Updated: 11 December 1998

Guide to DECthreads

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

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:

• Your program must allocate the memory that will be used for the new thread's stack.
• On DIGITAL UNIX, to create a new thread using a thread attributes object, the stackaddr attribute must be an address that points to the high-memory end of the memory region allocated for the stack. This address must point to the highest even-boundary quadword in the allocated memory region.

Also note that:

• If you use the pthread_attr_setstackaddr() routine to set a thread attributes object's stack address attribute and use that attributes object to create a new thread, DECthreads ignores the attributes object's guardsize attribute and provides no thread stack guard area for the new thread.
• If you use the same thread attributes object to create more than one thread and each created thread uses a nondefault stack address, you must use the pthread_attr_setstackaddr() routine to set a unique stack address attribute value for each new thread created using that attributes object.

Return Values

Associated Routines

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

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

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

Associated Routines

• pthread_attr_init()
• pthread_attr_getstacksize()
• pthread_create()

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

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

Associated Routines

• pthread_cleanup_pop()
• pthread_cleanup_push()
• pthread_create()
• pthread_exit()
• pthread_join()
• pthread_setcancelstate()
• pthread_setcanceltype()
• pthread_testcancel()

(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

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:

• The thread calls pthread_cleanup_pop() and specifies a nonzero value for the execute argument.
• The thread calls pthread_exit().
• The thread is canceled.
• An exception is raised and is caught when DECthreads unwinds the calling thread's stack to the lexical scope of the pthread_cleanup_push() and pthread_cleanup_pop() pair.

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

Associated Routines

• pthread_cancel()
• pthread_cleanup_push()
• pthread_create()
• pthread_exit()

(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

Arguments

routine

Routine executed as the cleanup handler.

arg

Argument passed 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:

• The thread calls pthread_cleanup_pop() and specifies a nonzero value for the execute argument.
• The thread calls pthread_exit().
• The thread is canceled.
• An exception is raised and is caught when DECthreads unwinds the calling thread's stack to the lexical scope of the pthread_cleanup_push() and pthread_cleanup_pop() pair.

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

Associated Routines

• pthread_cancel()
• pthread_cleanup_pop()
• pthread_create()
• pthread_exit()
• pthread_testcancel()

Destroys a condition variable attributes object.

Syntax

pthread_condattr_destroy(
attr );

Argument	Data Type	Access
attr	opaque pthread_condattr_t	write

C Binding

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

Associated Routines

• pthread_condattr_init()

Obtains the process-shared attribute of the specified condition variable attributes object.

This routine is for DIGITAL UNIX systems only.

Syntax

pthread_condattr_getpshared(
attr ,
pshared );

Argument	Data Type	Access
attr	opaque pthread_condattr_t	read
pshared	int	write

C Binding

Arguments

attr

Address of the condition variable attributes object whose process-shared attribute is obtained.

pshared

Receives the value of the process-shared attribute of the condition variable attributes object specified by attr.

Description

This routine obtains the value of the process-shared attribute of the condition variable attributes object specified by the attr argument and stores it in the location specified by the pshared argument. The specified attributes object must already be initialized at the time this routine is called.

Creating a condition variable whose process-shared attribute is set to PTHREAD_PROCESS_PRIVATE permits it to be operated upon by threads created within the same process as the thread that initialized that condition variable. If threads in other processes attempt to operate on such a condition variable, the behavior is undefined.

The default value of the process-shared attribute of an initialized condition variable attributes object is PTHREAD_PROCESS_PRIVATE.

Creating a condition variable whose process-shared attribute is set to PTHREAD_PROCESS_SHARED permits it to be operated upon by any thread that has access to the memory where that condition variable is allocated, even if it is allocated in memory that is shared by multiple processes.

Return Values

Associated Routines

• pthread_condattr_destroy()
• pthread_condattr_init()
• pthread_condattr_setpshared()
• pthread_cond_init()

Initializes a condition variable attributes object.

Syntax

pthread_condattr_init(
attr );

Argument	Data Type	Access
attr	opaque pthread_condattr_t	write

C Binding

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

Associated Routines

• pthread_condattr_destroy()
• pthread_cond_init()

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