Tcl_LinkVar(3tcl) Tcl Library Procedures Tcl_LinkVar(3tcl)
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NAME
Tcl_LinkVar, Tcl_UnlinkVar, Tcl_UpdateLinkedVar - link Tcl variable to
C variable
SYNOPSIS
#include <tcl.h>
int
Tcl_LinkVar(interp, varName, addr, type)
Tcl_UnlinkVar(interp, varName)
Tcl_UpdateLinkedVar(interp, varName)
ARGUMENTS
Tcl_Interp *interp (in) Interpreter that contains varName.
Also used by Tcl_LinkVar to return
error messages.
const char *varName (in) Name of global variable.
char *addr (in) Address of C variable that is to be
linked to varName.
int type (in) Type of C variable. Must be one of
TCL_LINK_INT, TCL_LINK_UINT,
TCL_LINK_CHAR, TCL_LINK_UCHAR,
TCL_LINK_SHORT, TCL_LINK_USHORT,
TCL_LINK_LONG, TCL_LINK_ULONG,
TCL_LINK_WIDE_INT,
TCL_LINK_WIDE_UINT, TCL_LINK_FLOAT,
TCL_LINK_DOUBLE, TCL_LINK_BOOLEAN, or
TCL_LINK_STRING, optionally OR'ed
with TCL_LINK_READ_ONLY to make Tcl
variable read-only.
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DESCRIPTION
Tcl_LinkVar uses variable traces to keep the Tcl variable named by var-
Name in sync with the C variable at the address given by addr. When-
ever the Tcl variable is read the value of the C variable will be re-
turned, and whenever the Tcl variable is written the C variable will be
updated to have the same value. Tcl_LinkVar normally returns TCL_OK;
if an error occurs while setting up the link (e.g. because varName is
the name of array) then TCL_ERROR is returned and the interpreter's re-
sult contains an error message.
The type argument specifies the type of the C variable, and must have
one of the following values, optionally OR'ed with TCL_LINK_READ_ONLY:
TCL_LINK_INT
The C variable is of type int. Any value written into the Tcl
variable must have a proper integer form acceptable to Tcl_Get-
IntFromObj; attempts to write non-integer values into varName
will be rejected with Tcl errors. Incomplete integer representa-
tions (like the empty string, '+', '-' or the hex/octal/binary
prefix) are accepted as if they are valid too.
TCL_LINK_UINT
The C variable is of type unsigned int. Any value written into
the Tcl variable must have a proper unsigned integer form ac-
ceptable to Tcl_GetWideIntFromObj and in the platform's defined
range for the unsigned int type; attempts to write non-integer
values (or values outside the range) into varName will be re-
jected with Tcl errors. Incomplete integer representations (like
the empty string, '+', '-' or the hex/octal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_CHAR
The C variable is of type char. Any value written into the Tcl
variable must have a proper integer form acceptable to Tcl_Get-
IntFromObj and be in the range of the char datatype; attempts to
write non-integer or out-of-range values into varName will be
rejected with Tcl errors. Incomplete integer representations
(like the empty string, '+', '-' or the hex/octal/binary prefix)
are accepted as if they are valid too.
TCL_LINK_UCHAR
The C variable is of type unsigned char. Any value written into
the Tcl variable must have a proper unsigned integer form ac-
ceptable to Tcl_GetIntFromObj and in the platform's defined
range for the unsigned char type; attempts to write non-integer
values (or values outside the range) into varName will be re-
jected with Tcl errors. Incomplete integer representations (like
the empty string, '+', '-' or the hex/octal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_SHORT
The C variable is of type short. Any value written into the Tcl
variable must have a proper integer form acceptable to Tcl_Get-
IntFromObj and be in the range of the short datatype; attempts
to write non-integer or out-of-range values into varName will be
rejected with Tcl errors. Incomplete integer representations
(like the empty string, '+', '-' or the hex/octal/binary prefix)
are accepted as if they are valid too.
TCL_LINK_USHORT
The C variable is of type unsigned short. Any value written
into the Tcl variable must have a proper unsigned integer form
acceptable to Tcl_GetIntFromObj and in the platform's defined
range for the unsigned short type; attempts to write non-integer
values (or values outside the range) into varName will be re-
jected with Tcl errors. Incomplete integer representations (like
the empty string, '+', '-' or the hex/octal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_LONG
The C variable is of type long. Any value written into the Tcl
variable must have a proper integer form acceptable to Tcl_Get-
LongFromObj; attempts to write non-integer or out-of-range val-
ues into varName will be rejected with Tcl errors. Incomplete
integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_ULONG
The C variable is of type unsigned long. Any value written into
the Tcl variable must have a proper unsigned integer form ac-
ceptable to Tcl_GetWideIntFromObj and in the platform's defined
range for the unsigned long type; attempts to write non-integer
values (or values outside the range) into varName will be re-
jected with Tcl errors. Incomplete integer representations (like
the empty string, '+', '-' or the hex/octal/binary prefix) are
accepted as if they are valid too.
TCL_LINK_DOUBLE
The C variable is of type double. Any value written into the
Tcl variable must have a proper real form acceptable to Tcl_Get-
DoubleFromObj; attempts to write non-real values into varName
will be rejected with Tcl errors. Incomplete integer or real
representations (like the empty string, '.', '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_FLOAT
The C variable is of type float. Any value written into the Tcl
variable must have a proper real form acceptable to Tcl_GetDou-
bleFromObj and must be within the range acceptable for a float;
attempts to write non-real values (or values outside the range)
into varName will be rejected with Tcl errors. Incomplete inte-
ger or real representations (like the empty string, '.', '+',
'-' or the hex/octal/binary prefix) are accepted as if they are
valid too.
TCL_LINK_WIDE_INT
The C variable is of type Tcl_WideInt (which is an integer type
at least 64-bits wide on all platforms that can support it.)
Any value written into the Tcl variable must have a proper inte-
ger form acceptable to Tcl_GetWideIntFromObj; attempts to write
non-integer values into varName will be rejected with Tcl er-
rors. Incomplete integer representations (like the empty string,
'+', '-' or the hex/octal/binary prefix) are accepted as if they
are valid too.
TCL_LINK_WIDE_UINT
The C variable is of type Tcl_WideUInt (which is an unsigned in-
teger type at least 64-bits wide on all platforms that can sup-
port it.) Any value written into the Tcl variable must have a
proper unsigned integer form acceptable to Tcl_GetWideIntFromObj
(it will be cast to unsigned); attempts to write non-integer
values into varName will be rejected with Tcl errors. Incomplete
integer representations (like the empty string, '+', '-' or the
hex/octal/binary prefix) are accepted as if they are valid too.
TCL_LINK_BOOLEAN
The C variable is of type int. If its value is zero then it
will read from Tcl as “0”; otherwise it will read from Tcl as
“1”. Whenever varName is modified, the C variable will be set
to a 0 or 1 value. Any value written into the Tcl variable must
have a proper boolean form acceptable to Tcl_GetBooleanFromObj;
attempts to write non-boolean values into varName will be re-
jected with Tcl errors.
TCL_LINK_STRING
The C variable is of type char *. If its value is not NULL then
it must be a pointer to a string allocated with Tcl_Alloc or ck-
alloc. Whenever the Tcl variable is modified the current C
string will be freed and new memory will be allocated to hold a
copy of the variable's new value. If the C variable contains a
NULL pointer then the Tcl variable will read as “NULL”.
If the TCL_LINK_READ_ONLY flag is present in type then the variable
will be read-only from Tcl, so that its value can only be changed by
modifying the C variable. Attempts to write the variable from Tcl will
be rejected with errors.
Tcl_UnlinkVar removes the link previously set up for the variable given
by varName. If there does not exist a link for varName then the proce-
dure has no effect.
Tcl_UpdateLinkedVar may be invoked after the C variable has changed to
force the Tcl variable to be updated immediately. In many cases this
procedure is not needed, since any attempt to read the Tcl variable
will return the latest value of the C variable. However, if a trace
has been set on the Tcl variable (such as a Tk widget that wishes to
display the value of the variable), the trace will not trigger when the
C variable has changed. Tcl_UpdateLinkedVar ensures that any traces on
the Tcl variable are invoked.
Note that, as with any call to a Tcl interpreter, Tcl_UpdateLinkedVar
must be called from the same thread that created the interpreter. The
safest mechanism is to ensure that the C variable is only ever updated
from the same thread that created the interpreter (possibly in response
to an event posted with Tcl_ThreadQueueEvent), but when it is necessary
to update the variable in a separate thread, it is advised that
Tcl_AsyncMark be used to indicate to the thread hosting the interpreter
that it is ready to run Tcl_UpdateLinkedVar.
SEE ALSO
Tcl_TraceVar(3tcl)
KEYWORDS
boolean, integer, link, read-only, real, string, trace, variable
Tcl 7.5 Tcl_LinkVar(3tcl)
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