The first call to () that specifies a path removes visibility of the entire filesystem from all other filesystem-related system calls (such as open(2), chmod(2) and rename(2)), except for the specified path and permissions .

The () system call remains capable of traversing to any path in the filesystem, so additional calls can set permissions at other points in the filesystem hierarchy.

After establishing a collection of path and permissions rules, future calls to () can be disabled by passing two NULL arguments. Alternatively, pledge(2) may be used to remove the "unveil" promise.

The permissions argument points to a string consisting of zero or more of the following characters:

r Make path available for read operations, corresponding to the pledge(2) promise "rpath". w Make path available for write operations, corresponding to the pledge(2) promise "wpath". x Make path available for execute operations, corresponding to the pledge(2) promise "exec". c Allow path to be created and removed, corresponding to the pledge(2) promise "cpath".

A path that is a directory will enable all filesystem access underneath path using permissions if and only if no more specific matching () exists at a lower level. Directories are remembered at the time of a call to (). This means that a directory that is removed and recreated after a call to () will appear to not exist.

Non-directory paths are remembered by name within their containing directory, and so may be created, removed, or re-created after a call to () and still appear to exist.

Attempts to access paths not allowed by () will result in an error of EACCES when the permissions argument does not match the attempted operation. ENOENT is returned for paths for which no () permissions qualify. After a process has terminated, lastcomm(1) will mark it with the ‘U’ flag if file access was prevented by ().

() use can be tricky because programs misbehave badly when their files unexpectedly disappear. In many cases it is easier to unveil the directories in which an application makes use of files.