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Tcl_Obj(3) Tcl Library Procedures Tcl_Obj(3)
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NAME
Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount,
Tcl_IsShared, Tcl_InvalidateStringRep - manipulate Tcl values
SYNOPSIS
#include <tcl.h>
Tcl_Obj *
Tcl_NewObj()
Tcl_Obj *
Tcl_DuplicateObj(objPtr)
Tcl_IncrRefCount(objPtr)
Tcl_DecrRefCount(objPtr)
int
Tcl_IsShared(objPtr)
Tcl_InvalidateStringRep(objPtr)
ARGUMENTS
Tcl_Obj *objPtr (in) Points to a value; must have been the
result of a previous call to Tcl_NewObj.
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INTRODUCTION
This man page presents an overview of Tcl values (called Tcl_Objs for
historical reasons) and how they are used. It also describes generic
procedures for managing Tcl values. These procedures are used to
create and copy values, and increment and decrement the count of
references (pointers) to values. The procedures are used in
conjunction with ones that operate on specific types of values such as
Tcl_GetIntFromObj and Tcl_ListObjAppendElement. The individual
procedures are described along with the data structures they
manipulate.
Tcl's dual-ported values provide a general-purpose mechanism for
storing and exchanging Tcl values. They largely replace the use of
strings in Tcl. For example, they are used to store variable values,
command arguments, command results, and scripts. Tcl values behave
like strings but also hold an internal representation that can be
manipulated more efficiently. For example, a Tcl list is now
represented as a value that holds the list's string representation as
well as an array of pointers to the values for each list element.
Dual-ported values avoid most runtime type conversions. They also
improve the speed of many operations since an appropriate
representation is immediately available. The compiler itself uses Tcl
values to cache the instruction bytecodes resulting from compiling
scripts.
The two representations are a cache of each other and are computed
lazily. That is, each representation is only computed when necessary,
it is computed from the other representation, and, once computed, it is
saved. In addition, a change in one representation invalidates the
other one. As an example, a Tcl program doing integer calculations can
operate directly on a variable's internal machine integer
representation without having to constantly convert between integers
and strings. Only when it needs a string representing the variable's
value, say to print it, will the program regenerate the string
representation from the integer. Although values contain an internal
representation, their semantics are defined in terms of strings: an up-
to-date string can always be obtained, and any change to the value will
be reflected in that string when the value's string representation is
fetched. Because of this representation invalidation and regeneration,
it is dangerous for extension writers to access Tcl_Obj fields
directly. It is better to access Tcl_Obj information using procedures
like Tcl_GetStringFromObj and Tcl_GetString.
Values are allocated on the heap and are referenced using a pointer to
their Tcl_Obj structure. Values are shared as much as possible. This
significantly reduces storage requirements because some values such as
long lists are very large. Also, most Tcl values are only read and
never modified. This is especially true for procedure arguments, which
can be shared between the caller and the called procedure. Assignment
and argument binding is done by simply assigning a pointer to the
value. Reference counting is used to determine when it is safe to
reclaim a value's storage.
Tcl values are typed. A value's internal representation is controlled
by its type. Several types are predefined in the Tcl core including
integer, double, list, and bytecode. Extension writers can extend the
set of types by defining their own Tcl_ObjType structs.
THE TCL_OBJ STRUCTURE
Each Tcl value is represented by a Tcl_Obj structure which is defined
as follows.
typedef struct Tcl_Obj {
int refCount;
char *bytes;
int length;
const Tcl_ObjType *typePtr;
union {
long longValue;
double doubleValue;
void *otherValuePtr;
Tcl_WideInt wideValue;
struct {
void *ptr1;
void *ptr2;
} twoPtrValue;
struct {
void *ptr;
unsigned long value;
} ptrAndLongRep;
} internalRep;
} Tcl_Obj;
The bytes and the length members together hold a value's UTF-8 string
representation, which is a counted string not containing null bytes
(UTF-8 null characters should be encoded as a two byte sequence: 192,
128.) bytes points to the first byte of the string representation.
The length member gives the number of bytes. The byte array must
always have a null byte after the last data byte, at offset length;
this allows string representations to be treated as conventional null-
terminated C strings. C programs use Tcl_GetStringFromObj and
Tcl_GetString to get a value's string representation. If bytes is
NULL, the string representation is invalid.
A value's type manages its internal representation. The member typePtr
points to the Tcl_ObjType structure that describes the type. If
typePtr is NULL, the internal representation is invalid.
The internalRep union member holds a value's internal representation.
This is either a (long) integer, a double-precision floating-point
number, a pointer to a value containing additional information needed
by the value's type to represent the value, a Tcl_WideInt integer, two
arbitrary pointers, or a pair made up of an unsigned long integer and a
pointer.
The refCount member is used to tell when it is safe to free a value's
storage. It holds the count of active references to the value.
Maintaining the correct reference count is a key responsibility of
extension writers. Reference counting is discussed below in the
section STORAGE MANAGEMENT OF VALUES.
Although extension writers can directly access the members of a Tcl_Obj
structure, it is much better to use the appropriate procedures and
macros. For example, extension writers should never read or update
refCount directly; they should use macros such as Tcl_IncrRefCount and
Tcl_IsShared instead.
A key property of Tcl values is that they hold two representations. A
value typically starts out containing only a string representation: it
is untyped and has a NULL typePtr. A value containing an empty string
or a copy of a specified string is created using Tcl_NewObj or
Tcl_NewStringObj respectively. A value's string value is gotten with
Tcl_GetStringFromObj or Tcl_GetString and changed with
Tcl_SetStringObj. If the value is later passed to a procedure like
Tcl_GetIntFromObj that requires a specific internal representation, the
procedure will create one and set the value's typePtr. The internal
representation is computed from the string representation. A value's
two representations are duals of each other: changes made to one are
reflected in the other. For example, Tcl_ListObjReplace will modify a
value's internal representation and the next call to
Tcl_GetStringFromObj or Tcl_GetString will reflect that change.
Representations are recomputed lazily for efficiency. A change to one
representation made by a procedure such as Tcl_ListObjReplace is not
reflected immediately in the other representation. Instead, the other
representation is marked invalid so that it is only regenerated if it
is needed later. Most C programmers never have to be concerned with
how this is done and simply use procedures such as
Tcl_GetBooleanFromObj or Tcl_ListObjIndex. Programmers that implement
their own value types must check for invalid representations and mark
representations invalid when necessary. The procedure
Tcl_InvalidateStringRep is used to mark a value's string representation
invalid and to free any storage associated with the old string
representation.
Values usually remain one type over their life, but occasionally a
value must be converted from one type to another. For example, a C
program might build up a string in a value with repeated calls to
Tcl_AppendToObj, and then call Tcl_ListObjIndex to extract a list
element from the value. The same value holding the same string value
can have several different internal representations at different times.
Extension writers can also force a value to be converted from one type
to another using the Tcl_ConvertToType procedure. Only programmers
that create new value types need to be concerned about how this is
done. A procedure defined as part of the value type's implementation
creates a new internal representation for a value and changes its
typePtr. See the man page for Tcl_RegisterObjType to see how to create
a new value type.
EXAMPLE OF THE LIFETIME OF A VALUE
As an example of the lifetime of a value, consider the following
sequence of commands:
set x 123
This assigns to x an untyped value whose bytes member points to 123 and
length member contains 3. The value's typePtr member is NULL.
puts "x is $x"
x's string representation is valid (since bytes is non-NULL) and is
fetched for the command.
incr x
The incr command first gets an integer from x's value by calling
Tcl_GetIntFromObj. This procedure checks whether the value is already
an integer value. Since it is not, it converts the value by setting
the value's internal representation to the integer 123 and setting the
value's typePtr to point to the integer Tcl_ObjType structure. Both
representations are now valid. incr increments the value's integer
internal representation then invalidates its string representation (by
calling Tcl_InvalidateStringRep) since the string representation no
longer corresponds to the internal representation.
puts "x is now $x"
The string representation of x's value is needed and is recomputed.
The string representation is now 124 and both representations are again
valid.
STORAGE MANAGEMENT OF VALUES
Tcl values are allocated on the heap and are shared as much as possible
to reduce storage requirements. Reference counting is used to
determine when a value is no longer needed and can safely be freed. A
value just created by Tcl_NewObj or Tcl_NewStringObj has refCount 0.
The macro Tcl_IncrRefCount increments the reference count when a new
reference to the value is created. The macro Tcl_DecrRefCount
decrements the count when a reference is no longer needed and, if the
value's reference count drops to zero, frees its storage. A value
shared by different code or data structures has refCount greater than
1. Incrementing a value's reference count ensures that it will not be
freed too early or have its value change accidentally.
As an example, the bytecode interpreter shares argument values between
calling and called Tcl procedures to avoid having to copy values. It
assigns the call's argument values to the procedure's formal parameter
variables. In doing so, it calls Tcl_IncrRefCount to increment the
reference count of each argument since there is now a new reference to
it from the formal parameter. When the called procedure returns, the
interpreter calls Tcl_DecrRefCount to decrement each argument's
reference count. When a value's reference count drops less than or
equal to zero, Tcl_DecrRefCount reclaims its storage. Most command
procedures do not have to be concerned about reference counting since
they use a value's value immediately and do not retain a pointer to the
value after they return. However, if they do retain a pointer to a
value in a data structure, they must be careful to increment its
reference count since the retained pointer is a new reference.
Command procedures that directly modify values such as those for
lappend and linsert must be careful to copy a shared value before
changing it. They must first check whether the value is shared by
calling Tcl_IsShared. If the value is shared they must copy the value
by using Tcl_DuplicateObj; this returns a new duplicate of the original
value that has refCount 0. If the value is not shared, the command
procedure "owns" the value and can safely modify it directly. For
example, the following code appears in the command procedure that
implements linsert. This procedure modifies the list value passed to
it in objv[1] by inserting objc-3 new elements before index.
listPtr = objv[1];
if (Tcl_IsShared(listPtr)) {
listPtr = Tcl_DuplicateObj(listPtr);
}
result = Tcl_ListObjReplace(interp, listPtr, index, 0,
(objc-3), &(objv[3]));
As another example, incr's command procedure must check whether the
variable's value is shared before incrementing the integer in its
internal representation. If it is shared, it needs to duplicate the
value in order to avoid accidentally changing values in other data
structures.
SEE ALSO
Tcl_ConvertToType(3), Tcl_GetIntFromObj(3),
Tcl_ListObjAppendElement(3), Tcl_ListObjIndex(3),
Tcl_ListObjReplace(3), Tcl_RegisterObjType(3)
KEYWORDS
internal representation, value, value creation, value type, reference
counting, string representation, type conversion
Tcl 8.5 Tcl_Obj(3)