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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 varName  in  sync  with
       the  C variable at the address given by addr.  Whenever the Tcl variable is read the value
       of the C variable will be returned, and whenever the Tcl variable is written the  C  vari-
       able  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 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_GetIntFromObj;  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-inte- |
	      ger  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_GetIntFromObj 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_GetIntFromObj 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_GetLongFromObj; attempts to write non-inte- |
	      ger or out-of-range values 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_GetDoubleFromObj;  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_GetDoubleFromObj 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  vari-
	      able must have a proper integer 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-integer values into var- |
	      Name 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 procedure has no effect.

       Tcl_UpdateLinkedVar may be invoked after the C variable has changed to force the Tcl vari-
       able 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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