A NOTE has been added to this issue. ====================================================================== https://austingroupbugs.net/view.php?id=1798 ====================================================================== Reported By: eblake Assigned To: ====================================================================== Project: Issue 8 drafts Issue ID: 1798 Category: System Interfaces Type: Clarification Requested Severity: Objection Priority: normal Status: New Name: Eric Blake Organization: Red Hat User Reference: ebb.posix_getdents Section: XSH posix_getdents Page Number: 1567 Line Number: 52609 Final Accepted Text: ====================================================================== Date Submitted: 2024-01-22 15:13 UTC Last Modified: 2024-02-16 10:18 UTC ====================================================================== Summary: Must posix_getdents remember file offsets across exec? ======================================================================
---------------------------------------------------------------------- (0006658) corinna_vinschen (reporter) - 2024-02-16 10:18 https://austingroupbugs.net/view.php?id=1798#c6658 ---------------------------------------------------------------------- >From Cygwin's side, the problem is this: The underlying non-POSIXy kernel does not allow lseek(2) operations on directory descriptors, not even requesting a position within the directory. The only available seek-like operation is equivalent to lseek(dirfd, SEEK_SET, 0). Therefore we have to use a DIR* and the entire operation of position bookkeeping is performed in user space. If the standard strives to allow implementing posix_getdents() using DIR* under the hood, the standard should be clear on the subject that DIR is not dup(2)'able the same way as the dir descriptor given as argument to posix_getdents(). DIR is, by and large, a user-space object while the descriptor is a kernel object. DIR has never been meant as a dup'able object and there's no precedent for such a functionality. As such, there's no way to keep the dup'ed DIR* in sync after such a duplication. The same problem occurs with fork(2), which is just a more thorough dup(2) in terms of descriptors. Bottom line is, with user-space DIR* with enforced user space bookkeeping, there's no way after dup(2)/fork(2) to keep the directory position info in sync. Consequentially, there should be no assumption made how posix_getdents() behaves after dup(2) or fork(2). I.e. using the descriptors with posix_getdents() or readdir() in parallel should be undefined behaviour. If you're interested in code, I invite you to take a look into the current, preliminary implementation of posix_getdents() in Cygwin: https://cygwin.com/cgit/newlib-cygwin/commit/?id=62ca95721a14 As the commit outlines, the code does not try to keep track of the hidden DIR at all. Thanks, Corinna Issue History Date Modified Username Field Change ====================================================================== 2024-01-22 15:13 eblake New Issue 2024-01-22 15:13 eblake Name => Eric Blake 2024-01-22 15:13 eblake Organization => Red Hat 2024-01-22 15:13 eblake User Reference => ebb.posix_getdents 2024-01-22 15:13 eblake Section => XSH posix_getdents 2024-01-22 15:13 eblake Page Number => 1567 2024-01-22 15:13 eblake Line Number => 52609 2024-01-22 15:30 eblake Note Added: 0006632 2024-01-22 15:39 eblake Note Edited: 0006632 2024-02-16 10:18 corinna_vinschenNote Added: 0006658 ======================================================================
