Date: Mon, 25 Jan 2021 13:18:28 -0800 From: Kirk McKusick <mckusick@mckusick.com> To: Xin Li <delphij@delphij.net> Cc: d@delphij.net, freebsd-standards@freebsd.org, Jilles Tjoelker <jilles@freebsd.org>, Ed Maste <emaste@freebsd.org>, Konstantin Belousov <kostikbel@gmail.com> Subject: Re: _POSIX_C_SOURCE and S_IFMT macros Message-ID: <202101252118.10PLISQI065263@chez.mckusick.com> In-Reply-To: <YA62oUzcLwL/bUNh@kib.kiev.ua>
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> Date: Mon, 25 Jan 2021 14:16:33 +0200 > From: Konstantin Belousov <kostikbel@gmail.com> > To: d@delphij.net > Cc: freebsd-standards@freebsd.org, Jilles Tjoelker <jilles@freebsd.org>, > Ed Maste <emaste@freebsd.org>, Xin Li <delphij@delphij.net>, > Marshall Kirk McKusick <mckusick@freebsd.org> > Subject: Re: _POSIX_C_SOURCE and S_IFMT macros > = > On Mon, Jan 25, 2021 at 01:04:25AM -0800, Xin Li via freebsd-standards w= rote: >> Hi, >> = >> I recently discovered [1] a difference between FreeBSD's handling of >> _POSIX_C_SOURCE and what I have observe on Linux and macOS systems, >> basically, if a program defines _POSIX_C_SOURCE to 200809L, but did not >> define _XOPEN_SOURCE, we will not define S_IFMT when sys/stat.h is incl= uded. >> = >> It appears that the glibc version of sys/stat.h won't define it either, >> but their fcntl.h would define it by having _POSIX_C_SOURCE >=3D 200809= L >> to define _USE_XOPEN2K8 and expose it. >> = >> A quick glance on my Macbook suggests that they were defining the S_IFM= T >> macros unconditionally. >> = >> Based on my research, hiding these macros started in revision 40491 of >> CSRG repository ( >> https://svnweb.freebsd.org/csrg?view=3Drevision&revision=3D40491 ); Net= BSD >> chose to make the macro visible when _XOPEN_SOURCE in revision 1.34 and >> expanded the visibility scope to also cover _NETBSD_SOURCE. >> = >> Do we need to hide these with 200809L when _XOPEN_SOURCE is not defined= ? >> If I'm reading the standards correctly, I think we are required to >> define these in modern POSIX revisions? >> = >> = >> = >> [1] When building pigz (https://github.com/madler/pigz), I got: >> = >> cc -O3 -Wall -Wextra -Wno-unknown-pragmas -Wcast-qual -c pigz.c -o pig= z.o >> pigz.c:3817:47: error: use of undeclared identifier 'S_IFMT' >> if (stat(from, &st) !=3D 0 || (st.st_mode & S_IFMT) !=3D S_IFREG) >> ^ >> [...] > = > In my copy of IEEE Std 1003.1=E2=84=A2-2017 S_IFMT and related S_IF<type= > symbols > are specified as provided by sys/stat.h. fcntl.h (not sys/fcntl.h, whic= h > is not specified by POSIX at all) does not require export of S_IF*. > = > So I think S_IF symbols listed in POSIX standard should be provided by > sys/stat.h under some value of POSIX_C_SOURCE, but I did not dig to see > which should be the guard value. > = > Most likely 200809 is fine. My reading of the current POSIX standard agrees with Kostik's analysis that S_IF symbols listed in POSIX standard should be provided by sys/stat.h. I do not have a complete set of POSIX standards over time, so cannot help with what the cutover date should be. Kirk McKusick
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