Tcl_LinkVar(3)						      Tcl Library Procedures						    Tcl_LinkVar(3)

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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 varName 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 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_GetIntFro-
	      mObj;  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_GetInt- |
	      FromObj 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 accept- |
	      able 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_GetInt- |
	      FromObj 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 accept- |
	      able 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 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  accept- |
	      able  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_GetDouble-
	      FromObj;	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_GetDouble- |
	      FromObj 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 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_GetWideIntFromObj (it  will |
	      be cast to unsigned); attempts to write non-integer 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
       procedure 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)