'\"
'\" Copyright (c) 1990 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
.TH Tcl_Preserve 3tcl 7.5 Tcl "Tcl Library Procedures"
.\" The -*- nroff -*- definitions below are for supplemental macros used
.\" in Tcl/Tk manual entries.
.\"
.\" .AP type name in/out ?indent?
.\"	Start paragraph describing an argument to a library procedure.
.\"	type is type of argument (int, etc.), in/out is either "in", "out",
.\"	or "in/out" to describe whether procedure reads or modifies arg,
.\"	and indent is equivalent to second arg of .IP (shouldn't ever be
.\"	needed;  use .AS below instead)
.\"
.\" .AS ?type? ?name?
.\"	Give maximum sizes of arguments for setting tab stops.  Type and
.\"	name are examples of largest possible arguments that will be passed
.\"	to .AP later.  If args are omitted, default tab stops are used.
.\"
.\" .BS
.\"	Start box enclosure.  From here until next .BE, everything will be
.\"	enclosed in one large box.
.\"
.\" .BE
.\"	End of box enclosure.
.\"
.\" .CS
.\"	Begin code excerpt.
.\"
.\" .CE
.\"	End code excerpt.
.\"
.\" .VS ?version? ?br?
.\"	Begin vertical sidebar, for use in marking newly-changed parts
.\"	of man pages.  The first argument is ignored and used for recording
.\"	the version when the .VS was added, so that the sidebars can be
.\"	found and removed when they reach a certain age.  If another argument
.\"	is present, then a line break is forced before starting the sidebar.
.\"
.\" .VE
.\"	End of vertical sidebar.
.\"
.\" .DS
.\"	Begin an indented unfilled display.
.\"
.\" .DE
.\"	End of indented unfilled display.
.\"
.\" .SO ?manpage?
.\"	Start of list of standard options for a Tk widget. The manpage
.\"	argument defines where to look up the standard options; if
.\"	omitted, defaults to "options". The options follow on successive
.\"	lines, in three columns separated by tabs.
.\"
.\" .SE
.\"	End of list of standard options for a Tk widget.
.\"
.\" .OP cmdName dbName dbClass
.\"	Start of description of a specific option.  cmdName gives the
.\"	option's name as specified in the class command, dbName gives
.\"	the option's name in the option database, and dbClass gives
.\"	the option's class in the option database.
.\"
.\" .UL arg1 arg2
.\"	Print arg1 underlined, then print arg2 normally.
.\"
.\" .QW arg1 ?arg2?
.\"	Print arg1 in quotes, then arg2 normally (for trailing punctuation).
.\"
.\" .PQ arg1 ?arg2?
.\"	Print an open parenthesis, arg1 in quotes, then arg2 normally
.\"	(for trailing punctuation) and then a closing parenthesis.
.\"
.\"	# Set up traps and other miscellaneous stuff for Tcl/Tk man pages.
.if t .wh -1.3i ^B
.nr ^l \n(.l
.ad b
.\"	# Start an argument description
.de AP
.ie !"\\$4"" .TP \\$4
.el \{\
.   ie !"\\$2"" .TP \\n()Cu
.   el          .TP 15
.\}
.ta \\n()Au \\n()Bu
.ie !"\\$3"" \{\
\&\\$1 \\fI\\$2\\fP (\\$3)
.\".b
.\}
.el \{\
.br
.ie !"\\$2"" \{\
\&\\$1	\\fI\\$2\\fP
.\}
.el \{\
\&\\fI\\$1\\fP
.\}
.\}
..
.\"	# define tabbing values for .AP
.de AS
.nr )A 10n
.if !"\\$1"" .nr )A \\w'\\$1'u+3n
.nr )B \\n()Au+15n
.\"
.if !"\\$2"" .nr )B \\w'\\$2'u+\\n()Au+3n
.nr )C \\n()Bu+\\w'(in/out)'u+2n
..
.AS Tcl_Interp Tcl_CreateInterp in/out
.\"	# BS - start boxed text
.\"	# ^y = starting y location
.\"	# ^b = 1
.de BS
.br
.mk ^y
.nr ^b 1u
.if n .nf
.if n .ti 0
.if n \l'\\n(.lu\(ul'
.if n .fi
..
.\"	# BE - end boxed text (draw box now)
.de BE
.nf
.ti 0
.mk ^t
.ie n \l'\\n(^lu\(ul'
.el \{\
.\"	Draw four-sided box normally, but don't draw top of
.\"	box if the box started on an earlier page.
.ie !\\n(^b-1 \{\
\h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.el \}\
\h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
.\}
.\}
.fi
.br
.nr ^b 0
..
.\"	# VS - start vertical sidebar
.\"	# ^Y = starting y location
.\"	# ^v = 1 (for troff;  for nroff this doesn't matter)
.de VS
.if !"\\$2"" .br
.mk ^Y
.ie n 'mc \s12\(br\s0
.el .nr ^v 1u
..
.\"	# VE - end of vertical sidebar
.de VE
.ie n 'mc
.el \{\
.ev 2
.nf
.ti 0
.mk ^t
\h'|\\n(^lu+3n'\L'|\\n(^Yu-1v\(bv'\v'\\n(^tu+1v-\\n(^Yu'\h'-|\\n(^lu+3n'
.sp -1
.fi
.ev
.\}
.nr ^v 0
..
.\"	# Special macro to handle page bottom:  finish off current
.\"	# box/sidebar if in box/sidebar mode, then invoked standard
.\"	# page bottom macro.
.de ^B
.ev 2
'ti 0
'nf
.mk ^t
.if \\n(^b \{\
.\"	Draw three-sided box if this is the box's first page,
.\"	draw two sides but no top otherwise.
.ie !\\n(^b-1 \h'-1.5n'\L'|\\n(^yu-1v'\l'\\n(^lu+3n\(ul'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
.el \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c
.\}
.if \\n(^v \{\
.nr ^x \\n(^tu+1v-\\n(^Yu
\kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c
.\}
.bp
'fi
.ev
.if \\n(^b \{\
.mk ^y
.nr ^b 2
.\}
.if \\n(^v \{\
.mk ^Y
.\}
..
.\"	# DS - begin display
.de DS
.RS
.nf
.sp
..
.\"	# DE - end display
.de DE
.fi
.RE
.sp
..
.\"	# SO - start of list of standard options
.de SO
'ie '\\$1'' .ds So \\fBoptions\\fR
'el .ds So \\fB\\$1\\fR
.SH "STANDARD OPTIONS"
.LP
.nf
.ta 5.5c 11c
.ft B
..
.\"	# SE - end of list of standard options
.de SE
.fi
.ft R
.LP
See the \\*(So manual entry for details on the standard options.
..
.\"	# OP - start of full description for a single option
.de OP
.LP
.nf
.ta 4c
Command-Line Name:	\\fB\\$1\\fR
Database Name:	\\fB\\$2\\fR
Database Class:	\\fB\\$3\\fR
.fi
.IP
..
.\"	# CS - begin code excerpt
.de CS
.RS
.nf
.ta .25i .5i .75i 1i
..
.\"	# CE - end code excerpt
.de CE
.fi
.RE
..
.\"	# UL - underline word
.de UL
\\$1\l'|0\(ul'\\$2
..
.\"	# QW - apply quotation marks to word
.de QW
.ie '\\*(lq'"' ``\\$1''\\$2
.\"" fix emacs highlighting
.el \\*(lq\\$1\\*(rq\\$2
..
.\"	# PQ - apply parens and quotation marks to word
.de PQ
.ie '\\*(lq'"' (``\\$1''\\$2)\\$3
.\"" fix emacs highlighting
.el (\\*(lq\\$1\\*(rq\\$2)\\$3
..
.\"	# QR - quoted range
.de QR
.ie '\\*(lq'"' ``\\$1''\\-``\\$2''\\$3
.\"" fix emacs highlighting
.el \\*(lq\\$1\\*(rq\\-\\*(lq\\$2\\*(rq\\$3
..
.\"	# MT - "empty" string
.de MT
.QW ""
..
.BS
.SH NAME
Tcl_Preserve, Tcl_Release, Tcl_EventuallyFree \- avoid freeing storage while it is being used
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_Preserve\fR(\fIclientData\fR)
.sp
\fBTcl_Release\fR(\fIclientData\fR)
.sp
\fBTcl_EventuallyFree\fR(\fIclientData, freeProc\fR)
.SH ARGUMENTS
.AS Tcl_FreeProc clientData
.AP ClientData clientData in
Token describing structure to be freed or reallocated.  Usually a pointer
to memory for structure.
.AP Tcl_FreeProc *freeProc in
Procedure to invoke to free \fIclientData\fR.
.BE
.SH DESCRIPTION
.PP
These three procedures help implement a simple reference count mechanism
for managing storage.  They are designed to solve a problem
having to do with widget deletion, but are also useful in many other
situations.  When a widget is deleted, its
widget record (the structure holding information specific to the
widget) must be returned to the storage allocator.
However, it is possible that the widget record is in active use
by one of the procedures on the stack at the time of the deletion.
This can happen, for example, if the command associated with a button
widget causes the button to be destroyed:  an X event causes an
event-handling C procedure in the button to be invoked, which in
turn causes the button's associated Tcl command to be executed,
which in turn causes the button to be deleted, which in turn causes
the button's widget record to be de-allocated.
Unfortunately, when the Tcl command returns, the button's
event-handling procedure will need to reference the
button's widget record.
Because of this, the widget record must not be freed as part of the
deletion, but must be retained until the event-handling procedure has
finished with it.
In other situations where the widget is deleted, it may be possible
to free the widget record immediately.
.PP
\fBTcl_Preserve\fR and \fBTcl_Release\fR
implement short-term reference counts for their \fIclientData\fR
argument.
The \fIclientData\fR argument identifies an object and usually
consists of the address of a structure.
The reference counts guarantee that an object will not be freed
until each call to \fBTcl_Preserve\fR for the object has been
matched by calls to \fBTcl_Release\fR.
There may be any number of unmatched \fBTcl_Preserve\fR calls
in effect at once.
.PP
\fBTcl_EventuallyFree\fR is invoked to free up its \fIclientData\fR
argument.
It checks to see if there are unmatched \fBTcl_Preserve\fR calls
for the object.
If not, then \fBTcl_EventuallyFree\fR calls \fIfreeProc\fR immediately.
Otherwise \fBTcl_EventuallyFree\fR records the fact that \fIclientData\fR
needs eventually to be freed.
When all calls to \fBTcl_Preserve\fR have been matched with
calls to \fBTcl_Release\fR then \fIfreeProc\fR will be called by
\fBTcl_Release\fR to do the cleanup.
.PP
All the work of freeing the object is carried out by \fIfreeProc\fR.
\fIFreeProc\fR must have arguments and result that match the
type \fBTcl_FreeProc\fR:
.PP
.CS
typedef void \fBTcl_FreeProc\fR(
        char *\fIblockPtr\fR);
.CE
.PP
The \fIblockPtr\fR argument to \fIfreeProc\fR will be the
same as the \fIclientData\fR argument to \fBTcl_EventuallyFree\fR.
The type of \fIblockPtr\fR (\fBchar *\fR) is different than the type of the
\fIclientData\fR argument to \fBTcl_EventuallyFree\fR for historical
reasons, but the value is the same.
.PP
When the \fIclientData\fR argument to \fBTcl_EventuallyFree\fR
refers to storage allocated and returned by a prior call to
\fBTcl_Alloc\fR, \fBckalloc\fR, or another function of the Tcl library,
then the \fIfreeProc\fR argument should be given the special value of
\fBTCL_DYNAMIC\fR.
.PP
This mechanism can be used to solve the problem described above
by placing \fBTcl_Preserve\fR and \fBTcl_Release\fR calls around
actions that may cause undesired storage re-allocation.  The
mechanism is intended only for short-term use (i.e. while procedures
are pending on the stack);  it will not work efficiently as a
mechanism for long-term reference counts.
The implementation does not depend in any way on the internal
structure of the objects being freed;  it keeps the reference
counts in a separate structure.
.SH "SEE ALSO"
Tcl_Interp, Tcl_Alloc
.SH KEYWORDS
free, reference count, storage