1SD_BUS_MESSAGE_READ(3) sd_bus_message_read SD_BUS_MESSAGE_READ(3)
2
6 sd_bus_message_read, sd_bus_message_readv, sd_bus_message_peek_type -
7 Read a sequence of values from a message
8
10 #include <systemd/sd-bus.h>
11 int sd_bus_message_read(sd_bus_message *m, const char *types, ...);
13 int sd_bus_message_readv(sd_bus_message *m, const char *types,
15 va_list ap);
16 int sd_bus_message_peek_type(sd_bus_message *m, char *type,
18 const char **contents);
19
21 sd_bus_message_read() reads a sequence of fields from the D-Bus message
22 object m and advances the read position in the message. The type string
23 types describes the types of items expected in the message and the
24 field arguments that follow. The type string may be NULL or empty, in
25 which case nothing is read.
26 The type string is composed of the elements described in
28 sd_bus_message_append(3), i.e. basic and container types. It must
29 contain zero or more single "complete types". The type string is
30 NUL-terminated.
31 For each type specified in the type string, one or more arguments need
33 to be specified after the types parameter, in the same order. The
34 arguments must be pointers to appropriate types (a pointer to int8_t
35 for a "y" in the type string, a pointer to int32_t for an "i", a
36 pointer to const char* for an "s", ...) which are set based on the
37 values in the message. As an exception, in case of array and variant
38 types, the first argument is an "input" argument that further specifies
39 how the message should be read. See the table below for a complete list
40 of allowed arguments and their types. Note that, if the basic type is a
41 pointer (e.g., const char * in the case of a string), the argument is a
42 pointer to a pointer, and also the pointer value that is written is
43 only borrowed and the contents must be copied if they are to be used
44 after the end of the message's lifetime. If the type is "h" (UNIX file
45 descriptor), the descriptor is not duplicated by this call and the
46 returned descriptor remains in possession of the message object, and
47 needs to be duplicated by the caller in order to keep an open reference
48 to it after the message object is freed.
49 Each argument may also be NULL, in which case the value is read and
51 ignored.
52 Table 1. Item type specifiers
54 ┌──────────┬──────────────────────────────┬────────────────┬───────────────┬───────────────┐
55 │Specifier │ Constant │ Description │ Type of the │ Types of the │
56 │ │ │ │ first │ subsequent │
57 │ │ │ │ argument │ arguments, if │
58 │ │ │ │ │ any │
59 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
60 │"y" │ SD_BUS_TYPE_BYTE │ 8-bit │ uint8_t * │ │
61 │ │ │ unsigned │ │ │
62 │ │ │ integer │ │ │
63 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
64 │"b" │ SD_BUS_TYPE_BOOLEAN │ boolean │ int * (NB: │ │
65 │ │ │ │ not bool *) │ │
66 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
67 │"n" │ SD_BUS_TYPE_INT16 │ 16-bit signed │ int16_t * │ │
68 │ │ │ integer │ │ │
69 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
70 │"q" │ SD_BUS_TYPE_UINT16 │ 16-bit │ uint16_t * │ │
71 │ │ │ unsigned │ │ │
72 │ │ │ integer │ │ │
73 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
74 │"i" │ SD_BUS_TYPE_INT32 │ 32-bit signed │ int32_t * │ │
75 │ │ │ integer │ │ │
76 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
77 │"u" │ SD_BUS_TYPE_UINT32 │ 32-bit │ uint32_t * │ │
78 │ │ │ unsigned │ │ │
79 │ │ │ integer │ │ │
80 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
81 │"x" │ SD_BUS_TYPE_INT64 │ 64-bit signed │ int64_t * │ │
82 │ │ │ integer │ │ │
83 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
84 │"t" │ SD_BUS_TYPE_UINT64 │ 64-bit │ uint64_t * │ │
85 │ │ │ unsigned │ │ │
86 │ │ │ integer │ │ │
87 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
88 │"d" │ SD_BUS_TYPE_DOUBLE │ IEEE 754 │ double * │ │
89 │ │ │ double │ │ │
90 │ │ │ precision │ │ │
91 │ │ │ floating-point │ │ │
92 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
93 │"s" │ SD_BUS_TYPE_STRING │ UTF-8 string │ const char ** │ │
94 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
95 │"o" │ SD_BUS_TYPE_OBJECT_PATH │ D-Bus object │ const char ** │ │
96 │ │ │ path string │ │ │
97 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
98 │"g" │ SD_BUS_TYPE_SIGNATURE │ D-Bus │ const char ** │ │
99 │ │ │ signature │ │ │
100 │ │ │ string │ │ │
101 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
102 │"h" │ SD_BUS_TYPE_UNIX_FD │ UNIX file │ int * │ │
103 │ │ │ descriptor │ │ │
104 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
105 │"a" │ SD_BUS_TYPE_ARRAY │ array │ int, which │ n sets of │
106 │ │ │ │ specifies the │ arguments │
107 │ │ │ │ expected │ appropriate │
108 │ │ │ │ length n of │ for the array │
109 │ │ │ │ the array │ element type │
110 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
111 │"v" │ SD_BUS_TYPE_VARIANT │ variant │ signature │ arguments │
112 │ │ │ │ string │ appropriate │
113 │ │ │ │ │ for the types │
114 │ │ │ │ │ specified by │
115 │ │ │ │ │ the signature │
116 ├──────────┼──────────────────────────────┼────────────────┼───────────────┴───────────────┤
117 │"(" │ SD_BUS_TYPE_STRUCT_BEGIN │ array start │ arguments appropriate for │
118 ├──────────┼──────────────────────────────┼────────────────┤ the structure elements │
119 │")" │ SD_BUS_TYPE_STRUCT_END │ array end │ │
120 ├──────────┼──────────────────────────────┼────────────────┼───────────────┬───────────────┤
121 │"{" │ SD_BUS_TYPE_DICT_ENTRY_BEGIN │ dictionary │ arguments │ arguments │
122 │ │ │ entry start │ appropriate │ appropriate │
123 ├──────────┼──────────────────────────────┼────────────────┤ for the first │ for the │
124 │"}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary │ type in the │ second type │
125 │ │ │ entry end │ pair │ in the pair │
126 └──────────┴──────────────────────────────┴────────────────┴───────────────┴───────────────┘
127 If objects of the specified types are not present at the current
129 position in the message, an error is returned.
130 The sd_bus_message_readv() is equivalent to the sd_bus_message_read(),
132 except that it is called with a "va_list" instead of a variable number
133 of arguments. This function does not call the va_end() macro. Because
134 it invokes the va_arg() macro, the value of ap is undefined after the
135 call.
136 sd_bus_message_peek_type() determines the type of the next element in m
138 to be read by sd_bus_message_read() or similar functions. On success,
139 the type is stored in type, if it is not NULL. If the type is a
140 container type, the type of its elements is stored in contents, if it
141 is not NULL. If this function successfully determines the type of the
142 next element in m, it returns a positive integer. If there are no more
143 elements to be read, it returns zero.
144
146 On success, these functions return a non-negative integer. On failure,
147 they return a negative errno-style error code.
148 Errors
150 Returned errors may indicate the following problems:
151 -EINVAL
153 Specified type string is invalid or the message parameter is NULL.
154 -ENXIO
156 The message does not contain the specified type at current
157 position.
158 -EBADMSG
160 The message cannot be parsed.
161 -EBUSY
163 When reading from a container, this error will be returned if
164 unread elements are left in the container.
165
167 Functions described here are available as a shared library, which can
168 be compiled against and linked to with the libsystemd pkg-config(1)
169 file.
170 The code described here uses getenv(3), which is declared to be not
172 multi-thread-safe. This means that the code calling the functions
173 described here must not call setenv(3) from a parallel thread. It is
174 recommended to only do calls to setenv() from an early phase of the
175 program when no other threads have been started.
176
178 Read a single basic type (a 64-bit integer):
179 sd_bus_message *m;
181 int64_t x;
182 sd_bus_message_read(m, "x", &x);
184 Read a boolean value:
186 sd_bus_message *m;
188 int x; /* Do not use C99 'bool' type here, it's typically smaller
189 in memory and would cause memory corruption */
190 sd_bus_message_read(m, "b", &x);
192 Read all types of integers:
194 uint8_t y;
196 int16_t n;
197 uint16_t q;
198 int32_t i;
199 uint32_t u;
200 int32_t x;
201 uint32_t t;
202 double d;
203 sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);
205 Read a structure composed of a string and a D-Bus path:
207 const char *s, *p;
209 sd_bus_message_read(m, "(so)", &s, &p);
211 Read a variant, with the real type "gt" (signature, unsigned integer):
213 const char *s;
215 uint64_t *v;
216 sd_bus_message_read(m, "v", "gt", &s, &v);
218 Read a dictionary containing three pairs of type {integer=>string}:
220 int i, j, k;
222 const char *s, *t, *u;
223 sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u);
225 Read a single file descriptor, and duplicate it in order to keep it
228 open after the message is freed.
229 sd_bus_message *m;
231 int fd, fd_copy;
232 sd_bus_message_read(m, "h", &fd);
234 fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);
235
237 systemd(1), sd-bus(3), sd_bus_message_read_basic(3),
238 sd_bus_message_skip(3), sd_bus_message_append(3),
239 sd_bus_message_enter_container(3)
240systemd 254 SD_BUS_MESSAGE_READ(3)