NAME¶

Tcl_NewByteArrayObj, Tcl_SetByteArrayObj, Tcl_GetBytesFromObj, Tcl_GetByteArrayFromObj, Tcl_SetByteArrayLength - manipulate a Tcl value as an array of bytes

SYNOPSIS¶

#include <tcl.h>

Tcl_Obj *
Tcl_NewByteArrayObj(bytes, numBytes)

void
Tcl_SetByteArrayObj(objPtr, bytes, numBytes)

unsigned char *
Tcl_GetBytesFromObj(interp, objPtr, numBytesPtr)

unsigned char *
Tcl_GetByteArrayFromObj(objPtr, numBytesPtr)

unsigned char *
Tcl_SetByteArrayLength(objPtr, numBytes)

ARGUMENTS¶

const unsigned char *bytes (in)

The array of bytes used to initialize or set a byte-array value. May be NULL even if numBytes is non-zero.

Tcl_Size numBytes (in)

The number of bytes in the array.

Tcl_Obj *objPtr (in/out)

For Tcl_SetByteArrayObj, this points to an unshared value to be overwritten by a byte-array value. For Tcl_GetBytesFromObj, Tcl_GetByteArrayFromObj and Tcl_SetByteArrayLength, this points to the value from which to extract an array of bytes.

Tcl_Interp *interp (in)

Interpreter to use for error reporting.

Tcl_Size | int *numBytesPtr (out)

Points to space where the number of bytes in the array may be written. May be (Tcl_Size *)NULL when not used. If it points to a variable which type is not Tcl_Size, a compiler warning will be generated. If your extensions is compiled with -DTCL_8_API, this function will return NULL for byte arrays larger than INT_MAX (which should trigger proper error-handling), otherwise expect it to crash.

DESCRIPTION¶

These routines are used to create, modify, store, transfer, and retrieve arbitrary binary data in Tcl values. Specifically, data that can be represented as a sequence of arbitrary byte values is supported. This includes data read from binary channels, values created by the binary command, encrypted data, or other information representable as a finite byte sequence.

A byte is an 8-bit quantity with no inherent meaning. When the 8 bits are interpreted as an integer value, the range of possible values is (0-255). The C type best suited to store a byte is the unsigned char. An unsigned char array of size N stores an aribtrary binary value of size N bytes. We call this representation a byte-array. Here we document the routines that allow us to operate on Tcl values as byte-arrays.

All Tcl values must correspond to a string representation. When a byte-array value must be processed as a string, the sequence of N bytes is transformed into the corresponding sequence of N characters, where each byte value transforms to the same character codepoint value in the range (U+0000 - U+00FF). Obtaining the string representation of a byte-array value (by calling Tcl_GetStringFromObj) produces this string in Tcl's usual Modified UTF-8 encoding.

Tcl_NewByteArrayObj and Tcl_SetByteArrayObj create a new value or overwrite an existing unshared value, respectively, to hold a byte-array value of numBytes bytes. When a caller passes a non-NULL value of bytes, it must point to memory from which numBytes bytes can be read. These routines allocate numBytes bytes of memory, copy numBytes bytes from bytes into it, and keep the result in the internal representation of the new or overwritten value. When the caller passes a NULL value of bytes, the data copying step is skipped, and the bytes stored in the value are undefined. A bytes value of NULL is useful only when the caller will arrange to write known contents into the byte-array through a pointer retrieved by a call to one of the routines explained below. Tcl_NewByteArrayObj returns a pointer to the created value with a reference count of zero. Tcl_SetByteArrayObj overwrites and invalidates any old contents of the unshared objPtr as appropriate, and keeps its reference count (0 or 1) unchanged. The value produced by these routines has no string representation. Any memory allocation failure may cause a panic.

Tcl_GetBytesFromObj performs the opposite function of Tcl_SetByteArrayObj, providing access to read a byte-array from a Tcl value that was previously written into it. When objPtr is a value previously produced by Tcl_NewByteArrayObj or Tcl_SetByteArrayObj, then Tcl_GetBytesFromObj returns a pointer to the byte-array kept in the value's internal representation. If the caller provides a non-NULL value for numBytesPtr, it must point to memory where Tcl_GetBytesFromObj can write the number of bytes in the value's internal byte-array. With both pieces of information, the caller is able to retrieve any information about the contents of that byte-array that it seeks. When objPtr does not already contain an internal byte-array, Tcl_GetBytesFromObj will try to create one from the value's string representation. Any string value that does not include any character codepoints outside the range (U+0000 - U+00FF) will successfully translate to a unique byte-array value. With the created byte-array, the routine returns as before. For any string representation which does contain a forbidden character codepoint, the conversion fails, and Tcl_GetBytesFromObj returns NULL to signal that failure. On failure, nothing will be written to numBytesPtr, and if the interp argument is non-NULL, then error messages and codes are left in it recording the error.

Tcl_GetByteArrayFromObj performs exactly the same function as Tcl_GetBytesFromObj does when called with the interp argument passed the value NULL. This is incompatible with the way Tcl_GetByteArrayFromObj functioned in Tcl 8. Tcl_GetBytesFromObj is the more capable interface and should usually be favored for use over Tcl_GetByteArrayFromObj.

On success, both Tcl_GetByteFromObj and Tcl_GetByteArrayFromObj return a pointer into the internal representation of a Tcl_Obj. That pointer must not be freed by the caller, and should not be retained for use beyond the known time the internal representation of the value has not been disturbed. The pointer may be used to overwrite the byte contents of the internal representation, so long as the value is unshared and any string representation is invalidated.

On success, both Tcl_GetBytesFromObj and Tcl_GetByteArrayFromObj write the number of bytes in the byte-array value of objPtr to the space pointed to by numBytesPtr. This space may be of type Tcl_Size or of type int. It is recommended that callers provide a Tcl_Size space for this purpose. If the caller provides only an int space and the number of bytes in the byte-array value of objPtr is greater than INT_MAX, the routine will fail due to being unable to correctly report the byte-array size to the caller. The ability to provide an int space is best considered a migration aid for codebases constrained to continue operating with Tcl releases older than 8.7.

Tcl_SetByteArrayLength enables a caller to change the size of a byte-array in the internal representation of an unshared objPtr to become numBytes bytes. This is most often useful after the bytes of the internal byte-array have been directly overwritten and it has been discovered that the required size differs from the first estimate used in the allocation. Tcl_SetByteArrayLength returns a pointer to the resized byte-array. Because resizing the byte-array changes the internal representation, Tcl_SetByteArrayLength also invalidates any string representation in objPtr. If resizing grows the byte-array, the new byte values are undefined. If objPtr does not already possess an internal byte-array, one is produced in the same way that Tcl_GetBytesFromObj does, also returning NULL when any characters of the value in objPtr (up to numBytes of them) are not valid bytes.

REFERENCE COUNT MANAGEMENT¶

Tcl_NewByteArrayObj always returns a zero-reference object, much like Tcl_NewObj.

Tcl_SetByteArrayObj and Tcl_SetByteArrayLength do not modify the reference count of their objPtr arguments, but do require that the object be unshared.

Tcl_GetBytesFromObj and Tcl_GetByteArrayFromObj do not modify the reference count of objPtr; they only read.

SEE ALSO¶

Tcl_GetStringFromObj, Tcl_NewObj, Tcl_IncrRefCount, Tcl_DecrRefCount

KEYWORDS¶

value, binary data, byte array, utf, unicode