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sd_bus_message_read_basic - Read a basic type from a message


#include <systemd/sd-bus.h>

int sd_bus_message_read_basic(sd_bus_message *m, char type, void *p);


sd_bus_message_read_basic() reads a basic type from a message and advances the read position in the message. The set of basic types and their ascii codes passed in type are described in the D-Bus Specification[1].

If p is not NULL, it should contain a pointer to an appropriate object. For example, if type is 'y', the object passed in p should have type uint8_t *. If type is 's', the object passed in p should have type const char **. Note that, if the basic type is a pointer (e.g., const char * in the case of a string), the pointer is only borrowed and the contents must be copied if they are to be used after the end of the messages lifetime. Similarly, during the lifetime of such a pointer, the message must not be modified. If 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 (for example by calling "fcntl(fd, FD_DUPFD_CLOEXEC, 3)"). See the table below for a complete list of allowed types.

Table 1. Item type specifiers

Specifier Constant Description Expected C Type
"y" SD_BUS_TYPE_BYTE 8bit unsigned integer uint8_t *
"b" SD_BUS_TYPE_BOOLEAN boolean int * (NB: not bool *)
"n" SD_BUS_TYPE_INT16 16bit signed integer int16_t *
"q" SD_BUS_TYPE_UINT16 16bit unsigned integer uint16_t *
"i" SD_BUS_TYPE_INT32 32bit signed integer int32_t *
"u" SD_BUS_TYPE_UINT32 32bit unsigned integer uint32_t *
"x" SD_BUS_TYPE_INT64 64bit signed integer int64_t *
"t" SD_BUS_TYPE_UINT64 64bit unsigned integer uint64_t *
"d" SD_BUS_TYPE_DOUBLE IEEE 754 double precision floating-point double *
"s" SD_BUS_TYPE_STRING UTF-8 string const char **
"o" SD_BUS_TYPE_OBJECT_PATH D-Bus object path string const char **
"g" SD_BUS_TYPE_SIGNATURE D-Bus signature string const char **
"h" SD_BUS_TYPE_UNIX_FD UNIX file descriptor int *

If there is no object of the specified type at the current position in the message, an error is returned.


On success, sd_bus_message_read_basic() returns a positive integer. If the end of the currently opened array has been reached, it returns 0. On failure, it returns a negative errno-style error code.


Returned errors may indicate the following problems:


Specified type string is invalid or the message parameter is NULL.


The message does not contain the specified type at current position.


The message cannot be parsed.


systemd(1), sd-bus(3), sd_bus_message_append_basic(3), sd_bus_message_skip(3), sd_bus_message_read(3)


D-Bus Specification
systemd 247