How do you check whether a path is absolute or relative, using C on Linux?
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Answer
On Unix like systems (incl. Linux, macOS)
If it starts with a slash it’s absolute, otherwise it’s relative.
That is because everything is part of a single tree starting at the root (/) and file systems are mounted somewhere in this tree.
On Windows
Windows uses backslashes (though slashes are also supported these days) and drive letters. But it’s bit more complex:
- A path starting with a drive letter followed by a colon not followed by a backslash (or slash) can be relative to that drive’s current directory.
- A path starting with a backslash (or slash) is absolute but on the current drive, so in that sense it is in fact relative (to the top of the current drive).
- A path starting with 2 backslashes is an UNC path (pointing to a network location) which is always absolute, except when the path starts with
\?which is a special case to support longer paths.
So on Windows it’s best to use the PathIsRelative() (PathIsRelativeA/PathIsRelativeW) function to determine if the path is relative or not.