Tcl_DStringInit(), Tcl_DStringAppend(), Tcl_DStringAppendElement(), Tcl_DStringStartSublist(), Tcl_DStringEndSublist(), Tcl_DStringLength(), Tcl_DStringValue(), Tcl_DStringSetLength(), Tcl_DStringFree(), Tcl_DStringResult(), Tcl_DStringGetResult()

manipulate dynamic strings 

Tcl Library Procedures


#include <tcl.h>


char * Tcl_DStringAppend(dsPtr, string, length)

char * Tcl_DStringAppendElement(dsPtr, string)



int Tcl_DStringLength(dsPtr)

char * Tcl_DStringValue(dsPtr)

Tcl_DStringSetLength(dsPtr, newLength)

Tcl_DStringTrunc(dsPtr, newLength)


Tcl_DStringResult(interp, dsPtr)

Tcl_DStringGetResult(interp, dsPtr)


Tcl_DString *dsPtr (in/out) 

Pointer to structure that is used to manage a dynamic string.

char *string (in) 

Pointer to characters to add to dynamic string.

int length (in) 

Number of characters from string to add to dynamic string. If -1, add all characters up to null terminating character.

int newLength (in) 

New length for dynamic string, not including null terminating character.

Tcl_Interp *interp (in/out) 

Interpreter whose result is to be set from or moved to the dynamic string.


Dynamic strings provide a mechanism for building up arbitrarily long strings by gradually appending information. If the dynamic string is short then there will be no memory allocation overhead; as the string gets larger, additional space will be allocated as needed.

Tcl_DStringInit() initializes a dynamic string to zero length. The Tcl_DString structure must have been allocated by the caller. No assumptions are made about the current state of the structure; anything already in it is discarded. If the structure has been used previously, Tcl_DStringFree() should be called first to free up any memory allocated for the old string.

Tcl_DStringAppend() adds new information to a dynamic string, allocating more memory for the string if needed. If length is less than zero then everything in string is appended to the dynamic string; otherwise length specifies the number of bytes to append. Tcl_DStringAppend() returns a pointer to the characters of the new string. The string can also be retrieved from the string field of the Tcl_DString structure.

Tcl_DStringAppendElement() is similar to Tcl_DStringAppend() except that it doesn't take a length argument (it appends all of string) and it converts the string to a proper list element before appending. Tcl_DStringAppendElement() adds a separator space before the new list element unless the new list element is the first in a list or sub-list (that is, either the current string is empty, or it contains the single character {, or the last two characters of the current string are {). Tcl_DStringAppendElement() returns a pointer to the characters of the new string.

Tcl_DStringStartSublist() and Tcl_DStringEndSublist() can be used to create nested lists. To append a list element that is itself a sublist, first call Tcl_DStringStartSublist(), then call Tcl_DStringAppendElement() for each of the elements in the sublist, then call Tcl_DStringEndSublist() to end the sublist. Tcl_DStringStartSublist() appends a space character if needed, followed by an open brace; Tcl_DStringEndSublist() appends a close brace. Lists can be nested to any depth.

Tcl_DStringLength() is a macro that returns the current length of a dynamic string (not including the terminating null character). Tcl_DStringValue() is a macro that returns a pointer to the current contents of a dynamic string.

Tcl_DStringSetLength() changes the length of a dynamic string. If newLength is less than the string's current length, then the string is truncated. If newLength is greater than the string's current length, then the string will become longer and new space will be allocated for the string if needed. However, Tcl_DStringSetLength() will not initialize the new space except to provide a terminating null character; it is up to the caller to fill in the new space. Tcl_DStringSetLength() does not free up the string's storage space even if the string is truncated to zero length, so Tcl_DStringFree() will still need to be called.

Tcl_DStringTrunc changes the length of a dynamic string. This procedure is now deprecated. Tcl_DStringSetLength() should be used instead.

Tcl_DStringFree() should be called when you're finished using the string. It frees up any memory that was allocated for the string and reinitializes the string's value to an empty string.

Tcl_DStringResult() sets the result of interp to the value of the dynamic string given by dsPtr. It does this by moving a pointer from dsPtr to the interpreter's result. This saves the cost of allocating new memory and copying the string. Tcl_DStringResult() also reinitializes the dynamic string to an empty string.

Tcl_DStringGetResult() does the opposite of Tcl_DStringResult(). It sets the value of dsPtr to the result of interp and it clears interp's result. If possible it does this by moving a pointer rather than by copying the string.


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


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