Updated: 11 December 1998

Guide to DECthreads

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

This routine is for DIGITAL UNIX systems only.

Syntax

pthread_condattr_setpshared(
attr ,
pshared );

Argument	Data Type	Access
attr	opaque pthread_condattr_t	write
pshared	int	read

C Binding

Arguments

attr

Address of the condition variable attributes object whose process-shared attribute is to be modified.

pshared

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

Description

This routine uses the value specified in the pshared argument to set the process-shared attribute of the condition variable attributes object specified in the attr argument.

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 of differing 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_getpshared()
• pthread_cond_init()

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

Arguments

cond

Condition variable upon which the threads (to be awakened) are waiting.

Description

This routine unblocks all threads waiting on the specified condition variable cond. Calling this routine implies that data guarded by the associated mutex has changed, so that it might be possible for one or more waiting threads to proceed. The threads that are unblocked shall contend for the mutex according to their respective scheduling policies (if applicable).

If only one of the threads waiting on a condition variable may be able to proceed, but any single thread can proceed, then use pthread_cond_signal() instead.

Whether the associated mutex is locked or unlocked, you can still call this routine. However, if predictable scheduling behavior is required, that mutex should then be locked by the thread calling the pthread_cond_broadcast() routine.

If no threads are waiting on the specified condition variable, this routine takes no action. The broadcast does not propagate to the next condition variable wait.

Return Values

Associated Routines

• pthread_cond_destroy()
• pthread_cond_init()
• pthread_cond_signal()
• pthread_cond_timedwait()
• pthread_cond_wait()

Destroys a condition variable.

Syntax

pthread_cond_destroy(
cond );

Argument	Data Type	Access
cond	opaque pthread_cond_t	write

C Binding

Arguments

cond

Condition variable to be destroyed.

Description

This routine destroys the condition variable specified by cond. This effectively uninitializes the condition variable. Call this routine when a condition variable will no longer be referenced. Destroying a condition variable allows DECthreads to reclaim internal memory associated with the condition variable.

It is safe to destroy an initialized condition variable upon which no threads are currently blocked. Attempting to destroy a condition variable upon which other threads are blocked results in unpredictable behavior.

The results of this routine are unpredictable, if the condition variable specified in cond does not exist or is not initialized.

Return Values

Associated Routines

• pthread_cond_broadcast()
• pthread_cond_init()
• pthread_cond_signal()
• pthread_cond_timedwait()
• pthread_cond_wait()

Obtains the object name from a condition variable object.

Syntax

pthread_cond_getname_np(
cond ,
name ,
len );

Argument	Data Type	Access
cond	opaque pthread_cond_t	read
name	char	write
len	opaque size_t	read

C Binding

Arguments

cond

Address of the condition variable object whose object name is to be obtained.

name

Location to store the obtained object name.

len

Length in bytes of buffer at the location specified by name.

Description

This routine copies the object name from the condition variable object specified by the cond argument to the buffer at the location specified by the name argument. Before calling this routine, your program must allocate the buffer indicated by name.

The object name is a C language string and provides an identifier that is meaningful to a person debugging a multithreaded application based on DECthreads. The maximum number of characters in the object name is 31.

If the specified condition variable object has not been previously set with an object name, this routine copies a C language null string into the buffer at location name.

Return Values

Associated Routines

• pthread_cond_setname_np()

Initializes a condition variable.

Syntax

pthread_cond_init(
cond ,
attr );

Argument	Data Type	Access
cond	opaque pthread_cond_t	write
attr	opaque pthread_condattr_t	read

C Binding

Arguments

cond

Condition variable to be initialized.

attr

Condition variable attributes object that defines the characteristics of the condition variable to be initialized.

Description

This routine initializes the condition variable cond with attributes referenced by attr. If attr is NULL, the default condition variable attributes are used.

A condition variable is a synchronization object used in conjunction with a mutex. A mutex controls access to data that is shared among threads; a condition variable allows threads to wait for that data to enter a defined state.

Condition variables are not owned by a particular thread. Any associated storage is not automatically deallocated when the creating thread terminates.

Use the DECthreads macro PTHREAD_COND_INITIALIZER to initialize statically allocated condition variables to the default condition variable attributes. To call this macro, enter:
pthread_cond_t condition = PTHREAD_COND_INITIALIZER

When statically initialized, a condition variable should not also be initialized using pthread_cond_init(). Also, a statically initialized condition variable need not be destroyed using pthread_cond_destroy().

Under certain circumstances it might be impossible to wait upon a statically initialized condition variable when the process virtual address space (or some other memory limit) is nearly exhausted. In such a case pthread_cond_wait() or pthread_cond_timedwait() can return [ENOMEM]. To avoid this possibility, initialize critical condition variables using pthread_cond_init().

Return Values

Associated Routines

• pthread_cond_broadcast()
• pthread_cond_destroy()
• pthread_cond_signal()
• pthread_cond_timedwait()
• pthread_cond_wait()

Changes the object name in a condition variable object.

Syntax

pthread_cond_setname_np(
cond ,
name ,
mbz );

Argument	Data Type	Access
cond	opaque pthread_cond_t	write
name	char	read
mbz	void	read

C Binding

Arguments

cond

Address of the condition variable object whose object name is to be changed.

name

Object name value to copy into the condition variable object.

mbz

(Must be zero) Argument for use by DECthreads.

Description

This routine changes the object name in the condition variable object specified by the cond argument to the value specified by the name argument. To set a new condition variable object's object name, call this routine immediately after initializing the condition variable object.

The object name is a C language string and provides an identifier that is meaningful to a person debugging a multithreaded application based on DECthreads. The maximum number of characters in the object name is 31.

Return Values

Associated Routines

• pthread_cond_getname_np()

Wakes at least one thread that is waiting on the specified condition variable.

Syntax

pthread_cond_signal(
cond );

Argument	Data Type	Access
cond	opaque pthread_cond_t	modify

C Binding

Arguments

cond

Condition variable to be signaled.

Description

This routine unblocks at least one thread waiting on the specified condition variable cond. Calling this routine implies that data guarded by the associated mutex has changed, thus it might be possible for one of the waiting threads to proceed. In general, only one thread will be released.

If no threads are waiting on the specified condition variable, this routine takes no action. The signal does not propagate to the next condition variable wait.

This routine should be called when any thread waiting on the specified condition variable might find its predicate true, but only one thread should proceed. If more than one thread can proceed, or if any thread would not be able to proceed, then you must use pthread_cond_broadcast().

The scheduling policy determines which thread is awakened. For policies SCHED_FIFO and SCHED_RR, a blocked thread is chosen in priority order, using first-in/first-out (FIFO) within priorities.

If the calling thread holds the lock to the target condition variable's associated mutex while setting the variable's wait predicate, that thread can call pthread_cond_signal() to signal the variable even after releasing that mutex. However, for more predictable scheduling behavior, call pthread_cond_signal() before releasing the target condition variable's associated mutex.

Return Values

Associated Routines

• pthread_cond_broadcast()
• pthread_cond_destroy()
• pthread_cond_init()
• pthread_cond_timedwait()
• pthread_cond_wait()

Wakes one thread that is waiting on the specified condition variable (called from interrupt level only).

Syntax

pthread_cond_signal_int_np(
cond );

Argument	Data Type	Access
cond	opaque pthread_cond_t	modify

C Binding

Arguments

cond

Condition variable to be signaled.

Description

This routine wakes one thread waiting on the specified condition variable. It can only be called from a software interrupt handler routine (such as from a DIGITAL UNIX signal handler or OpenVMS AST). Calling this routine implies that it might be possible for a single waiting thread to proceed.

The scheduling policies of the waiting threads determine which thread is awakened. For policies SCHED_FIFO and SCHED_RR, a blocked thread is chosen in priority order, using first-in/first-out (FIFO) within priorities.

This routine does not cause a thread blocked on a condition variable to resume execution immediately. A thread resumes execution at some time after the interrupt handler routine returns.

You can call this routine regardless of whether the associated mutex is locked (by some other thread). Never lock a mutex from an interrupt handler routine.

Note This routine allows you to signal a thread from a software interrupt handler. Do not call this routine from noninterrupt code. To signal a thread from the normal noninterrupt level, use pthread_cond_signal().

Return Values

Associated Routines

• pthread_cond_broadcast()
• pthread_cond_signal()
• pthread_cond_timedwait()
• pthread_cond_wait()

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