sd_bus_message_readv(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(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 messages lifetime.

       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 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          │ 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 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         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);

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

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       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
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       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
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systemd 246                                           SD_BUS_MESSAGE_READ(3)

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