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Tcl_LinkVar(3)		    Tcl Library Procedures		Tcl_LinkVar(3)

______________________________________________________________________________

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.
_________________________________________________________________

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
       returned, 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 inter‐
       preter's result 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.				       │

       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 │
	      acceptable  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 rejected with Tcl errors.				       │

       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.					       │

       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 │
	      acceptable  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 │
	      rejected with Tcl errors.					       │

       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.					       │

       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 │
	      rejected with Tcl errors.					       │

       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.	       │

       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 │
	      acceptable  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 rejected with Tcl errors.

       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.				       │

       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.

       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
	      errors.							       │

       TCL_LINK_WIDE_UINT						       │
	      The  C  variable	is  of type Tcl_WideUInt (which is an unsigned │
	      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 unsigned integer form acceptable to  Tcl_GetWideIntFro‐ │
	      mObj  (it will be cast to unsigned); attempts to write non-inte‐ │
	      ger values into varName will be rejected with Tcl errors.

       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
	      rejected 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
	      ckalloc.	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.

KEYWORDS
       boolean, integer, link, read-only, real, string, traces, variable

Tcl				      7.5			Tcl_LinkVar(3)
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