Updated: 11 December 1998

Guide to DECthreads

The global errno variable is not used by the DECthreads pthread interface routines. To indicate errors, the pthread routines return integer values to indicate the error condition.

Routine names with the _np suffix denote that the routine is not portable, with respect to the POSIX.1c standard. That is, the routine might not be available in implementations of the POSIX.1c standard other than DECthreads.

DECthreads is beginning to provide capabilities specified by the Open Group's (formerly X/Open) Single UNIX Specification, Version 2---also known as UNIX98. Some of the pthread interface routines that UNIX98 specifies are not present the IEEE POSIX 1003.1c-1995 standard; these routines include pthread_attr_getguardsize(), pthread_attr_setguardsize(), pthread_mutexattr_gettype(), and pthread_mutexattr_settype(). DECthreads does not designate these routines as nonportable---that is, their names do not use the _np suffix naming convention. While portable to other implementations of the Single UNIX Specification, Version 2, these routines are not portable to other implementations of the POSIX.1c standard.

Declares fork handler routines to be called when the calling thread's process forks a child process.

This routine is for DIGITAL UNIX systems only.

Syntax

pthread_atfork(
prepare ,
parent ,
child );

Argument	Data Type	Access
prepare	Handler	read
parent	Handler	read
child	Handler	read

C Binding

Arguments

prepare

Address of a routine that performs the fork preparation handling. This routine is called in the parent process before creating the child process.

parent

Address of a routine that performs the fork parent handling. This routine is called in the parent process after creating the child process and before returning to the caller of fork(2).

child

Address of a routine that performs the fork child handling. This routine is called in the child process before returning to the caller of fork(2).

Description

This routine allows a main program or library to control resources during a DIGITAL UNIX fork(2) operation by declaring fork handler routines, as follows:

• The fork handler routine specified in the prepare argument is called before fork(2) executes.
• The fork handler routine specified in the parent argument is called after fork(2) executes within the parent process.
• The fork handler routine specified in the child argument is called in the new child process after fork(2) executes.

Your program (or library) can use fork handlers to ensure that program context in the child process is consistent and meaningful. After fork(2) executes, only the calling thread exists in the child process, and the state of all memory in the parent process is replicated in the child process, including the states of any mutexes, condition variables, and so on.

For example, in the new child process there might exist locked mutexes that are copies of mutexes that were locked in the parent process by threads that do not exist in the child process. Therefore, any associated program state might be inconsistent in the child process.

The program can avoid this problem by calling pthread_atfork() to provide routines that acquire and release resources that are critical to the child process. For example, the prepare handler should lock all mutexes that you want to be usable in the child process. The parent handler just unlocks those mutexes. The child handler will also unlock them all---and might also create threads or reset any program state for the child process.

To illustrate, if your library uses the mutex my_mutex, you might provide pthread_atfork() handler routines coded as follows:

void my_prepare(void) { pthread_mutex_lock(&my_mutex); } void my_parent(void) { pthread_mutex_unlock(&my_mutex); } void my_child(void) { pthread_mutex_unlock(&my_mutex); /* Reinitialize state that doesn't apply...like heap owned */ /* by other threads */ } { . . . pthread_atfork(my_prepare, my_parent, my_child); . . fork(); }

If no fork handling is desired, you can set any of this routine's arguments to NULL.

Note It is not legal to call pthread_atfork() from within a fork handler routine. Doing so could cause a deadlock.

Return Values

Associated Routines

• pthread_create()

Destroys a thread attributes object.

Syntax

pthread_attr_destroy(
attr );

Argument	Data Type	Access
attr	opaque pthread_attr_t	modify

C Binding

Arguments

attr

Thread attributes object to be destroyed.

Description

This routine destroys a thread attributes object. Call this routine when a thread attributes object will no longer be referenced.

Threads that were created using this thread attributes object are not affected by the destruction of the thread attributes object.

The results of calling this routine are unpredictable if the value specified by the attr argument refers to a thread attributes object that does not exist.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_create()

Obtains the detachstate attribute of the specified thread attributes object.

Syntax

pthread_attr_getdetachstate(
attr ,
detachstate );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
detachstate	integer	write

C Binding

Arguments

attr

Thread attributes object whose detachstate attribute is obtained.

detachstate

Receives the value of the detachstate attribute.

Description

This routine obtains the detachstate attribute of a thread attributes object. This attribute specifies whether threads created using the specified thread attributes object are created in a detached state.

On successful completion, this routine returns a zero and the detachstate attribute is set in detachstate. A value of PTHREAD_CREATE_JOINABLE indicates the thread is not detached, and a value of PTHREAD_CREATE_DETACHED indicates the thread is detached.

See the pthread_attr_setdetachstate() description for information about the detachstate attribute.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_setdetachstate()

Obtains the guardsize attribute of the specified thread attributes object.

Syntax

pthread_attr_getguardsize(
attr ,
guardsize );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
guardsize	size_t	write

C Binding

Arguments

attr

Address of the thread attributes object whose guardsize attribute is obtained.

guardsize

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

Description

This routine obtains the value of the guardsize attribute of the thread attributes object specified in the attr argument and stores it in the location specified by the guardsize 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 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.

Note that the value of the guardsize attribute of a particular thread attributes object does not necessarily correspond to the actual size of the guard area of any existing thread in your multithreaded program.

Return Values

Associated Routines

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

Obtains the guardsize attribute of the specified thread attributes object.

Syntax

pthread_attr_getguardsize_np(
attr ,
guardsize );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
guardsize	size_t	write

C Binding

Arguments

attr

Address of the thread attributes object whose guardsize attribute is obtained.

guardsize

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

Description

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

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.

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

Note This routine has been superseded by the pthread_attr_getguardsize() routine, as specified by the Single UNIX Specification, Version 2.

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_setguardsize_np()

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

Syntax

pthread_attr_getinheritsched(
attr ,
inheritsched );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
inheritsched	integer	write

C Binding

Arguments

attr

Thread attributes object whose inherit scheduling attribute is obtained.

inheritsched

Receives the value of the inherit scheduling attribute. Refer to the description of the pthread_attr_setinheritsched() function for valid values.

Description

This routine obtains the value of the inherit scheduling attribute from the specified thread attributes object. The inherit scheduling attribute specifies whether threads created using the attributes object inherit the scheduling attributes of the creating thread, or use the scheduling attributes stored in the attributes object that is passed to pthread_create().

Return Values

Associated Routines

• pthread_attr_init()
• pthread_attr_setinheritsched()
• pthread_create()

Obtains the object name attribute from a thread attributes object.

Syntax

pthread_attr_getname_np(
attr ,
name ,
len ,
mbz );

Argument	Data Type	Access
attr	opaque pthread_attr_t	read
name	char	write
len	opaque size_t	read
mbz	void	write

C Binding

Arguments

attr

Address of the thread attributes object whose object name attribute is to be obtained.

name

Location to store the obtained object name.

len

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

mbz

(Must be zero) Location for use by DECthreads. On DIGITAL UNIX Alpha and OpenVMS Alpha platforms, the value to which this argument points must be a 64-bit pointer. If compiling with short pointers, ensure that you have allocated a 64-bit value to receive the result.

Description

This routine copies the object name attribute from the thread attributes object specified by the attr 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. A new thread created using the thread attributes object is initialized with the object name that was set in that attributes 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.

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

This routine contrasts with pthread_getname_np(), which obtains the object name from the thread object for an existing thread.

Return Values

Associated Routines

• pthread_getname_np()
• pthread_attr_setname_np()
• pthread_setname_np()

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