SD_BUS_CALL_METHOD(3)

NAME
sd_bus_call_method, sd_bus_call_methodv, sd_bus_call_method_async, sd_bus_call_method_asyncv - Initialize a bus message object and invoke the corresponding D-Bus method call

SYNOPSIS

#include <systemd/sd-bus.h>

typedef int (*sd_bus_message_handler_t)(sd_bus_message *m, void *userdata, sd_bus_error *ret_error);

int sd_bus_call_method(sd_bus *bus, const char *destination, const char *path, const char *interface, const char *member, sd_bus_error *ret_error, sd_bus_message **reply, const char *types, ...);

int sd_bus_call_methodv(sd_bus *bus, const char *destination, const char *path, const char *interface, const char *member, sd_bus_error *ret_error, sd_bus_message **reply, const char *types, va_list ap);

int sd_bus_call_method_async(sd_bus *bus, sd_bus_slot **slot, const char *destination, const char *path, const char *interface, const char *member, sd_bus_message_handler_t callback, void *userdata, const char *types, ...);

int sd_bus_call_method_asyncv(sd_bus *bus, sd_bus_slot **slot, const char *destination, const char *path, const char *interface, const char *member, sd_bus_message_handler_t callback, void *userdata, const char *types, va_list ap);

DESCRIPTION

sd_bus_call_method() is a convenience function for initializing a bus message object and calling the corresponding D-Bus method. It combines the sd_bus_message_new_method_call(3), sd_bus_message_append(3) and sd_bus_call(3) functions into a single function call.

sd_bus_call_method_async() is a convenience function for initializing a bus message object and calling the corresponding D-Bus method asynchronously. It combines the sd_bus_message_new_method_call(3), sd_bus_message_append(3) and sd_bus_call_async(3) functions into a single function call.

RETURN VALUE

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

  Errors

See the man pages of sd_bus_message_new_method_call(3), sd_bus_message_append(3), sd_bus_call(3) and sd_bus_call_async(3) for a list of possible errors.

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-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;
    }

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.

SEE ALSO

systemd(1), sd-bus(3), sd_bus_message_new_method_call(3), sd_bus_message_append(3), sd_bus_call(3), sd_bus_set_property(3), sd_bus_emit_signal(3)