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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | HISTORY | SEE ALSO | COLOPHON |
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SD_BUS_MESSAGE_READ(3) sd_bus_message_read SD_BUS_MESSAGE_READ(3)
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 │ 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 │ int16_t * │ │
│ │ │ signed │ │ │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
│ "q" │ SD_BUS_TYPE_UINT16 │ 16-bit │ uint16_t * │ │
│ │ │ unsigned │ │ │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
│ "i" │ SD_BUS_TYPE_INT32 │ 32-bit │ int32_t * │ │
│ │ │ signed │ │ │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
│ "u" │ SD_BUS_TYPE_UINT32 │ 32-bit │ uint32_t * │ │
│ │ │ unsigned │ │ │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
│ "x" │ SD_BUS_TYPE_INT64 │ 64-bit │ int64_t * │ │
│ │ │ signed │ │ │
│ │ │ 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 │ arguments │
│ │ │ │ the expected │ appropriate │
│ │ │ │ length n of │ for the │
│ │ │ │ the array │ 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 │
│ ")" │ SD_BUS_TYPE_STRUCT_END │ array end │ elements │
├───────────┼──────────────────────────────┼────────────────┼──────────────┬──────────────┤
│ "{" │ SD_BUS_TYPE_DICT_ENTRY_BEGIN │ dictionary │ arguments │ arguments │
│ │ │ entry start │ appropriate │ appropriate │
├───────────┼──────────────────────────────┼────────────────┤ for the │ for the │
│ "}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary │ first type │ second type │
│ │ │ entry end │ in the 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.
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.
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.
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)
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have
a bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2024-06-14. (At that
time, the date of the most recent commit that was found in the
repository was 2024-06-13.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 257~devel SD_BUS_MESSAGE_READ(3)
Pages that refer to this page: sd-bus(3), sd_bus_message_at_end(3), sd_bus_message_open_container(3), sd_bus_message_read_array(3), sd_bus_message_read_basic(3), sd_bus_message_read_strv(3), sd_bus_message_rewind(3), sd_bus_message_skip(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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