Provided by: libsystemd-dev_256.5-2ubuntu4_amd64 
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 argument │ subsequent │
│ │ │ │ │ arguments, if │
│ │ │ │ │ any │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"y" │ SD_BUS_TYPE_BYTE │ 8-bit unsigned │ uint8_t * │ │
│ │ │ 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 unsigned │ uint16_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"i" │ SD_BUS_TYPE_INT32 │ 32-bit signed │ int32_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"u" │ SD_BUS_TYPE_UINT32 │ 32-bit unsigned │ uint32_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"x" │ SD_BUS_TYPE_INT64 │ 64-bit signed │ int64_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"t" │ SD_BUS_TYPE_UINT64 │ 64-bit unsigned │ uint64_t * │ │
│ │ │ 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 signature │ const char ** │ │
│ │ │ 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 length │ appropriate for │
│ │ │ │ n of the array │ 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 │ arguments │ arguments │
│ │ │ start │ appropriate for │ appropriate for │
├──────────┼──────────────────────────────┼──────────────────┤ the first type │ the second type │
│"}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary entry │ in the pair │ in the pair │
│ │ │ 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.
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.
Added in version 251.
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);
HISTORY
sd_bus_message_read() and sd_bus_message_readv() were added in version 240.
sd_bus_message_peek_type() was added in version 246.
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)