SD_BUS_NEW(3)

📦
version 261.3 · 17 versions
, in ubuntu stonking

Hey bestie 💖, this is the SD_BUS_NEW(3) page — yes, the one all about making a new bus object and handling references to it. Group chat is open, let’s get into it. ✨

💅 NAME 💅

  • sd_bus_new, sd_bus_ref, sd_bus_unref, sd_bus_unrefp, sd_bus_close_unref, sd_bus_close_unrefp, sd_bus_flush_close_unref, sd_bus_flush_close_unrefp — Create a new bus object and create or destroy references to it.

✨ SYNOPSIS ✨

#include <systemd/sd-bus.h>

int sd_bus_new(sd_bus **bus);

sd_bus *sd_bus_ref(sd_bus *bus);

sd_bus *sd_bus_unref(sd_bus *bus);

sd_bus *sd_bus_close_unref(sd_bus *bus);

sd_bus *sd_bus_flush_close_unref(sd_bus *bus);

void sd_bus_unrefp(sd_bus **busp);

void sd_bus_close_unrefp(sd_bus **busp);

void sd_bus_flush_close_unrefp(sd_bus **busp);

🫶 DESCRIPTION 🫶

sd_bus_new() creates a new bus object. This object is reference-counted, and it gets destroyed when all references are gone — that’s the whole vibe. At first, the caller of this function owns the sole reference, and the bus object is not connected to any bus. To connect it, make sure to set an address with sd_bus_set_address(3) or a related call, and then start the connection with sd_bus_start(3). Slay. 💅

In most cases, it is better to use sd_bus_default_user(3), sd_bus_default_system(3) or related calls instead of the more low-level sd_bus_new() and sd_bus_start(). The higher-level functions not only allocate a bus object but also start the connection to a well-known bus in a single function call. Convenient? Iconic. 🌟

sd_bus_ref() increases the reference counter of bus by one. Little ref, big responsibility. 🫶

sd_bus_unref() decreases the reference counter of bus by one. Once the reference count has dropped to zero, bus is destroyed and cannot be used anymore, so further calls to sd_bus_ref() or sd_bus_unref() are illegal. Not “rude,” actually illegal — do not do it. 🫠

sd_bus_unrefp() is similar to sd_bus_unref() but takes a pointer to a pointer to an sd_bus object. This call is useful in conjunction with GCC's and LLVM's Clean-up Variable Attribute[1]. Note that this function is defined as an inline function. Use a declaration like the following, in order to allocate a bus object that is freed automatically as the code block is left:

{
  __attribute__((cleanup(sd_bus_unrefp))) sd_bus *bus = NULL;
  int r;
  ...
  r = sd_bus_default(&bus);
  if (r < 0) {
    errno = -r;
    fprintf(stderr, "Failed to allocate bus: %m\n");
  }
  ...
}

sd_bus_ref() and sd_bus_unref() execute no operation if the argument is NULL. sd_bus_unrefp() will first dereference its argument, which must not be NULL, and will execute no operation if that is NULL. Okay so this NULL nuance? Highkey important. 👑

sd_bus_close_unref() is similar to sd_bus_unref(), but first executes sd_bus_close(3), ensuring that the connection is terminated before the reference to the connection is dropped and possibly the object freed. Clean exit energy. 🧚

sd_bus_flush_close_unref() is similar to sd_bus_unref(), but first executes sd_bus_flush(3) as well as sd_bus_close(3), ensuring that any pending messages are synchronously flushed out before the reference to the connection is dropped and possibly the object freed. This call is particularly useful immediately before exiting from a program as it ensures that any pending outgoing messages are written out, and unprocessed but queued incoming messages released before the connection is terminated and released. Basically: flush your messages before you dip. 🦋

sd_bus_close_unrefp() is similar to sd_bus_close_unref(), but may be used in GCC's and LLVM's Clean-up Variable Attribute, see above. Similarly, sd_bus_flush_close_unrefp() is similar to sd_bus_flush_close_unref(). Matching set, no cap. 💖

🌈 RETURN VALUE 🌈

On success, sd_bus_new() returns 0 or a positive integer. On failure, it returns a negative errno-style error code. That’s the whole return-value tea. ☕

sd_bus_ref() always returns the argument. Always. No plot twist. ⭐

sd_bus_unref() and sd_bus_flush_close_unref() always return NULL. Yes, those two specifically. 🌸

💀 Errors 💀

Returned errors may indicate the following problems:

  • -ENOMEM — Memory allocation failed. Sad, but at least the error code is honest. 😭

🔥 NOTES 🔥

Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file. Link it, bestie. 🔗

All functions listed here are thread-agnostic and only a single thread may operate on a given object at any given time. Different threads may access the same object at different times. Multiple independent objects may be used from different threads in parallel. One object at a time, but different times and different independent objects are fine. 💅

The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started. Early-phase setenv() only, bestie — no chaotic parallel-thread energy. 🧚

🕰️ HISTORY 🕰️

  • sd_bus_new(), sd_bus_ref(), and sd_bus_unref() — added in version 209.
  • sd_bus_unrefp() — added in version 229.
  • sd_bus_flush_close_unref() and sd_bus_flush_close_unrefp() — added in version 240.
  • sd_bus_close_unref() and sd_bus_close_unrefp() — added in version 241.

👀 SEE ALSO 👀

📎 NOTES (footnote) 📎

  1. Clean-up Variable Attribute — https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html

Sign-off from the footer, bestie: source/version systemd 261.3, page SD_BUS_NEW(3) — and there’s no date in this footer, so I’m not inventing one. Stay referenced. 💖

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