D-Bus : Transmit a Data Array in Simple and Useful Form

     In lots situation, One would send data via a simple data structure :  a byte array contain real data, and a integer to note this data's length. This concept would appear when one uses the D-Bus, of Course.  For D-Bus, D-BUS_TYPE_ARRAY is different from the others data types, for it is not way to know the data length if there is no additional information. (length of DBUS_TYPE_STRING could be known by seeking where is the null flag.)

    In this example, I demonstrate How to send a data array via D-Bus.

   server.c :

 
/* server.c */

#include <dbus/dbus.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

DBusHandlerResult filter_func(DBusConnection *connection, 
      DBusMessage *message, void *usr_data)
{
    DBusMessage *reply;
    dbus_bool_t handled = false;
    char *pReadData;
    int len;
    unsigned char i;
     
    DBusError dberr;
    
    dbus_error_init(&dberr);
    
    printf("pReadData = %x\n", (unsigned int)pReadData);
    
    if(FALSE == dbus_message_get_args(message, &dberr, DBUS_TYPE_ARRAY,
  DBUS_TYPE_BYTE, &pReadData, &len,  DBUS_TYPE_INVALID) && 0 != len)
 {
  //printf("len = %d\n");
  //printf("receiver data error\n");
  return DBUS_HANDLER_RESULT_NOT_YET_HANDLED;
 }
 
 if(0 == len)
  return DBUS_HANDLER_RESULT_HANDLED;
    
    printf("len = %d, ", len);
    
    for( i = 0; i < len; i++)
  printf("%#2x ", (unsigned char)pReadData[i]);
 printf("\n");    
    handled = true;
    
    printf("pReadData = %x\n", (unsigned int)pReadData);
    
    /*if one free pReadData, it will crash!*/
    //dbus_free_string_array((char**)&pReadData);
    
    return (handled ? DBUS_HANDLER_RESULT_HANDLED :
  DBUS_HANDLER_RESULT_NOT_YET_HANDLED);
  
}/*filter_func*/


int main(int argc, char *argv[])
{
    DBusError dberr;
    DBusConnection *dbconn;

    dbus_error_init(&dberr);
    dbconn = dbus_bus_get(DBUS_BUS_SESSION, &dberr);

    if (!dbus_connection_add_filter(dbconn, filter_func, NULL, NULL)) {
        return -1;
    }

    dbus_bus_add_match(dbconn, "type='signal',interface='gaiger.Drstein.Demonstration'", &dberr);
    
    while(dbus_connection_read_write_dispatch(dbconn, -1)) {
        /* loop */
    }
    
    return 0;
}/*main*/

    Take a notice: the buffer which containers received data is allocated by D-Bus, you could not release it manually. D-Bus would manage this buffer automatically, it would not occur memory leaking.


client.c :

#include <dbus/dbus.h>
#include <stdbool.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int db_send(DBusConnection *dbconn)
{
    DBusMessage *dbmsg;
    char *pSendData;
    int len;
    unsigned char i;

 pSendData = (char *)malloc(256);
 
    dbmsg = dbus_message_new_signal("/client/signal/Object", 
  "gaiger.Drstein.Demonstration", "Test");
  
 len = 6;
 for(i = 0; i < len; i++)
  pSendData[i] = (unsigned char)i;
   
    if (!dbus_message_append_args(dbmsg, DBUS_TYPE_ARRAY, DBUS_TYPE_BYTE, 
  &pSendData,  len, DBUS_TYPE_INVALID)) 
 {
  return -1;
    }

    if (!dbus_connection_send(dbconn, dbmsg, NULL)) {
        return -1;
    }
    
    dbus_connection_flush(dbconn);
    printf("send message : len = %d, ", len );
    
 for( i = 0; i < len; i++)
  printf("%#x ", (unsigned char)pSendData[i]);
    
 printf("\n"); 
    dbus_message_unref(dbmsg);    
    free(pSendData);
   
    return 0;
}/**/


int main(int argc, char *argv[])
{
 unsigned int i;
    DBusError dberr;
    DBusConnection *dbconn;

    dbus_error_init(&dberr);

    dbconn = dbus_bus_get(DBUS_BUS_SESSION, &dberr);

#if(1)    
    for(i = 0; i < 3; i++)
  db_send(dbconn);
#else
    while(dbus_connection_read_write_dispatch(dbconn, -1)) {
        db_send(dbconn);
    }
#endif  
  
    dbus_connection_unref(dbconn);
    return 0;
}

    The two codes are consistent,  client could send data to server. Hope it is useful.

Note the line in server.c:

 if(FALSE == dbus_message_get_args(message, &dberr, DBUS_TYPE_ARRAY,
 DBUS_TYPE_BYTE, &pReadData, &len,  DBUS_TYPE_INVALID) && 0 != len)

    The type of the length variable should be int. It is, inside the Dbus library, the lengh pointer has been assume as a pointer to an integer (int*). If you put the other integer types as length,  The values in vicinity of the passed length may be marred,  it would let to crash.

   More detail please ref this post in stack overflow.

If you do not know how to rewrite Makefile for the 2 codes, you could refer to this post I wrote.