sd_bus_message_read, sd_bus_message_readv, sd_bus_message_peek_type — Read a sequence of values from a message
#include <systemd/sd-bus.h>
int sd_bus_message_read( | sd_bus_message *m, |
const char *types, | |
...) ; |
int sd_bus_message_readv( | sd_bus_message *m, |
const char *types, | |
va_list ap) ; |
int sd_bus_message_peek_type( | sd_bus_message *m, |
char *type, | |
const char **contents) ; |
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 of
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 message's lifetime. If the type is
"h
" (UNIX file descriptor), the descriptor is not duplicated by this call and the
returned descriptor remains in possession of the message object, and needs to be duplicated by the caller
in order to keep an open reference to it after the message object is freed.
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 | 8-bit unsigned integer | uint8_t * | |
"b " | SD_BUS_TYPE_BOOLEAN | boolean | int * (NB: not bool *) | |
"n " | SD_BUS_TYPE_INT16 | 16-bit signed integer | int16_t * | |
"q " | SD_BUS_TYPE_UINT16 | 16-bit unsigned integer | uint16_t * | |
"i " | SD_BUS_TYPE_INT32 | 32-bit signed integer | int32_t * | |
"u " | SD_BUS_TYPE_UINT32 | 32-bit unsigned integer | uint32_t * | |
"x " | SD_BUS_TYPE_INT64 | 64-bit signed integer | int64_t * | |
"t " | SD_BUS_TYPE_UINT64 | 64-bit unsigned integer | uint64_t * | |
"d " | SD_BUS_TYPE_DOUBLE | IEEE 754 double precision floating-point | double * | |
"s " | SD_BUS_TYPE_STRING | UTF-8 string | const char ** | |
"o " | SD_BUS_TYPE_OBJECT_PATH | D-Bus object path string | const char ** | |
"g " | SD_BUS_TYPE_SIGNATURE | D-Bus signature string | 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.
sd_bus_message_peek_type()
determines the type of the next element in
m
to be read by sd_bus_message_read()
or similar functions.
On success, the type is stored in type
, if it is not NULL
.
If the type is a container type, the type of its elements is stored in contents
,
if it is not NULL
. If this function successfully determines the type of the next
element in m
, it returns a positive integer. If there are no more elements to be
read, it returns zero.
On success, these functions return a non-negative integer. On failure, they return a negative errno-style error code.
Returned errors may indicate the following problems:
Functions described here are available as a shared
library, which can be compiled against and linked to with the
libsystemd
pkg-config(1)
file.
The code described here uses
getenv(3),
which is declared to be not multi-thread-safe. This means that the code calling the functions described
here must not call
setenv(3)
from a parallel thread. It is recommended to only do calls to setenv()
from an early phase of the program when no other threads have been started.
Read a single basic type (a 64-bit integer):
sd_bus_message *m; int64_t x; sd_bus_message_read(m, "x", &x);
Read a boolean value:
sd_bus_message *m; int x; /* Do not use C99 'bool' type here, it's typically smaller in memory and would cause memory corruption */ sd_bus_message_read(m, "b", &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);
Read a single file descriptor, and duplicate it in order to keep it open after the message is freed.
sd_bus_message *m; int fd, fd_copy; sd_bus_message_read(m, "h", &fd); fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);
sd_bus_message_read()
and
sd_bus_message_readv()
were added in version 240.
sd_bus_message_peek_type()
was added in version 246.