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 │ 8bit unsigned │ uint8_t * │ │
61 │ │ │ integer │ │ │
62 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
63 │"b" │ SD_BUS_TYPE_BOOLEAN │ boolean │ int * (NB: │ │
64 │ │ │ │ not bool *) │ │
65 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
66 │"n" │ SD_BUS_TYPE_INT16 │ 16bit signed │ int16_t * │ │
67 │ │ │ integer │ │ │
68 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
69 │"q" │ SD_BUS_TYPE_UINT16 │ 16bit │ uint16_t * │ │
70 │ │ │ unsigned │ │ │
71 │ │ │ integer │ │ │
72 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
73 │"i" │ SD_BUS_TYPE_INT32 │ 32bit signed │ int32_t * │ │
74 │ │ │ integer │ │ │
75 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
76 │"u" │ SD_BUS_TYPE_UINT32 │ 32bit │ uint32_t * │ │
77 │ │ │ unsigned │ │ │
78 │ │ │ integer │ │ │
79 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
80 │"x" │ SD_BUS_TYPE_INT64 │ 64bit signed │ int64_t * │ │
81 │ │ │ integer │ │ │
82 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
83 │"t" │ SD_BUS_TYPE_UINT64 │ 64bit │ uint64_t * │ │
84 │ │ │ unsigned │ │ │
85 │ │ │ integer │ │ │
86 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
87 │"d" │ SD_BUS_TYPE_DOUBLE │ IEEE 754 │ double * │ │
88 │ │ │ double │ │ │
89 │ │ │ precision │ │ │
90 │ │ │ floating-point │ │ │
91 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
92 │"s" │ SD_BUS_TYPE_STRING │ UTF-8 string │ const char ** │ │
93 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
94 │"o" │ SD_BUS_TYPE_OBJECT_PATH │ D-Bus object │ const char ** │ │
95 │ │ │ path string │ │ │
96 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
97 │"g" │ SD_BUS_TYPE_SIGNATURE │ D-Bus │ const char ** │ │
98 │ │ │ signature │ │ │
99 │ │ │ string │ │ │
100 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
101 │"h" │ SD_BUS_TYPE_UNIX_FD │ UNIX file │ int * │ │
102 │ │ │ descriptor │ │ │
103 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
104 │"a" │ SD_BUS_TYPE_ARRAY │ array │ int, which │ n sets of │
105 │ │ │ │ specifies the │ arguments │
106 │ │ │ │ expected │ appropriate │
107 │ │ │ │ length n of │ for the array │
108 │ │ │ │ the array │ element type │
109 ├──────────┼──────────────────────────────┼────────────────┼───────────────┼───────────────┤
110 │"v" │ SD_BUS_TYPE_VARIANT │ variant │ signature │ arguments │
111 │ │ │ │ string │ appropriate │
112 │ │ │ │ │ for the types │
113 │ │ │ │ │ specified by │
114 │ │ │ │ │ the signature │
115 ├──────────┼──────────────────────────────┼────────────────┼───────────────┴───────────────┤
116 │"(" │ SD_BUS_TYPE_STRUCT_BEGIN │ array start │ arguments appropriate for │
117 ├──────────┼──────────────────────────────┼────────────────┤ the structure elements │
118 │")" │ SD_BUS_TYPE_STRUCT_END │ array end │ │
119 ├──────────┼──────────────────────────────┼────────────────┼───────────────┬───────────────┤
120 │"{" │ SD_BUS_TYPE_DICT_ENTRY_BEGIN │ dictionary │ arguments │ arguments │
121 │ │ │ entry start │ appropriate │ appropriate │
122 ├──────────┼──────────────────────────────┼────────────────┤ for the first │ for the │
123 │"}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary │ type in the │ second type │
124 │ │ │ entry end │ pair │ in the pair │
125 └──────────┴──────────────────────────────┴────────────────┴───────────────┴───────────────┘
126 If objects of the specified types are not present at the current
128 position in the message, an error is returned.
129 The sd_bus_message_readv() is equivalent to the sd_bus_message_read(),
131 except that it is called with a "va_list" instead of a variable number
132 of arguments. This function does not call the va_end() macro. Because
133 it invokes the va_arg() macro, the value of ap is undefined after the
134 call.
135 sd_bus_message_peek_type() determines the type of the next element in m
137 to be read by sd_bus_message_read() or similar functions. On success,
138 the type is stored in type, if it is not NULL. If the type is a
139 container type, the type of its elements is stored in contents, if it
140 is not NULL. If this function successfully determines the type of the
141 next element in m, it returns a positive integer. If there are no more
142 elements to be read, it returns zero.
143
145 On success, these functions return a non-negative integer. On failure,
146 they return a negative errno-style error code.
147 Errors
149 Returned errors may indicate the following problems:
150 -EINVAL
152 Specified type string is invalid or the message parameter is NULL.
153 -ENXIO
155 The message does not contain the specified type at current
156 position.
157 -EBADMSG
159 The message cannot be parsed.
160 -EBUSY
162 When reading from a container, this error will be returned if
163 unread elements are left in the container.
164
166 These APIs are implemented as a shared library, which can be compiled
167 and linked to with the libsystemd pkg-config(1) file.
168
170 Read a single basic type (a 64-bit integer):
171 sd_bus_message *m;
173 int64_t x;
174 sd_bus_message_read(m, "x", &x);
176 Read a boolean value:
178 sd_bus_message *m;
180 int x; /* Do not use C99 'bool' type here, it's typically smaller
181 in memory and would cause memory corruption */
182 sd_bus_message_read(m, "b", &x);
184 Read all types of integers:
186 uint8_t y;
188 int16_t n;
189 uint16_t q;
190 int32_t i;
191 uint32_t u;
192 int32_t x;
193 uint32_t t;
194 double d;
195 sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);
197 Read a structure composed of a string and a D-Bus path:
199 const char *s, *p;
201 sd_bus_message_read(m, "(so)", &s, &p);
203 Read a variant, with the real type "gt" (signature, unsigned integer):
205 const char *s;
207 uint64_t *v;
208 sd_bus_message_read(m, "v", "gt", &s, &v);
210 Read a dictionary containing three pairs of type {integer=>string}:
212 int i, j, k;
214 const char *s, *t, *u;
215 sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u);
217 Read a single file descriptor, and duplicate it in order to keep it
220 open after the message is freed.
221 sd_bus_message *m;
223 int fd, fd_copy;
224 sd_bus_message_read(m, "h", &fd);
226 fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);
227
229 systemd(1), sd-bus(3), sd_bus_message_read_basic(3),
230 sd_bus_message_skip(3), sd_bus_message_append(3),
231 sd_bus_message_enter_container(3)
232systemd 251 SD_BUS_MESSAGE_READ(3)