sd_bus_message_new_method_return(3)

1SD_BUS_MESSAGE_NEW_METHODs_dC_AbLuLs(_3m)essage_new_metShDo_dB_UcSa_lMlESSAGE_NEW_METHOD_CALL(3)
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NAME

6       sd_bus_message_new_method_call, sd_bus_message_new_method_return -
7       Create a method call message
8

SYNOPSIS

10       #include <systemd/sd-bus.h>
11
12       int sd_bus_message_new_method_call(sd_bus *bus, sd_bus_message **m,
13                                          const char *destination,
14                                          const char *path,
15                                          const char *interface,
16                                          const char *member);
17
18       int sd_bus_message_new_method_return(sd_bus_message *call,
19                                            sd_bus_message **m);
20

DESCRIPTION

22       The sd_bus_message_new_method_call() function creates a new bus message
23       object that encapsulates a D-Bus method call, and returns it in the m
24       output parameter. The call will be made on the destination destination,
25       path path, on the interface interface, member member.
26
27       Briefly, the destination is a dot-separated name that identifies a
28       service connected to the bus. The path is a slash-separated identifier
29       of an object within the destination that resembles a file system path.
30       The meaning of this path is defined by the destination. The interface
31       is a dot-separated name that resembles a Java interface name that
32       identifies a group of methods and signals supported by the object
33       identified by path. Methods and signals are collectively called members
34       and are identified by a simple name composed of ASCII letters, numbers,
35       and underscores. See the D-Bus Tutorial[1] for an in-depth explanation.
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37       The destination parameter may be NULL. The interface parameter may be
38       NULL, if the destination has only a single member with the given name
39       and there is no ambiguity if the interface name is omitted.
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41       Note that this is a low level interface. See sd_bus_call_method(3) for
42       a more convenient way of calling D-Bus methods.
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44       The sd_bus_message_new_method_return() function creates a new bus
45       message object that is a reply to the method call call and returns it
46       in the m output parameter. The call parameter must be a method call
47       message. The sender of call is used as the destination.
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RETURN VALUE

50       On success, these functions return a non-negative integer. On failure,
51       they return a negative errno-style error code.
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53   Errors
54       Returned errors may indicate the following problems:
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56       -EINVAL
57           The output parameter m is NULL.
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59           The destination parameter is non-null and is not a valid D-Bus
60           service name ("org.somewhere.Something"), the path parameter is not
61           a valid D-Bus path ("/an/object/path"), the interface parameter is
62           non-null and is not a valid D-Bus interface name
63           ("an.interface.name"), or the member parameter is not a valid D-Bus
64           member ("Name").
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66           The call parameter is not a method call object.
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68       -ENOTCONN
69           The bus parameter bus is NULL or the bus is not connected.
70
71       -ENOMEM
72           Memory allocation failed.
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74       -EPERM
75           The call parameter is not sealed.
76
77       -EOPNOTSUPP
78           The call message does not have a cookie.
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NOTES

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

85       Example 1. Make a call to a D-Bus method that takes a single parameter
86
87           /* SPDX-License-Identifier: CC0-1.0 */
88
89           #include <stdio.h>
90           #include <string.h>
91           #include <unistd.h>
92           #include <sys/types.h>
93
94           #include <systemd/sd-bus.h>
95           #define _cleanup_(f) __attribute__((cleanup(f)))
96
97           /* This is equivalent to:
98            * busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \
99            *       org.freedesktop.systemd1.Manager GetUnitByPID $$
100            *
101            * Compile with 'cc -lsystemd print-unit-path.c'
102            */
103
104           #define DESTINATION "org.freedesktop.systemd1"
105           #define PATH        "/org/freedesktop/systemd1"
106           #define INTERFACE   "org.freedesktop.systemd1.Manager"
107           #define MEMBER      "GetUnitByPID"
108
109           static int log_error(int error, const char *message) {
110             fprintf(stderr, "%s: %s\n", message, strerror(-error));
111             return error;
112           }
113
114           static int print_unit_path(sd_bus *bus) {
115             _cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL;
116             _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
117             _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
118             int r;
119
120             r = sd_bus_message_new_method_call(bus, &m,
121                                                DESTINATION, PATH, INTERFACE, MEMBER);
122             if (r < 0)
123               return log_error(r, "Failed to create bus message");
124
125             r = sd_bus_message_append(m, "u", (unsigned) getpid());
126             if (r < 0)
127               return log_error(r, "Failed to append to bus message");
128
129             r = sd_bus_call(bus, m, -1, &error, &reply);
130             if (r < 0)
131               return log_error(r, "Call failed");
132
133             const char *ans;
134             r = sd_bus_message_read(reply, "o", &ans);
135             if (r < 0)
136               return log_error(r, "Failed to read reply");
137
138             printf("Unit path is \"%s\".\n", ans);
139
140             return 0;
141           }
142
143           int main(int argc, char **argv) {
144             _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
145             int r;
146
147             r = sd_bus_open_system(&bus);
148             if (r < 0)
149               return log_error(r, "Failed to acquire bus");
150
151             print_unit_path(bus);
152           }
153
154       This defines a minimally useful program that will open a connection to
155       the bus, create a message object, send it, wait for the reply, and
156       finally extract and print the answer. It does error handling and proper
157       memory management.
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NOTES

164        1. D-Bus Tutorial
165           https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts
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169systemd 250                                  SD_BUS_MESSAGE_NEW_METHOD_CALL(3)