Index · Directives systemd 250


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


#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);


sd_bus_new() creates a new bus object. This object is reference-counted, and will be destroyed when all references are gone. Initially, the caller of this function owns the sole reference and the bus object will not be connected to any bus. To connect it to a bus, 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).

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.

sd_bus_ref() increases the reference counter of bus by one.

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.

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. Note that this function is defined as 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)
    fprintf(stderr, "Failed to allocate bus: %s\n", strerror(-r));

sd_bus_ref() and sd_bus_unref() execute no operation if the passed in bus object address is NULL. sd_bus_unrefp() will first dereference its argument, which must not be NULL, and will execute no operation if that is NULL.

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.

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.

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().

Return Value

On success, sd_bus_new() returns 0 or a positive integer. On failure, it returns a negative errno-style error code.

sd_bus_ref() always returns the argument.

sd_bus_unref() and sd_bus_flush_close_unref() always return NULL.


Returned errors may indicate the following problems:


Memory allocation failed.


These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.

See Also

systemd(1), sd-bus(3), sd_bus_default_user(3), sd_bus_default_system(3), sd_bus_open_user(3), sd_bus_open_system(3), sd_bus_close(3)