EXTATTR(2)

💅 EXTATTR(2) — the system calls manual, bestie edition ✨

okay so this is EXTATTR(2), a System Calls Manual page, and it's literally all about the system calls that manipulate VFS extended attributes. yes we're going there. grab a snack 🍓

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🌸 NAME 🌸

extattr_get_fd, extattr_set_fd, extattr_delete_fd, extattr_list_fd, extattr_get_file, extattr_set_file, extattr_delete_file, extattr_list_file, extattr_get_link, extattr_set_link, extattr_delete_link, extattr_list_link — system calls to manipulate VFS extended attributes 👑

📚 LIBRARY 📚

Standard C Library (libc, -lc) 🫶

✨ SYNOPSIS ✨

#include <sys/types.h>
#include <sys/extattr.h>

ssize_t
extattr_get_fd(int fd, int attrnamespace, const char *attrname,
    void *data, size_t nbytes);

ssize_t
extattr_set_fd(int fd, int attrnamespace, const char *attrname,
    const void *data, size_t nbytes);

int
extattr_delete_fd(int fd, int attrnamespace, const char *attrname);

ssize_t
extattr_list_fd(int fd, int attrnamespace, void *data, size_t nbytes);

ssize_t
extattr_get_file(const char *path, int attrnamespace,
    const char *attrname, void *data, size_t nbytes);

ssize_t
extattr_set_file(const char *path, int attrnamespace,
    const char *attrname, const void *data, size_t nbytes);

int
extattr_delete_file(const char *path, int attrnamespace,
    const char *attrname);

ssize_t
extattr_list_file(const char *path, int attrnamespace, void *data,
    size_t nbytes);

ssize_t
extattr_get_link(const char *path, int attrnamespace,
    const char *attrname, void *data, size_t nbytes);

ssize_t
extattr_set_link(const char *path, int attrnamespace,
    const char *attrname, const void *data, size_t nbytes);

int
extattr_delete_link(const char *path, int attrnamespace,
    const char *attrname);

ssize_t
extattr_list_link(const char *path, int attrnamespace, void *data,
    size_t nbytes);

like yes, that's twelve whole functions, we love a big family 💖

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🌈 DESCRIPTION 🌈

Named extended attributes are meta-data associated with vnodes representing files and directories. They exist as "name=value" pairs, living inside a set of namespaces. it's giving little sticky notes on your files, fr fr 🎀

The extattr_get_file() system call retrieves the value of the specified extended attribute into a buffer pointed to by data of size nbytes. The extattr_set_file() system call sets the value of the specified extended attribute to the data described by data. The extattr_delete_file() system call deletes the extended attribute specified. The extattr_list_file() returns a list of attributes present in the requested namespace. 📋

okay so about that list — each list entry consists of a single byte containing the length of the attribute name, followed by the attribute name. And the attribute name is not terminated by ASCII 0 (nul). periodt. no nul for you 💀

The extattr_get_file(), and extattr_list_file() calls consume the data and nbytes arguments in the style of read(2); extattr_set_file() consumes these arguments in the style of write(2). 🦋

If data is NULL in a call to extattr_get_file() and extattr_list_file() then the size of defined extended attribute data will be returned, rather than the quantity read — which is iconic because it permits applications to test the size of the data without performing a read. The extattr_delete_link(), extattr_get_link(), and extattr_set_link() system calls behave in the same way as their _file counterparts, except that they do not follow symlinks. 🚫🔗

The extattr_get_fd(), extattr_set_fd(), extattr_delete_fd(), and extattr_list_fd(), calls are identical to their "_file" counterparts except for the first argument. The "_fd" functions take a file descriptor, while the "_file" functions take a path. Both arguments describe a file associated with the extended attribute that should be manipulated. twins but make it file descriptors 💫

The following arguments are common to all the system calls described here:

• attrnamespace — the namespace in which the extended attribute resides; see extattr(9)
• attrname — the name of the extended attribute

Named extended attribute semantics vary by file system implementing the call. Not all operations may be supported for a particular attribute. Additionally, the format of the data in data is attribute-specific. so like, read the room (and the filesystem) 🔍

For more information on named extended attributes, please see extattr(9). go read it, it's the sequel 🌟

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⚠️ CAVEAT ⚠️

This interface is under active development, and as such is subject to change as applications are adapted to use it. Developers are discouraged from relying on its stability.

bestie. it's under construction. do NOT build your whole personality on it 😭🚧

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🫶 RETURN VALUES 🫶

If successful, the extattr_get_file(), extattr_set_file(), and extattr_list_file() calls return the number of bytes that were read or written from the data, respectively, or if data was NULL, then extattr_get_file() and extattr_list_file() return the number of bytes available to read. If any of the calls are unsuccessful, the value -1 is returned and the global variable errno is set to indicate the error. 💀➡️📉

The extattr_delete_file() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. zero is the slay, negative one is the nay 🎀

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💥 ERRORS 💥

The following errors may be returned by the system calls themselves. Additionally, the file system implementing the call may return any other errors it desires. yes, truly, it can just do that 🙈

• [EFAULT] — The attrnamespace and attrname arguments, or the memory range defined by data and nbytes point outside the process's allocated address space.
• [ENAMETOOLONG] — The attribute name was longer than EXTATTR_MAXNAMELEN.

The extattr_get_fd(), extattr_set_fd(), extattr_delete_fd(), and extattr_list_fd() system calls may also fail if:

• [EBADF] — The file descriptor referenced by fd was invalid.

Additionally, the extattr_get_file(), extattr_set_file(), and extattr_delete_file() calls may also fail due to the following errors:

• [ENOATTR] — The requested attribute was not defined for this file.
• [ENOTDIR] — A component of the path prefix is not a directory.
• [ENAMETOOLONG] — A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
• [ENOENT] — A component of the path name that must exist does not exist.
• [EACCES] — Search permission is denied for a component of the path prefix.

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👀 SEE ALSO 👀

extattr(3), getextattr(8), setextattr(8), extattr(9), VOP_GETEXTATTR(9), VOP_SETEXTATTR(9) 🦋✨

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📜 HISTORY 📜

Extended attribute support was developed as part of the TrustedBSD Project, and introduced in FreeBSD 5.0. It was developed to support security extensions requiring additional labels to be associated with each file or directory. a whole origin story, we love a glow-up 💜

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🐛 BUGS 🐛

In earlier versions of this API, passing an empty string for the attribute name to extattr_get_fd(), extattr_get_file(), or extattr_get_link() would return the list of attributes defined for the target object. This interface has been deprecated in preference to using the explicit list API, and should not be used. old behavior, new rules, do not pass go 🚫

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and that's the whole page, bestie — you ate 💅✨

this one came out of the Debian camp, dated January 29, 2008, so yeah, vintage but still serving. mwah, go forth and get those attributes 🎀🫶