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
162 These APIs are implemented as a shared library, which can be compiled
163 and linked to with the libsystemd pkg-config(1) file.
164
166 Read a single basic type (a 64-bit integer):
167 sd_bus_message *m;
169 int64_t x;
170 sd_bus_message_read(m, "x", &x);
172 Read a boolean value:
174 sd_bus_message *m;
176 int x; /* Do not use C99 'bool' type here, it's typically smaller
177 in memory and would cause memory corruption */
178 sd_bus_message_read(m, "b", &x);
180 Read all types of integers:
182 uint8_t y;
184 int16_t n;
185 uint16_t q;
186 int32_t i;
187 uint32_t u;
188 int32_t x;
189 uint32_t t;
190 double d;
191 sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);
193 Read a structure composed of a string and a D-Bus path:
195 const char *s, *p;
197 sd_bus_message_read(m, "(so)", &s, &p);
199 Read a variant, with the real type "gt" (signature, unsigned integer):
201 const char *s;
203 uint64_t *v;
204 sd_bus_message_read(m, "v", "gt", &s, &v);
206 Read a dictionary containing three pairs of type {integer=>string}:
208 int i, j, k;
210 const char *s, *t, *u;
211 sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u);
213 Read a single file descriptor, and duplicate it in order to keep it
216 open after the message is freed.
217 sd_bus_message *m;
219 int fd, fd_copy;
220 sd_bus_message_read(m, "h", &fd);
222 fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);
223
225 systemd(1), sd-bus(3), sd_bus_message_read_basic(3),
226 sd_bus_message_skip(3), sd_bus_message_append(3),
227 sd_bus_message_enter_container(3)
228systemd 248 SD_BUS_MESSAGE_READ(3)