1SD_BUS_MESSAGE_READ(3)        sd_bus_message_read       SD_BUS_MESSAGE_READ(3)
2
3
4

NAME

6       sd_bus_message_read, sd_bus_message_readv - Read a sequence of values
7       from a message
8

SYNOPSIS

10       #include <systemd/sd-bus.h>
11
12       int sd_bus_message_read(sd_bus_message *m, char char *types, ...);
13
14       int sd_bus_message_readv(sd_bus_message *m, char char *types,
15                                va_list ap);
16

DESCRIPTION

18       sd_bus_message_read() reads a sequence of fields from the D-Bus message
19       object m and advances the read position in the message. The type string
20       types describes the types of items expected in the message and the
21       field arguments that follow. The type string may be NULL or empty, in
22       which case nothing is read.
23
24       The type string is composed of the elements described in
25       sd_bus_message_append(3), i.e. basic and container types. It must
26       contain zero or more single "complete types". The type string is
27       NUL-terminated.
28
29       For each type specified in the type string, one or more arguments need
30       to be specified after the types parameter, in the same order. The
31       arguments must be pointers to appropriate types (a pointer to int8_t
32       for a "y" in the type string, a pointer to int32_t for an "i", a
33       pointer to const char* for an "s", ...) which are set based on the
34       values in the message. As an exception, in case or array and variant
35       types, the first argument is an "input" argument that further specifies
36       how the message should be read. See the table below for a complete list
37       of allowed arguments and their types. Note that, if the basic type is a
38       pointer (e.g., const char * in the case of a string), the argument is a
39       pointer to a pointer, and also the pointer value that is written is
40       only borrowed and the contents must be copied if they are to be used
41       after the end of the messages lifetime.
42
43       Each argument may also be NULL, in which case the value is read and
44       ignored.
45
46       Table 1. Item type specifiers
47       ┌──────────┬──────────────────────────────┬────────────────┬───────────────┬───────────────┐
48Specifier Constant                     Description    Type of the   Types of the  
49       │          │                              │                │ first         subsequent    
50       │          │                              │                │ argument      arguments, if 
51       │          │                              │                │               │ any           
52       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
53       │"y"       │ SD_BUS_TYPE_BYTE             │ 8bit unsigned  │ uint8_t *     │               │
54       │          │                              │ integer        │               │               │
55       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
56       │"b"       │ SD_BUS_TYPE_BOOLEAN          │ boolean        │ int * (NB:    │               │
57       │          │                              │                │ not bool *)   │               │
58       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
59       │"n"       │ SD_BUS_TYPE_INT16            │ 16bit signed   │ int16_t *     │               │
60       │          │                              │ integer        │               │               │
61       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
62       │"q"       │ SD_BUS_TYPE_UINT16           │ 16bit          │ uint16_t *    │               │
63       │          │                              │ unsigned       │               │               │
64       │          │                              │ integer        │               │               │
65       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
66       │"i"       │ SD_BUS_TYPE_INT32            │ 32bit signed   │ int32_t *     │               │
67       │          │                              │ integer        │               │               │
68       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
69       │"u"       │ SD_BUS_TYPE_UINT32           │ 32bit          │ uint32_t *    │               │
70       │          │                              │ unsigned       │               │               │
71       │          │                              │ integer        │               │               │
72       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
73       │"x"       │ SD_BUS_TYPE_INT64            │ 64bit signed   │ int64_t *     │               │
74       │          │                              │ integer        │               │               │
75       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
76       │"t"       │ SD_BUS_TYPE_UINT64           │ 64bit          │ uint64_t *    │               │
77       │          │                              │ unsigned       │               │               │
78       │          │                              │ integer        │               │               │
79       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
80       │"d"       │ SD_BUS_TYPE_DOUBLE           │ IEEE 754       │ double *      │               │
81       │          │                              │ double         │               │               │
82       │          │                              │ precision      │               │               │
83       │          │                              │ floating-point │               │               │
84       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
85       │"s"       │ SD_BUS_TYPE_STRING           │ UTF-8 string   │ const char ** │               │
86       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
87       │"o"       │ SD_BUS_TYPE_OBJECT_PATH      │ D-Bus object   │ const char ** │               │
88       │          │                              │ path string    │               │               │
89       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
90       │"g"       │ SD_BUS_TYPE_SIGNATURE        │ D-Bus          │ const char ** │               │
91       │          │                              │ signature      │               │               │
92       │          │                              │ string         │               │               │
93       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
94       │"h"       │ SD_BUS_TYPE_UNIX_FD          │ UNIX file      │ int *         │               │
95       │          │                              │ descriptor     │               │               │
96       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
97       │"a"       │ SD_BUS_TYPE_ARRAY            │ array          │ int, which    │ n sets of     │
98       │          │                              │                │ specifies the │ arguments     │
99       │          │                              │                │ expected      │ appropriate   │
100       │          │                              │                │ length n of   │ for the array │
101       │          │                              │                │ the array     │ element type  │
102       ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
103       │"v"       │ SD_BUS_TYPE_VARIANT          │ variant        │ signature     │ arguments     │
104       │          │                              │                │ string        │ appropriate   │
105       │          │                              │                │               │ for the types │
106       │          │                              │                │               │ specified by  │
107       │          │                              │                │               │ the signature │
108       ├──────────┼──────────────────────────────┼────────────────┼───────────────┴───────────────┤
109       │"("       │ SD_BUS_TYPE_STRUCT_BEGIN     │ array start    │ arguments appropriate for     │
110       ├──────────┼──────────────────────────────┼────────────────┤ the structure elements        │
111       │")"       │ SD_BUS_TYPE_STRUCT_END       │ array end      │                               │
112       ├──────────┼──────────────────────────────┼────────────────┼───────────────┬───────────────┤
113       │"{"       │ SD_BUS_TYPE_DICT_ENTRY_BEGIN │ dictionary     │ arguments     │ arguments     │
114       │          │                              │ entry start    │ appropriate   │ appropriate   │
115       ├──────────┼──────────────────────────────┼────────────────┤ for the first │ for the       │
116       │"}"       │ SD_BUS_TYPE_DICT_ENTRY_END   │ dictionary     │ type in the   │ second type   │
117       │          │                              │ entry end      │ pair          │ in the pair   │
118       └──────────┴──────────────────────────────┴────────────────┴───────────────┴───────────────┘
119
120       If objects of the specified types are not present at the current
121       position in the message, an error is returned.
122
123       The sd_bus_message_readv() is equivalent to the sd_bus_message_read(),
124       except that it is called with a "va_list" instead of a variable number
125       of arguments. This function does not call the va_end() macro. Because
126       it invokes the va_arg() macro, the value of ap is undefined after the
127       call.
128

RETURN VALUE

130       On success, sd_bus_message_read() and sd_bus_message_readv() return 0
131       or a positive integer. On failure, they return a negative errno-style
132       error code.
133
134   Errors
135       Returned errors may indicate the following problems:
136
137       -EINVAL
138           Specified type string is invalid or the message parameter is NULL.
139
140       -ENXIO
141           The message does not contain the specified type at current
142           position.
143
144       -EBADMSG
145           The message cannot be parsed.
146

NOTES

148       These APIs are implemented as a shared library, which can be compiled
149       and linked to with the libsystemd pkg-config(1) file.
150

EXAMPLES

152       Read a single basic type (a 64-bit integer):
153
154           sd_bus_message *m;
155           int64_t x;
156
157           sd_bus_message_read(m, "x", &x);
158
159       Read a boolean value:
160
161           sd_bus_message *m;
162           int x; /* Do not use C99 'bool' type here, it's typically smaller
163                     in memory and would cause memory corruption */
164
165           sd_bus_message_read(m, "b", &x);
166
167       Read all types of integers:
168
169           uint8_t y;
170           int16_t n;
171           uint16_t q;
172           int32_t i;
173           uint32_t u;
174           int32_t x;
175           uint32_t t;
176           double d;
177
178           sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);
179
180       Read a structure composed of a string and a D-Bus path:
181
182           const char *s, *p;
183
184           sd_bus_message_read(m, "(so)", &s, &p);
185
186       Read a variant, with the real type "gt" (signature, unsigned integer):
187
188           const char *s;
189           uint64_t *v;
190
191           sd_bus_message_read(m, "v", "gt", &s, &v);
192
193       Read a dictionary containing three pairs of type {integer=>string}:
194
195           int i, j, k;
196           const char *s, *t, *u;
197
198           sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u);
199
200

SEE ALSO

202       systemd(1), sd-bus(3), sd_bus_message_read_basic(3),
203       sd_bus_message_skip(3), sd_bus_message_append(3)
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206
207systemd 245                                             SD_BUS_MESSAGE_READ(3)
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