Tcl_LinkVar(3)

okay bestie, welcome to Tcl_LinkVar(3) — a Tcl Library Procedure moment ✨ and we're linking Tcl variables to C variables, no cap 💅

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🌸 NAME 🌸

Tcl_LinkArray, Tcl_LinkVar, Tcl_UnlinkVar, Tcl_UpdateLinkedVar — link Tcl variable to C variable. periodt.

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✨ SYNOPSIS ✨

       #include <tcl.h>

       int
       Tcl_LinkVar(interp, varName, addr, type)

       int                                                                      2
       Tcl_LinkArray(interp, varName, addr, type, size)                         2

       Tcl_UnlinkVar(interp, varName)

       Tcl_UpdateLinkedVar(interp, varName)

heads up, bestie: those little 2s are literally in the original synopsis, so I'm keeping them. character-for-character behavior 💅

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💖 ARGUMENTS 💖

• Tcl_Interp *interp (in) — the interpreter that contains varName. Also used by Tcl_LinkVar to return error messages. 🫶
• const char *varName (in) — name of global variable. 🌸
• void *addr (in) — address of C variable that is to be linked to varName. In Tcl_LinkArray, may be NULL to tell 2 Tcl to create the storage for the array 2 in the variable. 💫
• int type (in) — type of C variable for Tcl_LinkVar or type of array element for Tcl_LinkArray. 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 one of the extra ones listed below. In Tcl_LinkVar, the additional linked type TCL_LINK_STRING may be used. In Tcl_LinkArray, the additional linked 2 types TCL_LINK_CHARS and 2 TCL_LINK_BINARY may be used. All the above for both functions may be optionally OR'ed with TCL_LINK_READ_ONLY to make the Tcl variable read-only. 👑
• Tcl_Size size (in) — the number of elements in the C array. 2 Must be greater than zero. 💀

💖 DESCRIPTION 💖

okay so bestie, here's the whole vibe: Tcl_LinkVar uses variable traces to keep the Tcl variable named by varName in sync with the C variable sitting at the address given by addr. Like, a little long-distance relationship between your C code and your Tcl code, and honestly? it works. 🫶

Whenever the Tcl variable gets read, the value of the C variable gets handed back. Whenever the Tcl variable gets written, the C variable gets updated to have that same value. Two-way street, no cap.

Tcl_LinkVar normally returns TCL_OK. But if something goes sideways while setting up the link (e.g. because varName is the name of an array) then TCL_ERROR is returned and the interpreter's result contains an error message. 💀

Tcl_LinkArray is similar, but for arrays of fixed size (given by the size argument). When it's asked to allocate the backing C array storage (via the addr argument being NULL), it writes the address that it allocated to the Tcl interpreter result. ✨

The type argument specifies the type of the C variable, or the type of the elements of the C array, and it must have one of the following values, optionally OR'ed with TCL_LINK_READ_ONLY:

• TCL_LINK_INT — the C variable, or each element of the C array, 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too. 🙈
• TCL_LINK_UINT — the C variable, or each element of the C array, is of type unsigned int. Any value written into the Tcl variable must have a proper unsigned integer form acceptable to Tcl_GetWideIntFromObj and be 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_CHAR — the C variable, or each element of the C array, 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.

If you're using an array of these, consider using TCL_LINK_CHARS instead. 👀

• TCL_LINK_CHARS — the C array is of type char * and is mapped into Tcl as a string. Any value written into the Tcl variable must have the same length as the underlying storage. Only supported with Tcl_LinkArray.
• TCL_LINK_UCHAR — the C variable, or each element of the C array, is of type unsigned char. Any value written into the Tcl variable must have a proper unsigned integer form acceptable to Tcl_GetIntFromObj and be 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.

If you're using an array of these, consider using TCL_LINK_BINARY instead. 🧚

• TCL_LINK_BINARY — the C array is of type unsigned char * and is mapped into Tcl as a bytearray. Any value written into the Tcl variable must have the same length as the underlying storage. Only supported with Tcl_LinkArray.
• TCL_LINK_SHORT — the C variable, or each element of the C array, 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_USHORT — the C variable, or each element of the C array, is of type unsigned short. Any value written into the Tcl variable must have a proper unsigned integer form acceptable to Tcl_GetIntFromObj and be 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_LONG — the C variable, or each element of the C array, 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-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/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_ULONG — the C variable, or each element of the C array, is of type unsigned long. Any value written into the Tcl variable must have a proper unsigned integer form acceptable to Tcl_GetWideIntFromObj and be 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_DOUBLE — the C variable, or each element of the C array, 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. Incomplete integer or real representations (like the empty string, '.', '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too. 💅
• TCL_LINK_FLOAT — the C variable, or each element of the C array, 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. Incomplete integer or real representations (like the empty string, '.', '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_WIDE_INT — the C variable, or each element of the C array, 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. Incomplete integer representations (like the empty string, '+', '-' or the hex/octal/decimal/binary prefix) are accepted as if they are valid too. 👑
• TCL_LINK_WIDE_UINT — the C variable, or each element of the C array, 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 wideinteger form acceptable to Tcl_GetWideUIntFromObj; 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/decimal/binary prefix) are accepted as if they are valid too.
• TCL_LINK_BOOLEAN — the C variable, or each element of the C array, 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. 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". This is only supported by Tcl_LinkVar. 🌸

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okay so if the TCL_LINK_READ_ONLY flag is present in type, then the variable will be read-only from Tcl, so 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. (No drama, no error, just nothing. iconic tbh.)

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. 🧵✨

lowkey the most important part of this whole page, bestie. threads do not play. 💀

🔑 KEYWORDS 🔑 ✨

okay bestie, this is the lil tag cloud at the bottom — the words you'd smash into man -k or your search bar when you're like "where is that page about linking a variable to a C variable again??" 🙈 These are the searchable vibes this page is filed under:

• boolean — for when you want a variable that's just true/false energy 💖
• integer — whole numbers only, no decimals allowed, periodt 💅
• link — the whole point of this page honestly, the link between your Tcl variable and your C variable 🔗
• read-only — for the ones you can look at but cannot touch 🚫
• real — floating point girlies, decimals welcome 🍓
• string — just vibes, just text, anything goes 🎀
• trace — the behind-the-scenes magic that makes the link update ✨
• variable — the main character of this entire manpage, no cap 👑

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and that's a wrap on Tcl_LinkVar(3) 🫶 this lil icon of a page comes to you from Tcl 7.5 — yes, she's been around a while and she's still slaying 💅 no date stamp on this footer, just the version doing its thing, iconic behaviour fr fr 🌸

go link some variables bestie, you got this 💖✨🦋