Provided by: libsystemd-dev_249.11-0ubuntu3.12_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 first │ Types of the │
│ │ │ │ argument │ subsequent │
│ │ │ │ │ arguments, if any │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"y" │ SD_BUS_TYPE_BYTE │ 8bit unsigned │ uint8_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"b" │ SD_BUS_TYPE_BOOLEAN │ boolean │ int * (NB: not bool │ │
│ │ │ │ *) │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"n" │ SD_BUS_TYPE_INT16 │ 16bit signed │ int16_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"q" │ SD_BUS_TYPE_UINT16 │ 16bit unsigned │ uint16_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"i" │ SD_BUS_TYPE_INT32 │ 32bit signed │ int32_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"u" │ SD_BUS_TYPE_UINT32 │ 32bit unsigned │ uint32_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"x" │ SD_BUS_TYPE_INT64 │ 64bit signed │ int64_t * │ │
│ │ │ integer │ │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"t" │ SD_BUS_TYPE_UINT64 │ 64bit 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 path │ const char ** │ │
│ │ │ 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 arguments │
│ │ │ │ specifies the │ appropriate for the │
│ │ │ │ expected length n │ array element type │
│ │ │ │ of the array │ │
├──────────┼──────────────────────────────┼───────────────────┼─────────────────────┼─────────────────────┤
│"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 the │ appropriate for the │
├──────────┼──────────────────────────────┼───────────────────┤ first type in the │ second type in the │
│"}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary entry │ pair │ 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.
NOTES
These APIs are implemented as a shared library, which can be compiled and linked to with the
libsystemd pkg-config(1) file.
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)