Added fast alternate io.BytesIO implementation and its test suite.

Removed old test suite for StringIO.
Modified truncate() to imply a seek to given argument value.

2 files changed, 760 insertions, 48 deletions

diff --git a/Modules/_bytesio.c b/Modules/_bytesio.c
new file mode 100644
index 0000000..00cb06b
--- /dev/null
+++ b/Modules/_bytesio.c
@@ -0,0 +1,744 @@
+#include "Python.h"
+
+typedef struct {
+    PyObject_HEAD
+    char *buf;
+    Py_ssize_t pos;
+    Py_ssize_t string_size;
+    size_t buf_size;
+} BytesIOObject;
+
+#define CHECK_CLOSED(self)                                  \
+    if ((self)->buf == NULL) {                              \
+        PyErr_SetString(PyExc_ValueError,                   \
+                        "I/O operation on closed file.");   \
+        return NULL;                                        \
+    }
+
+/* Internal routine to get a line from the buffer of a BytesIO
+   object. Returns the length between the current position to the
+   next newline character. */
+static Py_ssize_t
+get_line(BytesIOObject *self, char **output)
+{
+    char *n;
+    const char *str_end;
+    Py_ssize_t len;
+
+    assert(self->buf != NULL);
+
+    /* Move to the end of the line, up to the end of the string, s. */
+    str_end = self->buf + self->string_size;
+    for (n = self->buf + self->pos;
+         n < str_end && *n != '\n';
+         n++);
+
+    /* Skip the newline character */
+    if (n < str_end)
+        n++;
+
+    /* Get the length from the current position to the end of the line. */
+    len = n - (self->buf + self->pos);
+    *output = self->buf + self->pos;
+
+    assert(len >= 0);
+    assert(self->pos < PY_SSIZE_T_MAX - len);
+    self->pos += len;
+
+    return len;
+}
+
+/* Internal routine for changing the size of the buffer of BytesIO objects.
+   The caller should ensure that the 'size' argument is non-negative.  Returns
+   0 on success, -1 otherwise. */
+static int
+resize_buffer(BytesIOObject *self, size_t size)
+{
+    /* Here, unsigned types are used to avoid dealing with signed integer
+       overflow, which is undefined in C. */
+    size_t alloc = self->buf_size;
+    char *new_buf = NULL;
+
+    assert(self->buf != NULL);
+
+    /* For simplicity, stay in the range of the signed type. Anyway, Python
+       doesn't allow strings to be longer than this. */
+    if (size > PY_SSIZE_T_MAX)
+        goto overflow;
+
+    if (size < alloc / 2) {
+        /* Major downsize; resize down to exact size. */
+        alloc = size + 1;
+    }
+    else if (size < alloc) {
+        /* Within allocated size; quick exit */
+        return 0;
+    }
+    else if (size <= alloc * 1.125) {
+        /* Moderate upsize; overallocate similar to list_resize() */
+        alloc = size + (size >> 3) + (size < 9 ? 3 : 6);
+    }
+    else {
+        /* Major upsize; resize up to exact size */
+        alloc = size + 1;
+    }
+
+    if (alloc > ((size_t)-1) / sizeof(char))
+        goto overflow;
+    new_buf = (char *)PyMem_Realloc(self->buf, alloc * sizeof(char));
+    if (new_buf == NULL) {
+        PyErr_NoMemory();
+        return -1;
+    }
+    self->buf_size = alloc;
+    self->buf = new_buf;
+
+    return 0;
+
+  overflow:
+    PyErr_SetString(PyExc_OverflowError,
+                    "new buffer size too large");
+    return -1;
+}
+
+/* Internal routine for writing a string of bytes to the buffer of a BytesIO
+   object. Returns the number of bytes wrote, or -1 on error. */
+static Py_ssize_t
+write_bytes(BytesIOObject *self, const char *bytes, Py_ssize_t len)
+{
+    assert(self->buf != NULL);
+    assert(self->pos >= 0);
+    assert(len >= 0);
+
+    /* This overflow check is not strictly necessary. However, it avoids us to
+       deal with funky things like comparing an unsigned and a signed
+       integer. */
+    if (self->pos > PY_SSIZE_T_MAX - len) {
+        PyErr_SetString(PyExc_OverflowError,
+                        "new position too large");
+        return -1;
+    }
+    if (self->pos + len > self->buf_size) {
+        if (resize_buffer(self, self->pos + len) < 0)
+            return -1;
+    }
+
+    if (self->pos > self->string_size) {
+        /* In case of overseek, pad with null bytes the buffer region between
+           the end of stream and the current position.
+
+          0   lo      string_size                           hi
+          |   |<---used--->|<----------available----------->|
+          |   |            <--to pad-->|<---to write--->    |
+          0   buf                   position
+        */
+        memset(self->buf + self->string_size, '\0',
+               (self->pos - self->string_size) * sizeof(char));
+    }
+
+    /* Copy the data to the internal buffer, overwriting some of the existing
+       data if self->pos < self->string_size. */
+    memcpy(self->buf + self->pos, bytes, len);
+    self->pos += len;
+
+    /* Set the new length of the internal string if it has changed. */
+    if (self->string_size < self->pos) {
+        self->string_size = self->pos;
+    }
+
+    return len;
+}
+
+static PyObject *
+bytesio_get_closed(BytesIOObject *self)
+{
+    if (self->buf == NULL)
+        Py_RETURN_TRUE;
+    else
+        Py_RETURN_FALSE;
+}
+
+/* Generic getter for the writable, readable and seekable properties */
+static PyObject *
+return_true(BytesIOObject *self)
+{
+    Py_RETURN_TRUE;
+}
+
+PyDoc_STRVAR(flush_doc,
+"flush() -> None.  Does nothing.");
+
+static PyObject *
+bytesio_flush(BytesIOObject *self)
+{
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(getval_doc,
+"getvalue() -> string.\n"
+"\n"
+"Retrieve the entire contents of the BytesIO object.");
+
+static PyObject *
+bytesio_getvalue(BytesIOObject *self)
+{
+    CHECK_CLOSED(self);
+    return PyString_FromStringAndSize(self->buf, self->string_size);
+}
+
+PyDoc_STRVAR(isatty_doc,
+"isatty() -> False.\n"
+"\n"
+"Always returns False since BytesIO objects are not connected\n"
+"to a tty-like device.");
+
+static PyObject *
+bytesio_isatty(BytesIOObject *self)
+{
+    CHECK_CLOSED(self);
+    Py_RETURN_FALSE;
+}
+
+PyDoc_STRVAR(tell_doc,
+"tell() -> current file position, an integer\n");
+
+static PyObject *
+bytesio_tell(BytesIOObject *self)
+{
+    CHECK_CLOSED(self);
+    return PyLong_FromSsize_t(self->pos);
+}
+
+PyDoc_STRVAR(read_doc,
+"read([size]) -> read at most size bytes, returned as a string.\n"
+"\n"
+"If the size argument is negative, read until EOF is reached.\n"
+"Return an empty string at EOF.");
+
+static PyObject *
+bytesio_read(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t size, n;
+    char *output;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:read", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        size = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* Read until EOF is reached, by default. */
+        size = -1;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    /* adjust invalid sizes */
+    n = self->string_size - self->pos;
+    if (size < 0 || size > n) {
+        size = n;
+        if (size < 0)
+            size = 0;
+    }
+
+    assert(self->buf != NULL);
+    output = self->buf + self->pos;
+    self->pos += size;
+
+    return PyString_FromStringAndSize(output, size);
+}
+
+
+PyDoc_STRVAR(read1_doc,
+"read1(size) -> read at most size bytes, returned as a string.\n"
+"\n"
+"If the size argument is negative or omitted, read until EOF is reached.\n"
+"Return an empty string at EOF.");
+
+static PyObject *
+bytesio_read1(BytesIOObject *self, PyObject *n)
+{
+    PyObject *arg, *res;
+
+    arg = PyTuple_Pack(1, n);
+    if (arg == NULL)
+        return NULL;
+    res  = bytesio_read(self, arg);
+    Py_DECREF(arg);
+    return res;
+}
+
+PyDoc_STRVAR(readline_doc,
+"readline([size]) -> next line from the file, as a string.\n"
+"\n"
+"Retain newline.  A non-negative size argument limits the maximum\n"
+"number of bytes to return (an incomplete line may be returned then).\n"
+"Return an empty string at EOF.\n");
+
+static PyObject *
+bytesio_readline(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t size, n;
+    char *output;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:readline", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        size = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* No size limit, by default. */
+        size = -1;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    n = get_line(self, &output);
+
+    if (size >= 0 && size < n) {
+        size = n - size;
+        n -= size;
+        self->pos -= size;
+    }
+
+    return PyString_FromStringAndSize(output, n);
+}
+
+PyDoc_STRVAR(readlines_doc,
+"readlines([size]) -> list of strings, each a line from the file.\n"
+"\n"
+"Call readline() repeatedly and return a list of the lines so read.\n"
+"The optional size argument, if given, is an approximate bound on the\n"
+"total number of bytes in the lines returned.\n");
+
+static PyObject *
+bytesio_readlines(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t maxsize, size, n;
+    PyObject *result, *line;
+    char *output;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:readlines", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        maxsize = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* No size limit, by default. */
+        maxsize = -1;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    size = 0;
+    result = PyList_New(0);
+    if (!result)
+        return NULL;
+
+    while ((n = get_line(self, &output)) != 0) {
+        line = PyString_FromStringAndSize(output, n);
+        if (!line)
+            goto on_error;
+        if (PyList_Append(result, line) == -1) {
+            Py_DECREF(line);
+            goto on_error;
+        }
+        Py_DECREF(line);
+        size += n;
+        if (maxsize > 0 && size >= maxsize)
+            break;
+    }
+    return result;
+
+  on_error:
+    Py_DECREF(result);
+    return NULL;
+}
+
+PyDoc_STRVAR(readinto_doc,
+"readinto(bytes) -> int.  Read up to len(b) bytes into b.\n"
+"\n"
+"Returns number of bytes read (0 for EOF), or None if the object\n"
+"is set not to block as has no data to read.");
+
+static PyObject *
+bytesio_readinto(BytesIOObject *self, PyObject *buffer)
+{
+    void *raw_buffer;
+    Py_ssize_t len;
+
+    CHECK_CLOSED(self);
+
+    if (PyObject_AsWriteBuffer(buffer, &raw_buffer, &len) == -1)
+        return NULL;
+
+    if (self->pos + len > self->string_size)
+        len = self->string_size - self->pos;
+
+    memcpy(raw_buffer, self->buf + self->pos, len);
+    assert(self->pos + len < PY_SSIZE_T_MAX);
+    assert(len >= 0);
+    self->pos += len;
+
+    return PyLong_FromSsize_t(len);
+}
+
+PyDoc_STRVAR(truncate_doc,
+"truncate([size]) -> int.  Truncate the file to at most size bytes.\n"
+"\n"
+"Size defaults to the current file position, as returned by tell().\n"
+"Returns the new size.  Imply an absolute seek to the position size.");
+
+static PyObject *
+bytesio_truncate(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t size;
+    PyObject *arg = Py_None;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "|O:truncate", &arg))
+        return NULL;
+
+    if (PyLong_Check(arg)) {
+        size = PyLong_AsSsize_t(arg);
+    }
+    else if (arg == Py_None) {
+        /* Truncate to current position if no argument is passed. */
+        size = self->pos;
+    }
+    else {
+        PyErr_Format(PyExc_TypeError, "integer argument expected, got '%s'",
+                     Py_TYPE(arg)->tp_name);
+        return NULL;
+    }
+
+    if (size < 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "negative size value %zd", size);
+        return NULL;
+    }
+
+    if (size < self->string_size) {
+        self->string_size = size;
+        if (resize_buffer(self, size) < 0)
+            return NULL;
+    }
+    self->pos = size;
+
+    return PyLong_FromSsize_t(size);
+}
+
+static PyObject *
+bytesio_iternext(BytesIOObject *self)
+{
+    char *next;
+    Py_ssize_t n;
+
+    CHECK_CLOSED(self);
+
+    n = get_line(self, &next);
+
+    if (!next || n == 0)
+        return NULL;
+
+    return PyString_FromStringAndSize(next, n);
+}
+
+PyDoc_STRVAR(seek_doc,
+"seek(pos, whence=0) -> int.  Change stream position.\n"
+"\n"
+"Seek to byte offset pos relative to position indicated by whence:\n"
+"     0  Start of stream (the default).  pos should be >= 0;\n"
+"     1  Current position - pos may be negative;\n"
+"     2  End of stream - pos usually negative.\n"
+"Returns the new absolute position.");
+
+static PyObject *
+bytesio_seek(BytesIOObject *self, PyObject *args)
+{
+    Py_ssize_t pos;
+    int mode = 0;
+
+    CHECK_CLOSED(self);
+
+    if (!PyArg_ParseTuple(args, "n|i:seek", &pos, &mode))
+        return NULL;
+
+    if (pos < 0 && mode == 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "negative seek value %zd", pos);
+        return NULL;
+    }
+
+    /* mode 0: offset relative to beginning of the string.
+       mode 1: offset relative to current position.
+       mode 2: offset relative the end of the string. */
+    if (mode == 1) {
+        if (pos > PY_SSIZE_T_MAX - self->pos) {
+            PyErr_SetString(PyExc_OverflowError,
+                            "new position too large");
+            return NULL;
+        }
+        pos += self->pos;
+    }
+    else if (mode == 2) {
+        if (pos > PY_SSIZE_T_MAX - self->string_size) {
+            PyErr_SetString(PyExc_OverflowError,
+                            "new position too large");
+            return NULL;
+        }
+        pos += self->string_size;
+    }
+    else if (mode != 0) {
+        PyErr_Format(PyExc_ValueError,
+                     "invalid whence (%i, should be 0, 1 or 2)", mode);
+        return NULL;
+    }
+
+    if (pos < 0)
+        pos = 0;
+    self->pos = pos;
+
+    return PyLong_FromSsize_t(self->pos);
+}
+
+PyDoc_STRVAR(write_doc,
+"write(str) -> int.  Write string str to file.\n"
+"\n"
+"Return the number of bytes written.");
+
+static PyObject *
+bytesio_write(BytesIOObject *self, PyObject *obj)
+{
+    const char *bytes;
+    Py_ssize_t size;
+    Py_ssize_t n = 0;
+
+    CHECK_CLOSED(self);
+
+    if (PyObject_AsReadBuffer(obj, (void *)&bytes, &size) < 0)
+        return NULL;
+
+    if (size != 0) {
+        n = write_bytes(self, bytes, size);
+        if (n < 0)
+            return NULL;
+    }
+
+    return PyLong_FromSsize_t(n);
+}
+
+PyDoc_STRVAR(writelines_doc,
+"writelines(sequence_of_strings) -> None.  Write the strings to the file.\n"
+"\n"
+"Note that newlines are not added.  The sequence can be any iterable object\n"
+"producing strings. This is equivalent to calling write() for each string.");
+
+static PyObject *
+bytesio_writelines(BytesIOObject *self, PyObject *v)
+{
+    PyObject *it, *item;
+    PyObject *ret;
+
+    CHECK_CLOSED(self);
+
+    it = PyObject_GetIter(v);
+    if (it == NULL)
+        return NULL;
+
+    while ((item = PyIter_Next(it)) != NULL) {
+        ret = bytesio_write(self, item);
+        Py_DECREF(item);
+        if (ret == NULL) {
+            Py_DECREF(it);
+            return NULL;
+        }
+        Py_DECREF(ret);
+    }
+    Py_DECREF(it);
+
+    /* See if PyIter_Next failed */
+    if (PyErr_Occurred())
+        return NULL;
+
+    Py_RETURN_NONE;
+}
+
+PyDoc_STRVAR(close_doc,
+"close() -> None.  Disable all I/O operations.");
+
+static PyObject *
+bytesio_close(BytesIOObject *self)
+{
+    if (self->buf != NULL) {
+        PyMem_Free(self->buf);
+        self->buf = NULL;
+    }
+    Py_RETURN_NONE;
+}
+
+static void
+bytesio_dealloc(BytesIOObject *self)
+{
+    if (self->buf != NULL) {
+        PyMem_Free(self->buf);
+        self->buf = NULL;
+    }
+    Py_TYPE(self)->tp_free(self);
+}
+
+static PyObject *
+bytesio_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
+{
+    BytesIOObject *self;
+
+    assert(type != NULL && type->tp_alloc != NULL);
+    self = (BytesIOObject *)type->tp_alloc(type, 0);
+    if (self == NULL)
+        return NULL;
+
+    self->string_size = 0;
+    self->pos = 0;
+    self->buf_size = 0;
+    self->buf = (char *)PyMem_Malloc(0);
+    if (self->buf == NULL) {
+        Py_DECREF(self);
+        return PyErr_NoMemory();
+    }
+
+    return (PyObject *)self;
+}
+
+static int
+bytesio_init(BytesIOObject *self, PyObject *args, PyObject *kwds)
+{
+    PyObject *initvalue = NULL;
+
+    if (!PyArg_ParseTuple(args, "|O:BytesIO", &initvalue))
+        return -1;
+
+    /* In case, __init__ is called multiple times. */
+    self->string_size = 0;
+    self->pos = 0;
+
+    if (initvalue && initvalue != Py_None) {
+        PyObject *res;
+        res = bytesio_write(self, initvalue);
+        if (res == NULL)
+            return -1;
+        Py_DECREF(res);
+        self->pos = 0;
+    }
+
+    return 0;
+}
+
+static PyGetSetDef bytesio_getsetlist[] = {
+    {"closed",  (getter)bytesio_get_closed, NULL,
+     "True if the file is closed."},
+    {0},            /* sentinel */
+};
+
+static struct PyMethodDef bytesio_methods[] = {
+    {"readable",   (PyCFunction)return_true,        METH_NOARGS, NULL},
+    {"seekable",   (PyCFunction)return_true,        METH_NOARGS, NULL},
+    {"writable",   (PyCFunction)return_true,        METH_NOARGS, NULL},
+    {"close",      (PyCFunction)bytesio_close,      METH_NOARGS, close_doc},
+    {"flush",      (PyCFunction)bytesio_flush,      METH_NOARGS, flush_doc},
+    {"isatty",     (PyCFunction)bytesio_isatty,     METH_NOARGS, isatty_doc},
+    {"tell",       (PyCFunction)bytesio_tell,       METH_NOARGS, tell_doc},
+    {"write",      (PyCFunction)bytesio_write,      METH_O, write_doc},
+    {"writelines", (PyCFunction)bytesio_writelines, METH_O, writelines_doc},
+    {"read1",      (PyCFunction)bytesio_read1,      METH_O, read1_doc},
+    {"readinto",   (PyCFunction)bytesio_readinto,   METH_O, readinto_doc},
+    {"readline",   (PyCFunction)bytesio_readline,   METH_VARARGS, readline_doc},
+    {"readlines",  (PyCFunction)bytesio_readlines,  METH_VARARGS, readlines_doc},
+    {"read",       (PyCFunction)bytesio_read,       METH_VARARGS, read_doc},
+    {"getvalue",   (PyCFunction)bytesio_getvalue,   METH_VARARGS, getval_doc},
+    {"seek",       (PyCFunction)bytesio_seek,       METH_VARARGS, seek_doc},
+    {"truncate",   (PyCFunction)bytesio_truncate,   METH_VARARGS, truncate_doc},
+    {NULL, NULL}        /* sentinel */
+};
+
+PyDoc_STRVAR(bytesio_doc,
+"BytesIO([buffer]) -> object\n"
+"\n"
+"Create a buffered I/O implementation using an in-memory bytes\n"
+"buffer, ready for reading and writing.");
+
+static PyTypeObject BytesIO_Type = {
+    PyVarObject_HEAD_INIT(NULL, 0)
+    "_bytesio._BytesIO",                       /*tp_name*/
+    sizeof(BytesIOObject),                     /*tp_basicsize*/
+    0,                                         /*tp_itemsize*/
+    (destructor)bytesio_dealloc,               /*tp_dealloc*/
+    0,                                         /*tp_print*/
+    0,                                         /*tp_getattr*/
+    0,                                         /*tp_setattr*/
+    0,                                         /*tp_compare*/
+    0,                                         /*tp_repr*/
+    0,                                         /*tp_as_number*/
+    0,                                         /*tp_as_sequence*/
+    0,                                         /*tp_as_mapping*/
+    0,                                         /*tp_hash*/
+    0,                                         /*tp_call*/
+    0,                                         /*tp_str*/
+    0,                                         /*tp_getattro*/
+    0,                                         /*tp_setattro*/
+    0,                                         /*tp_as_buffer*/
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,  /*tp_flags*/
+    bytesio_doc,                               /*tp_doc*/
+    0,                                         /*tp_traverse*/
+    0,                                         /*tp_clear*/
+    0,                                         /*tp_richcompare*/
+    0,                                         /*tp_weaklistoffset*/
+    PyObject_SelfIter,                         /*tp_iter*/
+    (iternextfunc)bytesio_iternext,            /*tp_iternext*/
+    bytesio_methods,                           /*tp_methods*/
+    0,                                         /*tp_members*/
+    bytesio_getsetlist,                        /*tp_getset*/
+    0,                                         /*tp_base*/
+    0,                                         /*tp_dict*/
+    0,                                         /*tp_descr_get*/
+    0,                                         /*tp_descr_set*/
+    0,                                         /*tp_dictoffset*/
+    (initproc)bytesio_init,                    /*tp_init*/
+    0,                                         /*tp_alloc*/
+    bytesio_new,                               /*tp_new*/
+};
+
+PyMODINIT_FUNC
+init_bytesio(void)
+{
+    PyObject *m;
+
+    if (PyType_Ready(&BytesIO_Type) < 0)
+        return;
+    m = Py_InitModule("_bytesio", NULL);
+    if (m == NULL)
+        return;
+    Py_INCREF(&BytesIO_Type);
+    PyModule_AddObject(m, "_BytesIO", (PyObject *)&BytesIO_Type);
+}
diff --git a/Modules/_fileio.c b/Modules/_fileio.c
index 9868e56..7966878 100644
--- a/Modules/_fileio.c
+++ b/Modules/_fileio.c
@@ -552,11 +552,10 @@ portable_lseek(int fd, PyObject *posobj, int whence)
 			PyErr_SetString(PyExc_TypeError, "an integer is required");
 			return NULL;
 		}
-#if !defined(HAVE_LARGEFILE_SUPPORT)
-		pos = PyLong_AsLong(posobj);
+#if defined(HAVE_LARGEFILE_SUPPORT)
+		pos = PyLong_AsLongLong(posobj);
 #else
-		pos = PyLong_Check(posobj) ?
-			PyLong_AsLongLong(posobj) : PyLong_AsLong(posobj);
+		pos = PyLong_AsLong(posobj);
 #endif
 		if (PyErr_Occurred())
 			return NULL;
@@ -572,10 +571,10 @@ portable_lseek(int fd, PyObject *posobj, int whence)
 	if (res < 0)
 		return PyErr_SetFromErrno(PyExc_IOError);
 
-#if !defined(HAVE_LARGEFILE_SUPPORT)
-	return PyLong_FromLong(res);
-#else
+#if defined(HAVE_LARGEFILE_SUPPORT)
 	return PyLong_FromLongLong(res);
+#else
+	return PyLong_FromLong(res);
 #endif
 }
 
@@ -622,48 +621,29 @@ fileio_truncate(PyFileIOObject *self, PyObject *args)
 		return NULL;
 
 	if (posobj == Py_None || posobj == NULL) {
+		/* Get the current position. */
                 posobj = portable_lseek(fd, NULL, 1);
                 if (posobj == NULL)
-                  return NULL;
+			return NULL;
         }
         else {
-		Py_INCREF(posobj);
+		/* Move to the position to be truncated. */
+                posobj = portable_lseek(fd, posobj, 0);
         }
 
-#if !defined(HAVE_LARGEFILE_SUPPORT)
-	pos = PyLong_AsLong(posobj);
+#if defined(HAVE_LARGEFILE_SUPPORT)
+	pos = PyLong_AsLongLong(posobj);
 #else
-	pos = PyLong_Check(posobj) ?
-		PyLong_AsLongLong(posobj) : PyLong_AsLong(posobj);
+	pos = PyLong_AsLong(posobj);
 #endif
-	if (PyErr_Occurred()) {
-		Py_DECREF(posobj);
+	if (PyErr_Occurred())
 		return NULL;
-	}
 
 #ifdef MS_WINDOWS
 	/* MS _chsize doesn't work if newsize doesn't fit in 32 bits,
 	   so don't even try using it. */
 	{
 		HANDLE hFile;
-		PyObject *pos2, *oldposobj;
-
-		/* store the current position */
-		oldposobj = portable_lseek(self->fd, NULL, 1);
-		if (oldposobj == NULL) {
-			Py_DECREF(posobj);
-			return NULL;
-		}
-
-		/* Have to move current pos to desired endpoint on Windows. */
-		errno = 0;
-		pos2 = portable_lseek(fd, posobj, SEEK_SET);
-		if (pos2 == NULL) {
-			Py_DECREF(posobj);
-			Py_DECREF(oldposobj);
-			return NULL;
-		}
-		Py_DECREF(pos2);
 
 		/* Truncate.  Note that this may grow the file! */
 		Py_BEGIN_ALLOW_THREADS
@@ -676,18 +656,6 @@ fileio_truncate(PyFileIOObject *self, PyObject *args)
 				errno = EACCES;
 		}
 		Py_END_ALLOW_THREADS
-
-		if (ret == 0) {
-			/* Move to the previous position in the file */
-			pos2 = portable_lseek(fd, oldposobj, SEEK_SET);
-			if (pos2 == NULL) {
-				Py_DECREF(posobj);
-				Py_DECREF(oldposobj);
-				return NULL;
-			}
-		}
-		Py_DECREF(pos2);
-		Py_DECREF(oldposobj);
 	}
 #else
 	Py_BEGIN_ALLOW_THREADS
@@ -697,7 +665,6 @@ fileio_truncate(PyFileIOObject *self, PyObject *args)
 #endif /* !MS_WINDOWS */
 
 	if (ret != 0) {
-		Py_DECREF(posobj);
 		PyErr_SetFromErrno(PyExc_IOError);
 		return NULL;
 	}
@@ -791,7 +758,8 @@ PyDoc_STRVAR(seek_doc,
 PyDoc_STRVAR(truncate_doc,
 "truncate([size: int]) -> None.	 Truncate the file to at most size bytes.\n"
 "\n"
-"Size defaults to the current file position, as returned by tell().");
+"Size defaults to the current file position, as returned by tell()."
+"The current file position is changed to the value of size.");
 #endif
 
 PyDoc_STRVAR(tell_doc,