SD_ELF_NOTE_DLOPEN(3)

💖 SD_ELF_NOTE_DLOPEN(3) 💖

okay bestie, pull up a chair, because this one is genuinely iconic. this is the manual page for SD_ELF_NOTE_DLOPEN(3) and its little constellation of sibling macros — SD_ELF_NOTE_DLOPEN_VENDOR, SD_ELF_NOTE_DLOPEN_TYPE, SD_ELF_NOTE_DLOPEN_PRIORITY_REQUIRED, SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED, and SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED — and they're all about embedding ELF .note.dlopen metadata for shared library dependencies. yes. that's the whole vibe. let's get into it 🦋

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✨ NAME ✨

SD_ELF_NOTE_DLOPEN, SD_ELF_NOTE_DLOPEN_VENDOR, SD_ELF_NOTE_DLOPEN_TYPE, SD_ELF_NOTE_DLOPEN_PRIORITY_REQUIRED, SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED, SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED — Embed ELF .note.dlopen metadata for shared library dependencies 💅

like. it literally says it in the name. no cap.

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🌸 SYNOPSIS 🌸

#include <systemd/sd-dlopen.h>

SD_ELF_NOTE_DLOPEN(feature, description, priority, soname...)

#define SD_ELF_NOTE_DLOPEN_VENDOR "FDO"

#define SD_ELF_NOTE_DLOPEN_TYPE UINT32_C(0x407c0c0a)

#define SD_ELF_NOTE_DLOPEN_PRIORITY_REQUIRED    "required"

#define SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED "recommended"

#define SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED   "suggested"

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🎀 DESCRIPTION 🎀

so SD_ELF_NOTE_DLOPEN() is a macro — not a function, not a daemon, not a whole thing you have to link against — that embeds a .note.dlopen ELF note section directly into your compiled binary. and what's it declaring? a weak dependency on a shared library that you load via dlopen(). that's the whole assignment. 💖

it implements the ELF dlopen metadata[1] spec, which is the part that makes package managers and build systems go "oh wait, there's a runtime dependency here that we literally cannot see through the regular ELF DT_NEEDED entries." sneaky? no. transparent, actually. that's the point. main character behavior. 👑

the macro takes these parameters:

• feature — a short string identifying the feature this library provides, like "XKB" or "PCRE2".

Added in version 261.

• description — a human-readable description of what the library is actually used for.

Added in version 261.

• priority — one of SD_ELF_NOTE_DLOPEN_PRIORITY_REQUIRED, SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED, or SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED, telling everyone how important this dependency is.

Added in version 261.

• soname... — one or more shared object names (sonames) for the library, e.g. "libfoo.so.1". you can pass multiple sonames as separate arguments — up to 5 — for libraries that have changed soname across versions. so like, when a lib rebrands itself every other release, you're covered. 🙈

Added in version 261.

and if you wanna read that embedded metadata back out of a compiled ELF binary:

systemd-analyze dlopen-metadata binary

that's it. that's the command. it's giving "inspect and verify." ✨

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🔥 EXAMPLES 🔥

Example 1. Single soname

#include <systemd/sd-dlopen.h>

SD_ELF_NOTE_DLOPEN("XKB", "Keyboard layout support",
                    SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED,
                    "libxkbcommon.so.0");

one lib, one soname, one clean little declaration. ate. 💅

Example 2. Multiple sonames for different library versions

SD_ELF_NOTE_DLOPEN("crypt", "Support for hashing passwords",
                    SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED,
                    "libcrypt.so.2", "libcrypt.so.1", "libcrypt.so.1.1");

see how it's listing libcrypt.so.2, libcrypt.so.1, and libcrypt.so.1.1 all at once? that's the "up to 5 sonames" thing in action, fr fr. 🌈

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🧚 NOTES 🧚

the macros described here are header-only and do not require runtime linkage against libsystemd(3). the only things you need are the installed header and the include path (as provided by pkg-config --cflags libsystemd). that's it. no extra linking, no drama, no runtime baggage. lowkey the best kind of dependency: none. 🫶

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👑 HISTORY 👑

SD_ELF_NOTE_DLOPEN(), SD_ELF_NOTE_DLOPEN_VENDOR, SD_ELF_NOTE_DLOPEN_TYPE, SD_ELF_NOTE_DLOPEN_PRIORITY_REQUIRED, SD_ELF_NOTE_DLOPEN_PRIORITY_RECOMMENDED, and SD_ELF_NOTE_DLOPEN_PRIORITY_SUGGESTED were added in version 261.

all of them. same version. a whole family arriving at once. iconic. ⭐

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🌟 SEE ALSO 🌟

• systemd(1)
• sd-dlopen(3)
• dlopen(3)

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📎 NOTES 📎

1. ELF dlopen metadata https://uapi-group.org/specifications/specs/elf_dlopen_metadata/

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and that's the whole page bestie!! written by the systemd girlies, version systemd 262.1, and this vibe was certified around the time of that footer date you'd normally see stamped on the bottom of the page. go forth and annotate your binaries, you absolute legend 💜✨🎀