Tcl_StringObj(3)

💖 Tcl_StringObj(3) — Tcl Library Procedures 💖

okay bestie, gather round 🫶 this is the manual page for Tcl's string-object squad, and it's giving main character energy for your C extensions. We're talking about manipulating Tcl values as strings, and yes, it's as iconic as it sounds. ✨

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✨ NAME ✨

This page is the whole girlboss lineup for functions that manipulate Tcl values as strings:

• Tcl_NewStringObj
• Tcl_NewUnicodeObj
• Tcl_SetStringObj
• Tcl_SetUnicodeObj
• Tcl_GetStringFromObj
• Tcl_GetString
• Tcl_GetUnicodeFromObj
• Tcl_GetUnicode
• Tcl_GetUniChar
• Tcl_GetCharLength
• Tcl_GetRange
• Tcl_AppendToObj
• Tcl_AppendUnicodeToObj
• Tcl_AppendObjToObj
• Tcl_AppendStringsToObj
• Tcl_AppendLimitedToObj
• Tcl_Format
• Tcl_AppendFormatToObj
• Tcl_ObjPrintf
• Tcl_AppendPrintfToObj
• Tcl_SetObjLength
• Tcl_AttemptSetObjLength
• Tcl_ConcatObj

Periodt. 💅

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🌸 SYNOPSIS 🌸

Here's the exact spellbook, no cap 📜:

#include <tcl.h>

Tcl_Obj *
Tcl_NewStringObj(bytes, length)

Tcl_Obj *
Tcl_NewUnicodeObj(unicode, numChars)

void
Tcl_SetStringObj(objPtr, bytes, length)

void
Tcl_SetUnicodeObj(objPtr, unicode, numChars)

char *
Tcl_GetStringFromObj(objPtr, lengthPtr)

char *
Tcl_GetString(objPtr)

Tcl_UniChar *
Tcl_GetUnicodeFromObj(objPtr, lengthPtr)

Tcl_UniChar *
Tcl_GetUnicode(objPtr)

int
Tcl_GetUniChar(objPtr, index)

Tcl_Size
Tcl_GetCharLength(objPtr)

Tcl_Obj *
Tcl_GetRange(objPtr, first, last)

void
Tcl_AppendToObj(objPtr, bytes, length)

void
Tcl_AppendUnicodeToObj(objPtr, unicode, numChars)

void
Tcl_AppendObjToObj(objPtr, appendObjPtr)

void
Tcl_AppendStringsToObj(objPtr, string, string, ... (char *)NULL)

void
Tcl_AppendLimitedToObj(objPtr, bytes, length, limit, ellipsis)

Tcl_Obj *
Tcl_Format(interp, format, objc, objv)

int
Tcl_AppendFormatToObj(interp, objPtr, format, objc, objv)

Tcl_Obj *
Tcl_ObjPrintf(format, ...)

void
Tcl_AppendPrintfToObj(objPtr, format, ...)

void
Tcl_SetObjLength(objPtr, newLength)

int
Tcl_AttemptSetObjLength(objPtr, newLength)

Tcl_Obj *
Tcl_ConcatObj(objc, objv)

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🎀 ARGUMENTS 🎀

• const char *bytes (in) — points to the first byte of an array of UTF-8-encoded bytes used to set or append to a string value. This byte array may contain embedded null characters unless numChars is negative. If your application needs null bytes, represent them as the two-byte sequence \300\200, use Tcl_ExternalToUtf to convert, or use Tcl_NewByteArrayObj if the string is a collection of uninterpreted bytes. 🦋
• Tcl_Size length (in) — the number of bytes to copy from bytes when initializing, setting, or appending to a string value. If negative, all bytes up to the first null are used. ✨
• const Tcl_UniChar *unicode (in) — points to the first byte of an array of Unicode characters used to set or append to a string value. This byte array may contain embedded null characters unless numChars is negative. 🎀
• Tcl_Size numChars (in) — the number of Unicode characters to copy from unicode when initializing, setting, or appending to a string value. If negative, all characters up to the first null character are used. 🌸
• Tcl_Size index (in) — the index of the Unicode character to return. 🔍
• Tcl_Size first (in) — the index of the first Unicode character in the Unicode range to be returned as a new value. If negative, behave the same as if the value was 0. 💖
• Tcl_Size last (in) — the index of the last Unicode character in the Unicode range to be returned as a new value. If negative, take all characters up to the last one available. 👑
• Tcl_Obj *objPtr (in/out) — a pointer to a value to read, or to an unshared value to modify. 🫶
• Tcl_Obj *appendObjPtr (in) — the value to append to objPtr in Tcl_AppendObjToObj. 💜
• Tcl_Size | int *lengthPtr (out) — the location where Tcl_GetStringFromObj will store the length of a value's string representation. May be (Tcl_Size *)NULL when not used. If it points to a variable whose type is not Tcl_Size, a compiler warning will be generated. If your extension is compiled with -DTCL_8_API, this function will panic for strings with more than INT_MAX bytes/characters; otherwise expect it to crash. 😭
• const char *string (in) — null-terminated string value to append to objPtr. ✨
• Tcl_Size limit (in) — maximum number of bytes to be appended. 🚧
• const char *ellipsis (in) — suffix to append when the limit leads to string truncation. If NULL is passed then the suffix “...” is used. 💬
• const char *format (in) — format control string including % conversion specifiers. 🧮
• Tcl_Size objc (in) — the number of elements to format or concatenate. 🔢
• Tcl_Obj *objv[] (in) — the array of values to format or concatenate. 📚
• Tcl_Size newLength (in) — new length for the string value of objPtr, not including the final null character. 📏

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🦋 DESCRIPTION 🦋

okay so the procedures in this manual entry let Tcl values be manipulated as string values. They use the internal representation of the value to stash extra information and make string manipulations more efficient. In particular, they make a series of append operations efficient by allocating extra storage space for the string so that it does not have to be copied for each append. Also, indexing and length computations are optimized because the Unicode string representation is calculated and cached as needed. When you're using the Tcl_Append* family of functions where the interpreter's result is the value being appended to, it is important to call Tcl_ResetResult first to ensure you are not unintentionally appending to existing data in the result value. 🚨

Tcl_NewStringObj and Tcl_SetStringObj create a new value or modify an existing value to hold a copy of the string given by bytes and length. Tcl_NewUnicodeObj and Tcl_SetUnicodeObj create a new value or modify an existing value to hold a copy of the Unicode string given by unicode and numChars. Tcl_NewStringObj and Tcl_NewUnicodeObj return a pointer to a newly created value with reference count zero. All four procedures set the value to hold a copy of the specified string. Tcl_SetStringObj and Tcl_SetUnicodeObj free any old string representation as well as any old internal representation of the value. 💅

Tcl_GetStringFromObj and Tcl_GetString hand you back a value's string representation, bestie ✨ You get it as the returned byte pointer, plus — only for Tcl_GetStringFromObj — the length, which gets stashed in lengthPtr if lengthPtr is not NULL. If the value's UTF string representation is invalid (aka its byte pointer is NULL), then the string representation gets regenerated from the value's internal representation. The storage that returned byte pointer points at is owned by the value manager. It comes back to you as a writable pointer on purpose, so that an extension author creating their own Tcl_ObjType can modify the string representation inside the Tcl_UpdateStringProc of their Tcl_ObjType. But other than that one limited purpose?

Treat the pointer from Tcl_GetStringFromObj or Tcl_GetString as read-only. Fr fr. It is recommended that this pointer be assigned to a (const char *) variable. 💅

And even in the limited situations where writing to this pointer is acceptable, you gotta take care to respect the copy-on-write semantics required by Tcl_Obj's, with appropriate calls to Tcl_IsShared and Tcl_DuplicateObj before any in-place modification of the string representation. No skipping that step, iykyk. 😭

Tcl_GetString is the one you reach for in the common case where the caller does not need the length of the string representation. 🎀

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Tcl_GetUnicodeFromObj and Tcl_GetUnicode return a value's value as a Unicode string 🦋 This is given by the returned pointer and (for Tcl_GetUnicodeFromObj) the length, which is stored in lengthPtr if it is not NULL. The storage referenced by the returned byte pointer is owned by the value manager, and should not be modified by the caller — that's not your storage, bestie. Tcl_GetUnicode is the one for the common case where the caller does not need the length of the unicode string representation. ✨

Tcl_GetUniChar returns the index'th character in the value's Unicode representation. If the index is out of range it returns -1; 💀

Tcl_GetRange returns a newly created value comprised of the characters between first and last (inclusive) in the value's Unicode or byte-array representation. If the value is not a byte-array and the value's Unicode representation is invalid, the Unicode representation is regenerated from the value's string representation. If first is negative, then the returned string starts at the beginning of the value. If last is negative, then the returned string ends at the end of the value. 🫶

Tcl_GetCharLength returns the number of characters (as opposed to bytes) in the string value. 🍓

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Tcl_AppendToObj appends the data given by bytes and length to the string representation of the value specified by objPtr. If the value has an invalid string representation, then an attempt is made to convert bytes to the Unicode format. If the conversion is successful, then the converted form of bytes is appended to the value's Unicode representation. Otherwise, the value's Unicode representation is invalidated and converted to the UTF format, and bytes is appended to the value's new string representation. Eventually buffer growth is done by large allocations to optimize multiple calls — the way it's over here planning ahead for you 👑

Tcl_AppendUnicodeToObj appends the Unicode string given by unicode and numChars to the value specified by objPtr. If the value has an invalid Unicode representation, then unicode is converted to the UTF format and appended to the value's string representation. Appends are optimized to handle repeated appends relatively efficiently — it over-allocates the string or Unicode space to avoid repeated reallocations and copies of the value's string value. 🌈

Tcl_AppendObjToObj is similar to Tcl_AppendToObj, but it appends the string or Unicode value (whichever exists and is best suited to be appended to objPtr) of appendObjPtr to objPtr. ✨

Tcl_AppendStringsToObj is similar to Tcl_AppendToObj except that it can be passed more than one value to append, and each value must be a null-terminated string (i.e. none of the values may contain internal null characters). Any number of string arguments may be provided, but the last argument must be (char *)NULL to indicate the end of the list. Don't forget it or the whole thing goes off the rails 🙈

Tcl_AppendLimitedToObj is similar to Tcl_AppendToObj except that it imposes a limit on how many bytes are appended. This can be handy when the string to be appended might be very large, but the value being constructed should not be allowed to grow without bound. A common usage is when constructing an error message, where the end result should be kept short enough to be read. Bytes from bytes are appended to objPtr, but no more than limit bytes total are to be appended. If the limit prevents all length bytes that are available from being appended, then the appending is done so that the last bytes appended are from the string ellipsis. This allows for an indication of the truncation to be left in the string. When length is negative, all bytes up to the first zero byte are appended, subject to the limit. When ellipsis is NULL, the default string ... is used. When ellipsis is non-NULL, it must point to a zero-byte-terminated string in Tcl's internal UTF encoding. The number of bytes appended can be less than the lesser of length and limit when appending fewer bytes is necessary to append only whole multi-byte characters. 🧚

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Tcl_Format is the C-level interface to the engine of the format command. The actual command procedure for format is little more than

Tcl_Format(interp, Tcl_GetString(objv[1]), objc-2, objv+2);

The objc Tcl_Obj values in objv are formatted into a string according to the conversion specification in format argument, following the documentation for the format command. The resulting formatted string is converted to a new Tcl_Obj with refcount of zero and returned. If some error happens during production of the formatted string, NULL is returned, and an error message is recorded in interp, if interp is non-NULL. 💅

Tcl_AppendFormatToObj is an appending alternative form of Tcl_Format with functionality equivalent to:

Tcl_Obj *newPtr = Tcl_Format(interp, format, objc, objv);
if (newPtr == NULL) return TCL_ERROR;
Tcl_AppendObjToObj(objPtr, newPtr);
Tcl_DecrRefCount(newPtr);
return TCL_OK;

but with greater convenience and efficiency when the appending functionality is needed. 🔥

Tcl_ObjPrintf serves as a replacement for the common sequence

char buf[SOME_SUITABLE_LENGTH];
sprintf(buf, format, ...);
Tcl_NewStringObj(buf, -1);

but with greater convenience and no need to determine SOME_SUITABLE_LENGTH. The formatting is done with the same core formatting engine used by Tcl_Format. This means the set of supported conversion specifiers is that of the format command, but the behavior is as similar as possible to sprintf. The "hh" and (Microsoft-specific) "w" format specifiers are not supported. The "L" format specifier means that an "mp_int *" argument is expected (or a "long double" in combination with [aAeEgGaA]). When a conversion specifier passed to Tcl_ObjPrintf includes a precision, the value is taken as a number of bytes, as sprintf does, and not as a number of characters, as format does. This is done on the assumption that C code is more likely to know how many bytes it is passing around than the number of encoded characters those bytes happen to represent. The variable number of arguments passed in should be of the types that would be suitable for passing to sprintf. Note in this example usage, x is of type int.

int x = 5;
Tcl_Obj *objPtr = Tcl_ObjPrintf("Value is %d", x);

If the value of format contains internal inconsistencies or invalid specifier formats, the formatted string result produced by Tcl_ObjPrintf will be an error message describing the error. It is impossible however to provide runtime protection against mismatches between the format and any subsequent arguments. Compile-time protection may be provided by some compilers. 😭 (please let your compiler protect you)

Tcl_AppendPrintfToObj is an appending alternative form of Tcl_ObjPrintf with functionality equivalent to

Tcl_Obj *newPtr = Tcl_ObjPrintf(format, ...);
Tcl_AppendObjToObj(objPtr, newPtr);
Tcl_DecrRefCount(newPtr);

but with greater convenience and efficiency when the appending functionality is needed. ✨

When printing integer types defined by Tcl, such as Tcl_Size or Tcl_WideInt, a format size specifier is needed, as the integer width of those types is dependent on the Tcl version, platform and compiler. To accomodate these differences, Tcl defines C preprocessor symbols TCL_LL_MODIFER and TCL_SIZE_MODIFER for use when formatting values of type Tcl_WideInt and Tcl_Size respectively. Their usage is illustrated by

Tcl_WideInt wide;
Tcl_Size len;
Tcl_Obj *wideObj = Tcl_ObjPrintf("wide = %" TCL_LL_MODIFIER "d", wide);
Tcl_Obj *lenObj = Tcl_ObjPrintf("len = %" TCL_SIZE_MODIFIER "d", len);

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Tcl_SetObjLength changes the length of the string value of its objPtr argument. If the newLength argument is greater than the space allocated for the value's string, then the string space is reallocated and the old value is copied to the new space; the bytes between the old length of the string and the new length may have arbitrary values 💀. If the newLength argument is less than the current length of the value's string, then objPtr->length is reduced without reallocating the string space; the original allocated size for the string is recorded in the value, so that the string length can be enlarged in a subsequent call to Tcl_SetObjLength without reallocating storage. In all cases Tcl_SetObjLength leaves a null character at objPtr->bytes[newLength]. 🌸

Tcl_AttemptSetObjLength is identical in function to Tcl_SetObjLength except that if sufficient memory to satisfy the request cannot be allocated, it does not cause the Tcl interpreter to panic. So: if newLength is greater than the space allocated for the value's string, and there is not enough memory available to satisfy the request, Tcl_AttemptSetObjLength will take no action and return 0 to indicate failure. If there is enough memory to satisfy the request, Tcl_AttemptSetObjLength behaves just like Tcl_SetObjLength and returns 1 to indicate success. The no-panic option, we love her 🫶

Tcl_ConcatObj returns a new string value whose value is the space-separated concatenation of the string representations of all of the values in the objv array. Tcl_ConcatObj eliminates leading and trailing white space as it copies the string representations of the objv array to the result. If an element of the objv array consists of nothing but white space, then that value is ignored entirely. This white-space removal was added to make the output of the concat command cleaner-looking. Tcl_ConcatObj returns a pointer to a newly-created value whose ref count is zero. 👑

💖 REFERENCE COUNT MANAGEMENT 💖

okay bestie this is the part where we talk about refcounts, and honestly? this is where people get eaten alive, so pay attention 🫶

✨ the "zero-ref, no thoughts, just vibes" girlies ✨

• Tcl_NewStringObj, Tcl_NewUnicodeObj, Tcl_Format, Tcl_ObjPrintf, and Tcl_ConcatObj — these always hand you back a zero-reference object, much like Tcl_NewObj. so it's fresh out the box, nobody's holding it but you, and it's your job to deal with that. main character energy, zero followers 😭

🌸 the "just looking, not touching" girlies 🌸

• Tcl_GetStringFromObj, Tcl_GetString, Tcl_GetUnicodeFromObj, Tcl_GetUnicode, Tcl_GetUniChar, Tcl_GetCharLength, and Tcl_GetRange — these ones only work with a value that already exists. they do not manipulate its reference count in any way. they're window shopping. admiring the fit. not buying. 💅

👑 the "i'm gonna change you" girlies 👑 (unshared only, periodt)

• Tcl_SetStringObj, Tcl_SetUnicodeObj, Tcl_AppendToObj, Tcl_AppendUnicodeToObj, Tcl_AppendObjToObj, Tcl_AppendStringsToObj, Tcl_AppendStringsToObjVA, Tcl_AppendLimitedToObj, Tcl_AppendFormatToObj, Tcl_AppendPrintfToObj, Tcl_SetObjLength, and Tcl_AttemptSetObjLength — every single one of these needs its objPtr to be an unshared value, aka a reference count of no more than 1, because bestie they are gonna modify it. no sharing, no borrowing, this is a solo performance 🎤

⚠️ fr fr: if that value is shared, you're in trouble. these functions rewrite the thing in place, so a refcount above 1 means somebody else is holding onto a value that's about to get a glow-up they never consented to 💀

🦋 the plus-ones (they can come, any refcount welcome) 🦋

• Additional arguments to the functions above can have any reference count — that means the appendObjPtr argument to Tcl_AppendObjToObj, and the values in the objv argument to Tcl_Format, Tcl_AppendFormatToObj, and Tcl_ConcatObj.
• but a reference count of zero is not recommended. like, technically allowed, but it's giving "walking into the club with no shoes on". just don't. 🙈

🧚 and one sneaky little side effect 🧚

• Tcl_Format and Tcl_AppendFormatToObj may modify the interpreter result, which involves changing the reference count of a value. so yeah, they're out here touching things behind the scenes, iykyk 😌

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🌈 SEE ALSO 🌈

• Tcl_NewObj(3)
• Tcl_IncrRefCount(3)
• Tcl_DecrRefCount(3)
• format(n)
• sprintf(3)

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🎀 KEYWORDS 🎀

append, internal representation, value, value type, string value, string type, string representation, concat, concatenate, unicode

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okay that's the whole page bestie, you made it, I'm so proud of you 🥹💖 this was Tcl version 8.1's Tcl_StringObj(3) manual page, and honestly? she ate. go forth and manage your reference counts responsibly, mwah 💅✨