Tcl_SetVar(3)

💅 Tcl_SetVar(3) — Tcl Library Procedures 💅

okay so bestie, this is the Tcl_SetVar(3) manual page from the Tcl Library Procedures, and it's literally all about manipulating Tcl variables. It's giving variable-handling main character energy, fr fr. No cap.

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🌸 NAME 🌸

Tcl_SetVar2Ex, Tcl_SetVar, Tcl_SetVar2, Tcl_ObjSetVar2, Tcl_GetVar2Ex, Tcl_GetVar, Tcl_GetVar2, Tcl_ObjGetVar2, Tcl_UnsetVar, Tcl_UnsetVar2 — manipulate Tcl variables

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✨ SYNOPSIS ✨

okay so here's the whole synopsis, bestie — it's a lot, but every line is iconic:

#include <tcl.h>

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 🎀

okay so these are the arguments, and they're all lowkey important:

• Tcl_Interp *interp (in) — Interpreter containing variable.
• const char *name1 (in) — Contains the name of an array variable (if name2 is non-NULL) or (if name2 is NULL) either the name of a scalar variable or a complete name including both variable name and index. May include :: namespace qualifiers to specify a variable in a particular namespace.
• const char *name2 (in) — If non-NULL, gives name of element within array; in this case name1 must refer to an array variable.
• Tcl_Obj *newValuePtr (in) — Points to a Tcl value containing the new value for the variable.
• int flags (in) — OR-ed combination of bits providing additional information. See below for valid values.
• const char *varName (in) — Name of variable. May include :: namespace qualifiers to specify a variable in a particular namespace. May refer to a scalar variable or an element of an array.
• const char *newValue (in) — New value for variable, specified as a null-terminated string. A copy of this value is stored in the variable.
• 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. May refer to a scalar variable or an element of an array variable.
• Tcl_Obj *part2Ptr (in) — If non-NULL, points to a value containing the name of an element within an array and part1Ptr must refer to an array variable.

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✨ DESCRIPTION ✨

okay bestie so these procedures are literally the whole toolkit for creating, modifying, reading and deleting Tcl variables straight from C code. yes you heard me. C. we're doing it. 💅

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🌸 the setter girlies

Tcl_SetVar2Ex, Tcl_SetVar, Tcl_SetVar2 and Tcl_ObjSetVar2 will either create a brand new variable or modify one that already exists. They set the given variable to the value given by newValuePtr or newValue, and they return a pointer to the variable's new value — which lives in Tcl's variable structure, bestie, that's where it's stored.

• 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 hand you back a string. These are usually less efficient than Tcl_ObjSetVar2, so like… noted.

⚠️ big heads up: the return value may be different from the newValuePtr or newValue argument you passed in, because write traces can modify things. no cap.

And if an error happens while setting the variable — like an array variable being referenced without giving an index into the array — then NULL is returned and an error message is left in interp's result if the TCL_LEAVE_ERR_MSG flag bit is set. 😭

🦋 the getter girlies

Tcl_GetVar2Ex, Tcl_GetVar, Tcl_GetVar2 and Tcl_ObjGetVar2 return the current value of a variable. Their arguments are treated in the exact same way as the arguments to the procedures we just talked about above. Under normal circumstances, the return value is a pointer to the variable's value.

• Tcl_GetVar2Ex and Tcl_ObjGetVar2 return the value as a pointer to a Tcl_Obj.
• Tcl_GetVar and Tcl_GetVar2 return the value as a string; this is usually less efficient, so Tcl_GetVar2Ex or Tcl_ObjGetVar2 are preferred. periodt.

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. 💀

🎀 the unsetter girlies

Tcl_UnsetVar and Tcl_UnsetVar2 may be used to remove a variable, so that future attempts to read the variable will return an error. The arguments to these procedures are treated in the same way as the arguments to the procedures above.

• Variable successfully removed? You get TCL_OK. slay. ✅
• Variable cannot be removed because it does not exist? You get TCL_ERROR, and an error message is left in interp's result if the TCL_LEAVE_ERR_MSG flag bit is set. 🙈
• Array element specified? The given element is removed but the array remains.
• Array name specified without an index? Then the entire array gets removed. gone. bye. 👋

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🧚 how you're allowed to name a variable

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

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. 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. 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).

(yes bestie the page says four ways and then lists three. we're just reporting what it says, iykyk 🫶)

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💖 the flags argument

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

• TCL_GLOBAL_ONLY — under normal circumstances the procedures look up variables like this: 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. And 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. so like, either you get the tea or you get silence. 💅
• 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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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 💜

okay this is the part where we talk about who's holding whose reference, and it's giving important, so listen up bestie.

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. so like, either way you're covered. 🫶

The part1Ptr argument to Tcl_ObjSetVar2 and Tcl_ObjGetVar2 can have any reference count. These functions never modify it. no touchy. 🙅

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. 💀💀💀 that's the bad ending, don't do that.

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🌈 SEE ALSO 🌈

• Tcl_GetObjResult
• Tcl_GetStringResult
• Tcl_TraceVar

⭐ 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 is literally called Tcl_SetVar(3) — go forth and set your variables, you absolute icon. mwah. 💖✨🎀🦋