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SD_BUS_MESSAGE_NEW_METHOD_CALL(3) sd_bus_message_new_method_call SD_BUS_MESSAGE_NEW_METHOD_CALL(3)

sd_bus_message_new_method_call, sd_bus_message_new_method_return - Create a method call message

#include <systemd/sd-bus.h>

int sd_bus_message_new_method_call(sd_bus *bus, sd_bus_message **m, const char *destination, const char *path, const char *interface, const char *member);

int sd_bus_message_new_method_return(sd_bus_message *call, sd_bus_message **m);

The sd_bus_message_new_method_call() function creates a new bus message object that encapsulates a D-Bus method call, and returns it in the m output parameter. The call will be made on the destination destination, path path, on the interface interface, member member.

Briefly, the destination is a dot-separated name that identifies a service connected to the bus. The path is a slash-separated identifier of an object within the destination that resembles a file system path. The meaning of this path is defined by the destination. The interface is a dot-separated name that resembles a Java interface name that identifies a group of methods and signals supported by the object identified by path. Methods and signals are collectively called members and are identified by a simple name composed of ASCII letters, numbers, and underscores. See the D-Bus Tutorial[1] for an in-depth explanation.

The destination parameter may be NULL. The interface parameter may be NULL, if the destination has only a single member with the given name and there is no ambiguity if the interface name is omitted.

The sd_bus_message_new_method_call() function creates a new bus message object that is a reply to the method call call and returns it in the m output parameter. The call parameter must be a method call message. The sender of call is used as the destination.

This function returns 0 if the message object was successfully created, and a negative errno-style error code otherwise.

Returned errors may indicate the following problems:

-EINVAL

The output parameter m is NULL.

The destination parameter is non-null and is not a valid D-Bus service name ("org.somewhere.Something"), the path parameter is not a valid D-Bus path ("/an/object/path"), the interface parameter is non-null and is not a valid D-Bus interface name ("an.interface.name"), or the member parameter is not a valid D-Bus member ("Name").

The call parameter is not a method call object.

-ENOTCONN

The bus parameter bus is NULL or the bus is not connected.

-ENOMEM

Memory allocation failed.

-EPERM

The call parameter is not sealed.

-EOPNOTSUPP

The call message does not have a cookie.

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

Example 1. Make a call to a D-Bus method that takes a single parameter

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <systemd/sd-bus.h>
#define _cleanup_(f) __attribute__((cleanup(f)))
/* This is equivalent to:
 * busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \
 *       org.freedesktop.systemd1.Manager GetUnitByPID $$
 *
 * Compile with 'cc -lsystemd print-unit-path.c'
 */
#define DESTINATION "org.freedesktop.systemd1"
#define PATH        "/org/freedesktop/systemd1"
#define INTERFACE   "org.freedesktop.systemd1.Manager"
#define MEMBER      "GetUnitByPID"
static int log_error(int error, const char *message) {
  fprintf(stderr, "%s: %s\n", message, strerror(-error));
  return error;
}
static int print_unit_path(sd_bus *bus) {
  _cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL;
  _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
  _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
  int r;
  r = sd_bus_message_new_method_call(bus, &m,
                                     DESTINATION, PATH, INTERFACE, MEMBER);
  if (r < 0)
    return log_error(r, "Failed to create bus message");
  r = sd_bus_message_append(m, "u", (unsigned) getpid());
  if (r < 0)
    return log_error(r, "Failed to append to bus message");
  r = sd_bus_call(bus, m, -1, &error, &reply);
  if (r < 0)
    return log_error(r, "Call failed");
  const char *ans;
  r = sd_bus_message_read(reply, "o", &ans);
  if (r < 0)
    return log_error(r, "Failed to read reply");
  printf("Unit path is \"%s\".\n", ans);
  return 0;
}
int main(int argc, char **argv) {
  _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
  int r;
  r = sd_bus_open_system(&bus);
  if (r < 0)
    return log_error(r, "Failed to acquire bus");
  print_unit_path(bus);
}

This defines a minimally useful program that will open a connection to the bus, create a message object, send it, wait for the reply, and finally extract and print the answer. It does error handling and proper memory management.

systemd(1), sd-bus(3), sd_bus_path_encode(3)

1.
D-Bus Tutorial
https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts
systemd 242