sd_bus_message_read(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | NOTES | EXAMPLES | SEE ALSO | COLOPHON

SD_BUS_MESSAGE_READ(3)     sd_bus_message_read    SD_BUS_MESSAGE_READ(3)

NAME         top

       sd_bus_message_read, sd_bus_message_readv,
       sd_bus_message_peek_type - Read a sequence of values from a
       message

SYNOPSIS         top

       #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         top

       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             │ 8bit           │ uint8_t *    │              │
       │          │                              │ unsigned       │              │              │
       │          │                              │ 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          │ uint16_t *   │              │
       │          │                              │ unsigned       │              │              │
       │          │                              │ integer        │              │              │
       ├──────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
       │"i"       │ SD_BUS_TYPE_INT32            │ 32bit signed   │ int32_t *    │              │
       │          │                              │ integer        │              │              │
       ├──────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
       │"u"       │ SD_BUS_TYPE_UINT32           │ 32bit          │ uint32_t *   │              │
       │          │                              │ unsigned       │              │              │
       │          │                              │ integer        │              │              │
       ├──────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
       │"x"       │ SD_BUS_TYPE_INT64            │ 64bit signed   │ int64_t *    │              │
       │          │                              │ integer        │              │              │
       ├──────────┼──────────────────────────────┼────────────────┼──────────────┼──────────────┤
       │"t"       │ SD_BUS_TYPE_UINT64           │ 64bit          │ 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.

RETURN VALUE         top

       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         top

       These APIs are implemented as a shared library, which can be
       compiled and linked to with the libsystemd pkg-config(1) file.

EXAMPLES         top

       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         top

       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)

COLOPHON         top

       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 2021-08-27.  (At that
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       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

systemd 249                                       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)