gh-71086: Document negative offset support in struct.pack_into() and unpack_from() (GH-153650)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
(cherry picked from commit 1706d14)

Author: serhiy-storchaka

Doc/library/struct.rst

@@ -68,6 +68,7 @@ The module defines the following exception and functions:
    Pack the values *v1*, *v2*, ... according to the format string *format* and
    write the packed bytes into the writable buffer *buffer* starting at
    position *offset*.  Note that *offset* is a required argument.
+   A negative *offset* counts from the end of *buffer*.
 
 
 .. function:: unpack(format, buffer)
@@ -84,6 +85,7 @@ The module defines the following exception and functions:
    string *format*.  The result is a tuple even if it contains exactly one
    item.  The buffer's size in bytes, starting at position *offset*, must be at
    least the size required by the format, as reflected by :func:`calcsize`.
+   A negative *offset* counts from the end of *buffer*.
 
 
 .. function:: iter_unpack(format, buffer)

Modules/_struct.c

@@ -1758,15 +1758,16 @@ Return a tuple containing unpacked values.
 Values are unpacked according to the format string Struct.format.
 
 The buffer's size in bytes, starting at position offset, must be
-at least Struct.size.
+at least Struct.size.  A negative offset counts from the end of the
+buffer.
 
 See help(struct) for more on format strings.
 [clinic start generated code]*/
 
 static PyObject *
 Struct_unpack_from_impl(PyStructObject *self, Py_buffer *buffer,
                         Py_ssize_t offset)
-/*[clinic end generated code: output=57fac875e0977316 input=cafd4851d473c894]*/
+/*[clinic end generated code: output=57fac875e0977316 input=3451f778ed8a5345]*/
 {
     _structmodulestate *state = get_struct_state_structinst(self);
     ENSURE_STRUCT_IS_READY(self);
@@ -2085,8 +2086,9 @@ PyDoc_STRVAR(s_pack_into__doc__,
 \n\
 Pack the values v1, v2, ... according to the format string S.format\n\
 and write the packed bytes into the writable buffer buf starting at\n\
-offset.  Note that the offset is a required argument.  See\n\
-help(struct) for more on format strings.");
+offset.  Note that the offset is a required argument.  A negative\n\
+offset counts from the end of the buffer.  See help(struct) for more\n\
+on format strings.");
 
 static PyObject *
 s_pack_into(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
@@ -2378,8 +2380,8 @@ PyDoc_STRVAR(pack_into_doc,
 \n\
 Pack the values v1, v2, ... according to the format string and write\n\
 the packed bytes into the writable buffer buf starting at offset.  Note\n\
-that the offset is a required argument.  See help(struct) for more\n\
-on format strings.");
+that the offset is a required argument.  A negative offset counts from\n\
+the end of the buffer.  See help(struct) for more on format strings.");
 
 static PyObject *
 pack_into(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
@@ -2433,14 +2435,15 @@ unpack_from
 Return a tuple containing values unpacked according to the format string.
 
 The buffer's size, minus offset, must be at least calcsize(format).
+A negative offset counts from the end of the buffer.
 
 See help(struct) for more on format strings.
 [clinic start generated code]*/
 
 static PyObject *
 unpack_from_impl(PyObject *module, PyStructObject *s_object,
                  Py_buffer *buffer, Py_ssize_t offset)
-/*[clinic end generated code: output=1042631674c6e0d3 input=6e80a5398e985025]*/
+/*[clinic end generated code: output=1042631674c6e0d3 input=e383ecb97728be2b]*/
 {
     return Struct_unpack_from_impl(s_object, buffer, offset);
 }