--- /dev/null
+/* 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-call-method.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) {
+ errno = -error;
+ fprintf(stderr, "%s: %m\n", message);
+ 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;
+ int r;
+
+ r = sd_bus_open_system(&bus);
+ if (r < 0)
+ return log_error(r, "Failed to acquire bus");
+
+ r = sd_bus_call_method(bus, DESTINATION, PATH, INTERFACE, MEMBER, &error, &reply, "u", (unsigned) getpid());
+ 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;
+}
<xi:include href="libsystemd-pkgconfig.xml" />
+ <refsect1>
+ <title>Examples</title>
+
+ <example>
+ <title>Make a call to a D-Bus method that takes a single parameter</title>
+
+ <programlisting><xi:include href="print-unit-path-call-method.c" parse="text" /></programlisting>
+ <para>This defines a minimally useful program that will open a connection to the bus, call a method,
+ wait for the reply, and finally extract and print the answer. It does error handling and proper
+ memory management.</para>
+ </example>
+ </refsect1>
+
<refsect1>
<title>See Also</title>