Thanks everyone for the work on this so far. My own expectation as a regular user of #pragma once can be summarised as follows: If a file (by which I mean a "regular file" according to stat, not a symlink etc) contains #pragma once, the preprocessor should behave as if that file has a unique include guard derived from the absolute path of that file, normalised appropriately e.g. case-insensitive on Windows.
In theory this definition would imply that some_header.h and a_symlink_to_that_header.h /should/ be recognised as the same file, and included only once. That's what gcc currently seems to do, but google suggests that older versions had different behaviour. I would argue that as soon as the user starts introducing symlinks/hard links/weird in-memory filesystems/generally getting creative with the concept of what a "file" is, that user can hardly be surprised if #pragma once does something unexpected. The same goes for putting it in a conditional or other clever tricks :) On Wed, 28 Jun 2023 at 20:29, draco <project.dr...@gmx.net> wrote: > Pragma once is implementation defined, so I ponder a bit on the definition… > > 1. pragma once is defined on the file level, while regular source code > can be amalgamated to a single source and in fact this is what the > preprocessor does > 2. does pragma once exclude the whole file from re-inclusion? And if > so, is this true for wherever the pragma in the file is set? Can i pragma > once in the middle of my header file? Can I pragma once conditionally? > What would be the consequences? > 3. does pragma once therefor refer to a certain file? Or to the file > content? What is a certain file? This is defined by the filesystem... On a > harddisk, it means the physically same blocks on the drive, but what does > it mean for different types off FS, lets say memory based? On linux we can > use inode as a proxy, but win32 is lacking inodes… that’s the current > problem to solve > 4. So, how can any compiler safely assure a files identity? Is there > any failsafe solution for the pragma once dilemma out there? > > Moreover, do we really have projects, where headerA.h is symlinked to > headerB.h (different filenames) and do we expect pragma once to exclude > headerB? Is this sane behavior? What usecases are implied here? > > Or can we safely assume, that two files with the same name, size and mtime > are identical, even on windows? > > So, the actual solution is good for linux (use dev/inode from stat) but > not for win32, where every file is read and hashed. On the other hand, > win32 does not use symlinks for files to myknowledge. > > Thiago proposes a normalized path based solution. I would say, even > easier, two files with the same tailname, size and mtime can be assumed to > be identical in this context. > > Michael > > > > _______________________________________________ > Tinycc-devel mailing list > Tinycc-devel@nongnu.org > https://lists.nongnu.org/mailman/listinfo/tinycc-devel >
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