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NAME

sd_bus_message_new_method_call, sd_bus_message_new_method_return - Create a method call message

SYNOPSIS

#include <systemd/sd-bus.h>

DESCRIPTION

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.

Note that this is a low level interface. See sd_bus_call_method(3) for a more convenient way of calling D-Bus methods.

The sd_bus_message_new_method_return() 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.

RETURN VALUE

On success, these functions return a non-negative integer. On failure, they return a negative errno-style error code.

Errors
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.

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.

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.

EXAMPLES

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

/* SPDX-License-Identifier: MIT-0 */

/* This is equivalent to:
* busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \
* org.freedesktop.systemd1.Manager GetUnitByPID $$
*
* Compile with 'cc print-unit-path.c -lsystemd'
*/

#include <errno.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

#include <systemd/sd-bus.h>

#define _cleanup_(f) __attribute__((cleanup(f)))
#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;
}

int main(int argc, char **argv) {
_cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
_cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
_cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL, *m = NULL;
int r;

r = sd_bus_open_system(&bus);
if (r < 0)
return log_error(r, "Failed to acquire bus");

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, MEMBER " 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;
}

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.

HISTORY

sd_bus_message_new_method_call() and sd_bus_message_new_method_return() were added in version 246.

SEE ALSO

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

NOTES

https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts