Tcl_SetVar(3tcl)

💅 Tcl_SetVar — the Tcl Library Procedures page, bestie edition 💅

okay so this is Tcl_SetVar(3tcl) from the Tcl Library Procedures manual, and it's a big one. Sit down, grab a strawberry 🍓, hydrate, because we're about to learn how Tcl variables get set, got, and yeeted from C code.

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✨ NAME ✨

the whole girl gang of variable-manipulating functions, all in one place:

• Tcl_SetVar2Ex
• Tcl_SetVar
• Tcl_SetVar2
• Tcl_ObjSetVar2
• Tcl_GetVar2Ex
• Tcl_GetVar
• Tcl_GetVar2
• Tcl_ObjGetVar2
• Tcl_UnsetVar
• Tcl_UnsetVar2

and what do they all do? they manipulate Tcl variables. that's it, that's the job description 💅

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✨ SYNOPSIS ✨

first things first, you gotta invite Tcl to the party:

#include <tcl.h>

and here's every single signature, verbatim, no notes:

Tcl_Obj *
Tcl_SetVar2Ex(interp, name1, name2, newValuePtr, flags)

const char *
Tcl_SetVar(interp, varName, newValue, flags)

const char *
Tcl_SetVar2(interp, name1, name2, newValue, flags)

Tcl_Obj *
Tcl_ObjSetVar2(interp, part1Ptr, part2Ptr, newValuePtr, flags)

Tcl_Obj *
Tcl_GetVar2Ex(interp, name1, name2, flags)

const char *
Tcl_GetVar(interp, varName, flags)

const char *
Tcl_GetVar2(interp, name1, name2, flags)

Tcl_Obj *
Tcl_ObjGetVar2(interp, part1Ptr, part2Ptr, flags)

int
Tcl_UnsetVar(interp, varName, flags)

int
Tcl_UnsetVar2(interp, name1, name2, flags)

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✨ ARGUMENTS ✨

ngl this is the part everyone skims, so let's make it fun. every single argument, every single detail, zero skips 🫶

• Tcl_Interp *interp (in) — the interpreter that's holding the variable. like, the whole universe this variable lives inside of 🌈
• const char *name1 (in) — this one's a multitasker. if name2 is non-NULL, it holds the name of an array variable. but if name2 is NULL, then it's either the name of a scalar variable or a complete name that includes both the variable name and the index. it may also include :: namespace qualifiers to point at a variable in a specific namespace 👑
• const char *name2 (in) — if it's non-NULL, it gives the name of an element within an array. and heads up: in that case, name1 must refer to an array variable. no exceptions, that's the vibe check 💀
• Tcl_Obj *newValuePtr (in) — points to a Tcl value that contains the new value for the variable. the glow-up, essentially ✨
• int flags (in) — an OR-ed combination of bits that provides additional information. valid values are listed below. bitwise OR-ing is literally the moment 🎀
• const char *varName (in) — the name of the variable. may include :: namespace qualifiers to specify a variable in a particular namespace. and it may refer to either a scalar variable or an element of an array 🦋
• const char *newValue (in) — the new value for the variable, given as a null-terminated string. important: a copy of this value gets stored in the variable. so go ahead and free your original, bestie, she's been duplicated 💖
• Tcl_Obj *part1Ptr (in) — points to a Tcl value containing the variable's name. the name may include a series of :: namespace qualifiers to specify a variable in a particular namespace. and it may refer to a scalar variable or an element of an array variable 🌸
• Tcl_Obj *part2Ptr (in) — if it's non-NULL, it points to a value containing the name of an element within an array, and part1Ptr must refer to an array variable. (yes, this is part1Ptr/part2Ptr doing the exact same dance as name1/name2 — twin energy fr fr 👯)

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that's the setup, the cast list, and the receipts. the actual behavior, flags, return values and all the drama? that's coming in part 2, so don't go anywhere 💜

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💖 DESCRIPTION 💖

okay bestie so these procedures are the whole vibe for working with Tcl variables straight from C code — creating them, modifying them, reading them, and yeeting them into the void. 🌸

Tcl_SetVar2Ex, Tcl_SetVar, Tcl_SetVar2, and Tcl_ObjSetVar2 will create a new variable or modify an existing one. They set the given variable to the value given by newValuePtr or newValue and return a pointer to the variable's new value, which lives in Tcl's variable structure. Tcl_SetVar2Ex and Tcl_ObjSetVar2 take the new value as a Tcl_Obj and hand you back a pointer to a Tcl_Obj. Tcl_SetVar and Tcl_SetVar2 take the new value as a string and give you back a string — and ngl, they're usually less efficient than Tcl_ObjSetVar2. 💅

Heads up bestie: the return value may be different than the newValuePtr or newValue argument you passed in, because write traces can do their little remix on it. 😭

If an error happens while setting the variable (like, say, an array variable is referenced without giving an index into the array), NULL is returned and an error message is left in interp's result if the TCL_LEAVE_ERR_MSG flag bit is set.

Tcl_GetVar2Ex, Tcl_GetVar, Tcl_GetVar2, and Tcl_ObjGetVar2 return the current value of a variable. The arguments to these get treated exactly the same way as the arguments to the procedures above. Under normal circumstances, the return value is a pointer to the variable's value. For Tcl_GetVar2Ex and Tcl_ObjGetVar2 that value comes back as a pointer to a Tcl_Obj. For Tcl_GetVar and Tcl_GetVar2 it comes back as a string — which, you guessed it, is usually less efficient, so Tcl_GetVar2Ex or Tcl_ObjGetVar2 are the preferred girlies. ✨

If an error occurs while reading the variable (e.g. the variable does not exist, or an array element is specified for a scalar variable), then NULL is returned and an error message is left in interp's result if the TCL_LEAVE_ERR_MSG flag bit is set.

Tcl_UnsetVar and Tcl_UnsetVar2 may be used to remove a variable, so that future attempts to read the variable will return an error. Their arguments are treated the same way as the arguments to the procedures above. If the variable is successfully removed then TCL_OK is returned. If the variable cannot be removed because it does not exist then TCL_ERROR is returned, and an error message is left in interp's result if the TCL_LEAVE_ERR_MSG flag bit is set. If an array element is specified, the given element is removed but the array remains. If an array name is specified without an index, then the entire array is removed. gone. periodt. 👑

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🎀 how to name a variable, four ways 🎀

The name of a variable may be specified to these procedures in four ways:

1. [1] If Tcl_SetVar, Tcl_GetVar, or Tcl_UnsetVar is invoked, the variable name is given as a single string, varName. If varName contains an open parenthesis and ends with a close parenthesis, then the value between the parentheses is treated as an index (which can have any string value) and the characters before the first open parenthesis are treated as the name of an array variable. If varName does not have parentheses as described above, then the entire string is treated as the name of a scalar variable.
2. [2] If the name1 and name2 arguments are provided and name2 is non-NULL, then an array element is specified and the array name and index have already been separated by the caller: name1 contains the name and name2 contains the index. An error is generated if name1 contains an open parenthesis and ends with a close parenthesis (array element) and name2 is non-NULL. 🚨
3. [3] If name2 is NULL, name1 is treated just like varName in case [1] above (it can be either a scalar or an array element variable name).

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🌈 the flags, and yes there's a lot of them 🌈

The flags argument may be used to specify any of several options to the procedures. It's an OR-ed combination of the following bits. 🫶

• TCL_GLOBAL_ONLY — Under normal circumstances the procedures look up variables as follows: if a procedure call is active in interp, the variable is looked up at the current level of procedure call. Otherwise, the variable is looked up first in the current namespace, then in the global namespace. But if this bit is set in flags, then the variable is looked up only in the global namespace even if there is a procedure call active. If both TCL_GLOBAL_ONLY and TCL_NAMESPACE_ONLY are given, TCL_GLOBAL_ONLY is ignored. 💀
• TCL_NAMESPACE_ONLY — If this bit is set in flags, then the variable is looked up only in the current namespace; if a procedure is active its variables are ignored, and the global namespace is also ignored unless it is the current namespace.
• TCL_LEAVE_ERR_MSG — If an error is returned and this bit is set in flags, then an error message will be left in the interpreter's result, where it can be retrieved with Tcl_GetObjResult or Tcl_GetStringResult. If this flag bit is not set then no error message is left and the interpreter's result will not be modified.
• TCL_APPEND_VALUE — If this bit is set then newValuePtr or newValue is appended to the current value instead of replacing it. If the variable is currently undefined, then the bit is ignored. This bit is only used by the Tcl_Set* procedures.
• TCL_LIST_ELEMENT — If this bit is set, then newValue is converted to a valid Tcl list element before setting (or appending to) the variable. A separator space is appended before the new list element unless the list element is going to be the first element in a list or sublist (i.e. the variable's current value is empty, or contains the single character “{”, or ends in “ }”). When appending, the original value of the variable must also be a valid list, so that the operation is the appending of a new list element onto a list. 🧚

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okay back to the girlies for a sec:

Tcl_GetVar and Tcl_GetVar2 return the current value of a variable. The arguments to these procedures are treated in the same way as the arguments to Tcl_SetVar and Tcl_SetVar2. Under normal circumstances, the return value is a pointer to the variable's value (which is stored in Tcl's variable structure and will not change before the next call to Tcl_SetVar or Tcl_SetVar2). Tcl_GetVar and Tcl_GetVar2 use the flag bits TCL_GLOBAL_ONLY and TCL_LEAVE_ERR_MSG, both of which have the same meaning as for Tcl_SetVar. If an error occurs in reading the variable (e.g. the variable does not exist or an array element is specified for a scalar variable), then NULL is returned.

Tcl_UnsetVar and Tcl_UnsetVar2 may be used to remove a variable, so that future calls to Tcl_GetVar or Tcl_GetVar2 for the variable will return an error. The arguments to these procedures are treated in the same way as the arguments to Tcl_GetVar and Tcl_GetVar2. If the variable is successfully removed then TCL_OK is returned. If the variable cannot be removed because it does not exist then TCL_ERROR is returned. If an array element is specified, the given element is removed but the array remains. If an array name is specified without an index, then the entire array is removed. ✨

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🦋 REFERENCE COUNT MANAGEMENT 🦋

bestie this part is the serious part, do not skim it, i am literally begging 🙈

The result of Tcl_SetVar2Ex, Tcl_ObjSetVar2, Tcl_GetVar2Ex, and Tcl_ObjGetVar2 is (if non-NULL) a value with a reference of at least 1, where that reference is held by the variable that the function has just operated upon.

The newValuePtr argument to Tcl_SetVar2Ex and Tcl_ObjSetVar2 may be an arbitrary reference count value. Its reference count is incremented on success. On failure, if its reference count is zero, it is decremented and freed so the caller need do nothing with it. 🫶

The part1Ptr argument to Tcl_ObjSetVar2 and Tcl_ObjGetVar2 can have any reference count. These functions never modify it.

⚠️ The part2Ptr argument to Tcl_ObjSetVar2 and Tcl_ObjGetVar2, if non-NULL, should not have a zero reference count, as these functions may retain a reference to it — particularly when it is used to create an array element that did not previously exist — and decrementing the reference count later would leave them pointing to a freed Tcl_Obj. no cap, that's a whole crash waiting to happen 💀

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🌟 SEE ALSO 🌟

Tcl_GetObjResult, Tcl_GetStringResult, Tcl_TraceVar

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💅 KEYWORDS 💅

array, get variable, interpreter, scalar, set, unset, value, variable

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and that's the whole page, bestie!! 🎀 this one's from Tcl 8.1, and the page itself is Tcl_SetVar(3tcl) — go forth and set your variables, you absolute icon 👑💖