The Austin Group is considering standardizing a subset of the Linux fcntl(F_GETOWN_EX), because of its ability to overcome the limitation that fcntl(F_GETOWN) must fail for some valid pids if pid_t is permitted to be wider than int (whether or not Linux ever reaches a point where pid_t is wider than int, POSIX did not want to make that restriction on other implementations). See http://austingroupbugs.net/view.php?id=1274 However, we've run into a minor issue which implies that man-pages and/or glibc is buggy: The man page for fcntl() (as of Fedora 30 man-pages-4.16-4.fc30) states: struct f_owner_ex { int type; pid_t pid; }; but in the headers under /usr/include, there are two different definitions, which raises the question on what the real type of 'type' should be: /usr/include/asm-generic/fcntl.h (from kernel-headers-5.2.9-200.fc30): struct f_owner_ex { int type; __kernel_pid_t pid; }; /usr/include/bits/fcntl-linux.h (from glibc-headers-2.29-15.fc30): struct f_owner_ex { enum __pid_type type; /* Owner type of ID. */ __pid_t pid; /* ID of owner. */ }; Note that an enum instead of an int matters as to whether this will complain when compiled: struct f_owner_ex s; int *foo = &s.type; Therefore, we want to confirm whether requiring the eventual POSIX definition to use enum f_pid_type (as currently worded in austingroupbugs.net/view.php?id=1274#c4536) is okay (in which case, there is a bug in the man page for documenting int instead of enum f_pid_type), or if POSIX should not bother defining enum f_pid_type (and instead just provide F_OWNER_PID and F_OWNER_PGRP as macros) with f_owner_ex being defined with an int (in which case, the glibc <fcntl.h> header needs a change to use int, and the Austin Group proposal needs to be tweaked to match). Note that the use of an enum in a public struct makes that struct dependent on ABI issues (if the library is compiled with one set of compiler flags where enums occupy the space of 'int', but an application compiles with a different set of flags where an enum occupies only the space of 'char', this could result in the application being unable to correctly call into libc), if that helps sway the decision on which of the two projects needs to change. However, the exact layout of the struct and any padding space was not deemed to be a showstopper (that is, similar to struct stat, the standard intends only to require that at least two members be present in f_owner_ex without any further restrictions on what layout those two members occupy). A side note was also raised during discussion: POSIX already standardizes the type idtype_t for use in waitid(), and on Linux, we happen to have P_PID==F_OWNER_PID==1 and P_PGID==F_OWNER_PGRP==2 (which are the only values that POSIX is considering adding), which on the surface looks like unnecessary duplication. So at one point, the question was raised whether POSIX should reuse the existing idtype_t instead of inventing something new for f_owner_ex. However, it was then pointed out that idtype_t also includes P_ALL (which on Linux is 0), and that Linux uses F_OWNER_TID==0 as an extension to what POSIX would require, but since Linux' F_OWNER_TID semantics for F_SETOWN_EX are not the same semantics as P_ALL in waitid(); furthermore, <fcntl.h> has free reign to add more F_* into the namespace but not P_*, where reuse of the idtype_t type would then require dragging in the <sys/wait.h> header just to populate f_owner_ex. Thus, this reuse of types was deemed unpalatable. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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