its stack, a guard area with a size greater than the default size might be required to detect stack overflow.
• Overflow protection of a thread's stack can potentially waste system resources, such as for an application that creates a large number of threads that will never overflow their stacks. Your multithreaded program can conserve system resources by "turning off" a thread's stack guard area---that is, by specifying a guardsize attribute of zero.

If a thread is created using a thread attributes object whose stackaddr attribute is set (using the pthread_attr_setstackaddr() routine), this routine ignores the object's guardsize attribute and provides no thread stack guard area for the new thread.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_getguardsize()
• pthread_attr_setstacksize()
• pthread_create()

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