x SuSE Linux 13.1-RELEASE x
x SuSE Linux 13.1-RELEASEx
SD_BUS_MESSAGE_READ(3) sd_bus_message_read SD_BUS_MESSAGE_READ(3)
NAME
sd_bus_message_read, sd_bus_message_readv, sd_bus_message_peek_type -
Read a sequence of values from a message
SYNOPSIS
#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);
DESCRIPTION
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 | Types of the |
| | | | first | subsequent |
| | | | argument | arguments, if |
| | | | | any |
+----------+------------------------------+----------------+---------------+---------------+
|"y" | SD_BUS_TYPE_BYTE | 8-bit | uint8_t * | |
| | | unsigned | | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"b" | SD_BUS_TYPE_BOOLEAN | boolean | int * (NB: | |
| | | | not bool *) | |
+----------+------------------------------+----------------+---------------+---------------+
|"n" | SD_BUS_TYPE_INT16 | 16-bit signed | int16_t * | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"q" | SD_BUS_TYPE_UINT16 | 16-bit | uint16_t * | |
| | | unsigned | | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"i" | SD_BUS_TYPE_INT32 | 32-bit signed | int32_t * | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"u" | SD_BUS_TYPE_UINT32 | 32-bit | uint32_t * | |
| | | unsigned | | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"x" | SD_BUS_TYPE_INT64 | 64-bit signed | int64_t * | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"t" | SD_BUS_TYPE_UINT64 | 64-bit | uint64_t * | |
| | | unsigned | | |
| | | integer | | |
+----------+------------------------------+----------------+---------------+---------------+
|"d" | SD_BUS_TYPE_DOUBLE | IEEE 754 | double * | |
| | | double | | |
| | | precision | | |
| | | floating-point | | |
+----------+------------------------------+----------------+---------------+---------------+
|"s" | SD_BUS_TYPE_STRING | UTF-8 string | const char ** | |
+----------+------------------------------+----------------+---------------+---------------+
|"o" | SD_BUS_TYPE_OBJECT_PATH | D-Bus object | const char ** | |
| | | path string | | |
+----------+------------------------------+----------------+---------------+---------------+
|"g" | SD_BUS_TYPE_SIGNATURE | D-Bus | const char ** | |
| | | signature | | |
| | | string | | |
+----------+------------------------------+----------------+---------------+---------------+
|"h" | SD_BUS_TYPE_UNIX_FD | UNIX file | int * | |
| | | descriptor | | |
+----------+------------------------------+----------------+---------------+---------------+
|"a" | SD_BUS_TYPE_ARRAY | array | int, which | n sets of |
| | | | specifies the | arguments |
| | | | expected | appropriate |
| | | | length n of | for the array |
| | | | the array | element type |
+----------+------------------------------+----------------+---------------+---------------+
|"v" | SD_BUS_TYPE_VARIANT | variant | signature | arguments |
| | | | string | 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 | arguments | arguments |
| | | entry start | appropriate | appropriate |
+----------+------------------------------+----------------+ for the first | for the |
|"}" | SD_BUS_TYPE_DICT_ENTRY_END | dictionary | type in the | second type |
| | | entry end | pair | in the pair |
+----------+------------------------------+----------------+---------------+---------------+
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.
RETURN VALUE
On success, these functions return a non-negative integer. On failure,
they return a negative errno-style error code.
Errors
Returned errors may indicate the following problems:
-EINVAL
Specified type string is invalid or the message parameter is NULL.
-ENXIO
The message does not contain the specified type at current
position.
-EBADMSG
The message cannot be parsed.
-EBUSY
When reading from a container, this error will be returned if
unread elements are left in the container.
NOTES
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.
EXAMPLES
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);
SEE ALSO
systemd(1), sd-bus(3), sd_bus_message_read_basic(3),
sd_bus_message_skip(3), sd_bus_message_append(3),
sd_bus_message_enter_container(3)
systemd 254 SD_BUS_MESSAGE_READ(3)
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