sd_bus_message_new_method_call(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
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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.
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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       The sd_bus_message_new_method_call() function creates a new bus message
42       object that is a reply to the method call call and returns it in the m
43       output parameter. The call parameter must be a method call message. The
44       sender of call is used as the destination.
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RETURN VALUE

47       This function returns 0 if the message object was successfully created,
48       and a negative errno-style error code otherwise.
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ERRORS

51       Returned errors may indicate the following problems:
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53       -EINVAL
54           The output parameter m is NULL.
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56           The destination parameter is non-null and is not a valid D-Bus
57           service name ("org.somewhere.Something"), the path parameter is not
58           a valid D-Bus path ("/an/object/path"), the interface parameter is
59           non-null and is not a valid D-Bus interface name
60           ("an.interface.name"), or the member parameter is not a valid D-Bus
61           member ("Name").
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63           The call parameter is not a method call object.
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65       -ENOTCONN
66           The bus parameter bus is NULL or the bus is not connected.
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68       -ENOMEM
69           Memory allocation failed.
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71       -EPERM
72           The call parameter is not sealed.
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74       -EOPNOTSUPP
75           The call message does not have a cookie.
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NOTES

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

82       Example 1. Make a call to a D-Bus method that takes a single parameter
83
84           #include <stdio.h>
85           #include <string.h>
86           #include <unistd.h>
87           #include <sys/types.h>
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89           #include <systemd/sd-bus.h>
90           #define _cleanup_(f) __attribute__((cleanup(f)))
91
92           /* This is equivalent to:
93            * busctl call org.freedesktop.systemd1 /org/freedesktop/systemd1 \
94            *       org.freedesktop.systemd1.Manager GetUnitByPID $$
95            *
96            * Compile with 'cc -lsystemd print-unit-path.c'
97            */
98
99           #define DESTINATION "org.freedesktop.systemd1"
100           #define PATH        "/org/freedesktop/systemd1"
101           #define INTERFACE   "org.freedesktop.systemd1.Manager"
102           #define MEMBER      "GetUnitByPID"
103
104           static int log_error(int error, const char *message) {
105             fprintf(stderr, "%s: %s\n", message, strerror(-error));
106             return error;
107           }
108
109           static int print_unit_path(sd_bus *bus) {
110             _cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL;
111             _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
112             _cleanup_(sd_bus_message_unrefp) sd_bus_message *reply = NULL;
113             int r;
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115             r = sd_bus_message_new_method_call(bus, &m,
116                                                DESTINATION, PATH, INTERFACE, MEMBER);
117             if (r < 0)
118               return log_error(r, "Failed to create bus message");
119
120             r = sd_bus_message_append(m, "u", (unsigned) getpid());
121             if (r < 0)
122               return log_error(r, "Failed to append to bus message");
123
124             r = sd_bus_call(bus, m, -1, &error, &reply);
125             if (r < 0)
126               return log_error(r, "Call failed");
127
128             const char *ans;
129             r = sd_bus_message_read(reply, "o", &ans);
130             if (r < 0)
131               return log_error(r, "Failed to read reply");
132
133             printf("Unit path is \"%s\".\n", ans);
134
135             return 0;
136           }
137
138           int main(int argc, char **argv) {
139             _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
140             int r;
141
142             r = sd_bus_open_system(&bus);
143             if (r < 0)
144               return log_error(r, "Failed to acquire bus");
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146             print_unit_path(bus);
147           }
148
149       This defines a minimally useful program that will open a connection to
150       the bus, create a message object, send it, wait for the reply, and
151       finally extract and print the answer. It does error handling and proper
152       memory management.
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NOTES

158        1. D-Bus Tutorial
159           https://dbus.freedesktop.org/doc/dbus-tutorial.html#concepts
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163systemd 241                                  SD_BUS_MESSAGE_NEW_METHOD_CALL(3)