--- name: "aa_features" section: "3" platform: "linux" package: "apparmor" version: "4.1.8-2" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/aa_features" --- # aa\_features(3) an opaque object representing a set of AppArmor kernel features aa\_features\_new - create a new aa\_features object based on a path aa\_features\_new\_from\_string - create a new aa\_features object based on a string aa\_features\_new\_from\_kernel - create a new aa\_features object based on the current kernel aa\_features\_ref - increments the ref count of an aa\_features object aa\_features\_unref - decrements the ref count and frees the aa\_features object when 0 aa\_features\_write\_to\_fd - write a string representation of an aa\_features object to a file descriptor aa\_features\_write\_to\_file - write a string representation of an aa\_features object to a file aa\_features\_is\_equal - equality test for two aa\_features objects aa\_features\_supports - provides aa\_features object support status aa\_features\_id - provides unique identifier for an aa\_features object aa\_features\_value - the value associated with a given feature. ## NAME aa\_features - an opaque object representing a set of AppArmor kernel features aa\_features\_new - create a new aa\_features object based on a path aa\_features\_new\_from\_string - create a new aa\_features object based on a string aa\_features\_new\_from\_kernel - create a new aa\_features object based on the current kernel aa\_features\_ref - increments the ref count of an aa\_features object aa\_features\_unref - decrements the ref count and frees the aa\_features object when 0 aa\_features\_write\_to\_fd - write a string representation of an aa\_features object to a file descriptor aa\_features\_write\_to\_file - write a string representation of an aa\_features object to a file aa\_features\_is\_equal - equality test for two aa\_features objects aa\_features\_supports - provides aa\_features object support status aa\_features\_id - provides unique identifier for an aa\_features object aa\_features\_value - the value associated with a given feature. ## SYNOPSIS **#include \** **typedef struct aa\_features aa\_features;** **int aa\_features\_new(aa\_features \*\*features, int dirfd, const char \*path);** **int aa\_features\_new\_from\_file(aa\_features \*\*features, int fd);** **int aa\_features\_new\_from\_string(aa\_features \*\*features, const char \*string, size\_t size);** **int aa\_features\_new\_from\_kernel(aa\_features \*\*features);** **aa\_features \*aa\_features\_ref(aa\_features \*features);** **void aa\_features\_unref(aa\_features \*features);** **int aa\_features\_write\_to\_fd(aa\_features \*features, int fd);** **int aa\_features\_write\_to\_file(aa\_features \*features, int dirfd, const char \*path);** **bool aa\_features\_is\_equal(aa\_features \*features1, aa\_features \*features2);** **bool aa\_features\_supports(aa\_features \*features, const char \*str);** **char \*aa\_features\_id(aa\_features \*features);** **char \*aa\_features\_value(aa\_features \*features, const char \*str, size\_t \*len);** Link with **-lapparmor** when compiling. ## DESCRIPTION The *aa\_features* object contains information about the AppArmor features supported by a kernel. The feature support information is based upon the files AppArmor represents in securityfs, which is typically found at /sys/kernel/security/apparmor/features/. That information may be parsed and turned into a string or flat file in order to represent a set of features of a kernel that is not currently running. The **aa\_features\_new()** function creates an *aa\_features* object based upon a directory file descriptor and path. The *path* can point to a file or directory. See the [**openat**(2)](/openat/2.md) man page for examples of *dirfd* and *path*. The allocated *features* object must be freed using **aa\_features\_unref()**. The **aa\_features\_new\_from\_file()** function is similar except that it accepts an open file as the argument. The allocated *features* object must be freed using **aa\_features\_unref()**. The **aa\_features\_new\_from\_string()** function is similar except that it accepts a NUL-terminated string representation of the AppArmor features as the *string* argument. The length of the features string, not counting the NUL-terminator, must be specified as the *size* argument. The allocated *features* object must be freed using **aa\_features\_unref()**. The **aa\_features\_new\_from\_kernel()** function creates an *aa\_features* object from the current running kernel. The allocated *features* object must be freed using **aa\_features\_unref()**. **aa\_features\_ref()** increments the reference count on the *features* object. **aa\_features\_unref()** decrements the reference count on the *features* object and releases all corresponding resources when the reference count reaches zero. The **aa\_features\_write\_to\_fd()** function writes a string representation of the *features* object to the file descriptor specified by the *fd*. The **aa\_features\_write\_to\_file()** function writes a string representation of the *features* object to the file specified by the *dirfd* and *path* combination. **aa\_features\_is\_equal()** can be used to detect if the *features1* and *features2* objects are equal. The definition of equality is private to libapparmor and may be changed in ways that do not break backward compatibility. The **aa\_features\_supports()** function can be used to query the *features* object to determine if a feature is supported. The *str* argument should be equal to the path, relative to the "apparmor/features/" directory of securityfs, of the feature to query. For example, to test if policy version 6 is supported, *str* would be "policy/versions/v6". The **aa\_features\_id()** function returns a string representation of an identifier that can be used to uniquely identify an *aa\_features* object. The mechanism for generating the string representation is internal to libapparmor and subject to change but an example implementation is applying a hash function to the features string. ## RETURN VALUE The **aa\_features\_new()** family of functions return 0 on success and *\*features* will point to an *aa\_features* object that must be freed by **aa\_features\_unref()**. -1 is returned on error, with errno set appropriately, and *\*features* will be set to NULL. **aa\_features\_ref()** returns the value of *features*. **aa\_features\_write\_to\_file()** returns 0 on success. -1 is returned on error, with errno set appropriately. **aa\_features\_is\_equal()** returns true if *features1* and *features2* are equal and false if they are not equal. **aa\_features\_supports()** returns true if the feature represented by *str* is supported and false if it is not supported. **aa\_features\_id()** returns a string identifying *features* which must be freed by the caller. NULL is returned on error, with errno set appropriately. **aa\_features\_value()** returns a null terminated string with is associated length which must be freed by the caller. NULL is returned on error, with errno set to ENOENT if the feature was not found, ENODIR if the specified feature does not resolve to a leaf feature. ## ERRORS The errno value will be set according to the underlying error in the *aa\_features* family of functions that return -1 or NULL on error. ## NOTES The **aa\_features\_id()** function can be found in libapparmor version 2.13. All the other aa\_feature functions described above are present in libapparmor version 2.10. **aa\_features\_unref()** saves the value of errno when called and restores errno before exiting in libapparmor version 2.12 and newer. ## BUGS None known. If you find any, please report them at \. ## SEE ALSO [**openat**(2)](/openat/2.md), [**aa-features-abi**(1)](/aa-features-abi/1.md) and \.