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gh-86768: implement os.lseek with SetFilePointer on Windows #133137

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gh-86768: implement os.lseek with SetFilePointer on Windows #133137

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43956d8
Implement os.lseek with SetFilePointerEx on Windows
aisk Apr 28, 2025
b1a5cbb
Add test
aisk Apr 29, 2025
7fe3490
Add news entry
aisk Apr 29, 2025
9a0d10f
Check error
aisk Apr 29, 2025
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8 changes: 8 additions & 0 deletions Lib/test/test_os.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2577,6 +2577,14 @@ def test_ftruncate(self):
def test_lseek(self):
self.check(os.lseek, 0, 0)

@unittest.skipUnless(hasattr(os, 'lseek'), 'test needs os.lseek()')
@unittest.skipUnless(hasattr(os, 'pipe'), "need os.pipe()")
def test_lseek_on_pipe(self):
rfd, wfd = os.pipe()
self.addCleanup(os.close, rfd)
self.addCleanup(os.close, wfd)
self.assertRaises(OSError, os.lseek, rfd, 123, os.SEEK_END)

@unittest.skipUnless(hasattr(os, 'read'), 'test needs os.read()')
def test_read(self):
self.check(os.read, 1)
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Windows/2025-04-29-17-55-55.gh-issue-86768.uIDTHc.rst
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@@ -0,0 +1,2 @@
:exc:`OSError` will be raised if call :func:`os.lseek` with non seekable
object on Windows.
43 changes: 40 additions & 3 deletions Modules/posixmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -219,6 +219,7 @@
# if defined(MS_WINDOWS_DESKTOP) || defined(MS_WINDOWS_SYSTEM)
# define HAVE_SYMLINK
# endif /* MS_WINDOWS_DESKTOP | MS_WINDOWS_SYSTEM */
extern int winerror_to_errno(int);
#endif


Expand Down Expand Up @@ -11406,7 +11407,7 @@ static Py_off_t
os_lseek_impl(PyObject *module, int fd, Py_off_t position, int how)
/*[clinic end generated code: output=971e1efb6b30bd2f input=f096e754c5367504]*/
{
Py_off_t result;
Py_off_t result = 0;

#ifdef SEEK_SET
/* Turn 0, 1, 2 into SEEK_{SET,CUR,END} */
Expand All @@ -11420,14 +11421,50 @@ os_lseek_impl(PyObject *module, int fd, Py_off_t position, int how)
Py_BEGIN_ALLOW_THREADS
_Py_BEGIN_SUPPRESS_IPH
#ifdef MS_WINDOWS
result = _lseeki64(fd, position, how);
switch (how) {
case SEEK_SET:
how = FILE_BEGIN;
break;
case SEEK_CUR:
how = FILE_CURRENT;
break;
case SEEK_END:
how = FILE_END;
break;
}
HANDLE h = (HANDLE)_get_osfhandle(fd);
if (h == INVALID_HANDLE_VALUE) {
result = -1;
}
if (result >= 0) {
if (GetFileType(h) != FILE_TYPE_DISK) {
// Only file is seekable
errno = ESPIPE;
result = -1;
}
}
if (result >= 0) {
LARGE_INTEGER distance, newdistance;
distance.QuadPart = position;
if (SetFilePointerEx(h, distance, &newdistance, how)) {
result = newdistance.QuadPart;
} else {
result = -1;
}
}
#else
result = lseek(fd, position, how);
#endif
_Py_END_SUPPRESS_IPH
Py_END_ALLOW_THREADS
if (result < 0)
if (result < 0) {
#ifdef MS_WINDOWS
if (errno == 0) {
errno = winerror_to_errno(GetLastError());
}
#endif
posix_error();
}

return result;
}
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