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 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.
On success, these functions return a non-negative integer. On failure, they return a negative errno-style error code.
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
/* SPDX-License-Identifier: CC0-1.0 */
#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.