sd_bus_message_read, sd_bus_message_readv — Read a sequence of values from a message
#include <systemd/sd-bus.h>
int sd_bus_message_read( | sd_bus_message *m, |
char char *types, | |
...) ; |
int sd_bus_message_readv( | sd_bus_message *m, |
char char *types, | |
va_list ap) ; |
sd_bus_message_read()
reads a sequence of fields from
the D-Bus message object m
and advances the read position
in the message. The type string types
describes the types
of items expected in the message and the field arguments that follow. The type
string may be NULL
or empty, in which case nothing is
read.
The type string is composed of the elements described in
sd_bus_message_append(3),
i.e. basic and container types. It must contain zero or more single "complete
types". The type string is NUL
-terminated.
For each type specified in the type string, one or more arguments need to be specified
after the types
parameter, in the same order. The arguments must be
pointers to appropriate types (a pointer to int8_t
for a "y
" in
the type string, a pointer to int32_t
for an "i
", a pointer to
const char*
for an "s
", ...) which are set based on the values in
the message. As an exception, in case or array and variant types, the first argument is an
"input" argument that further specifies how the message should be read. See the table below for
a complete list of allowed arguments and their types. Note that, if the basic type is a pointer
(e.g., const char *
in the case of a string), the argument is a pointer to a
pointer, and also the pointer value that is written is only borrowed and the contents must be
copied if they are to be used after the end of the messages lifetime.
Each argument may also be NULL
, in which case the value is read and
ignored.
Table 1. Item type specifiers
Specifier | Constant | Description | Type of the first argument | Types of the subsequent arguments, if any |
---|---|---|---|---|
"y " | SD_BUS_TYPE_BYTE | unsigned integer | uint8_t * | |
"b " | SD_BUS_TYPE_BOOLEAN | boolean | int * | |
"n " | SD_BUS_TYPE_INT16 | signed integer | int16_t * | |
"q " | SD_BUS_TYPE_UINT16 | unsigned integer | uint16_t * | |
"i " | SD_BUS_TYPE_INT32 | signed integer | int32_t * | |
"u " | SD_BUS_TYPE_UINT32 | unsigned integer | uint32_t * | |
"x " | SD_BUS_TYPE_INT64 | signed integer | int64_t * | |
"t " | SD_BUS_TYPE_UINT64 | unsigned integer | uint64_t * | |
"d " | SD_BUS_TYPE_DOUBLE | floating-point | double * | |
"s " | SD_BUS_TYPE_STRING | Unicode string | const char ** | |
"o " | SD_BUS_TYPE_OBJECT_PATH | object path | const char ** | |
"g " | SD_BUS_TYPE_SIGNATURE | signature | const char ** | |
"h " | SD_BUS_TYPE_UNIX_FD | UNIX file descriptor | int * | |
"a " | SD_BUS_TYPE_ARRAY | array | int, which specifies the expected length n of the array | n sets of arguments appropriate for the array element type |
"v " | SD_BUS_TYPE_VARIANT | variant | signature string | arguments appropriate for the types specified by the signature |
"( " | SD_BUS_TYPE_STRUCT_BEGIN | array start | arguments appropriate for the structure elements | |
") " | SD_BUS_TYPE_STRUCT_END | array end | ||
"{ " | SD_BUS_TYPE_DICT_ENTRY_BEGIN | dictionary entry start | arguments appropriate for the first type in the pair | arguments appropriate for the second type in the pair |
"} " | SD_BUS_TYPE_DICT_ENTRY_END | dictionary entry end |
If objects of the specified types are not present at the current position in the message, an error is returned.
The sd_bus_message_readv()
is equivalent to the
sd_bus_message_read()
, except that it is called with a
"va_list
" instead of a variable number of arguments. This
function does not call the va_end()
macro. Because it
invokes the va_arg()
macro, the value of
ap
is undefined after the call.
On success, sd_bus_message_read()
and
sd_bus_message_readv()
return 0 or a positive integer. On failure, they return a
negative errno-style error code.
These APIs are implemented as a shared
library, which can be compiled and linked to with the
libsystemd
pkg-config(1)
file.
Read a single basic type (a 64-bit integer):
sd_bus_message *m; int64_t x; sd_bus_message_read(m, "x", &x);
Read all types of integers:
uint8_t y; int16_t n; uint16_t q; int32_t i; uint32_t u; int32_t x; uint32_t t; double d; sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);
Read a structure composed of a string and a D-Bus path:
const char *s, *p; sd_bus_message_read(m, "(so)", &s, &p);
Read a variant, with the real type "gt" (signature, unsigned integer):
const char *s; uint64_t *v; sd_bus_message_read(m, "v", "gt", &s, &v);
Read a dictionary containing three pairs of type {integer=>string}:
int i, j, k; const char *s, *t, *u; sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u);