Help - Building a packet for W/L transmission

using cheap rf wireless modules , virtualwire library i`m trying send data 1 arduino can read in data processing sketch. figured best way decide on specific format , make packet before sending it.

i had decided on packet starting 1 command byte, followed number of data bytes, until comes terminating character.

the examples in virtualwire documentation sends strings this..

code: [select]

const char *msg = "hello";
vw_send((uint8_t *)msg, strlen(msg));

my problem building string send. lets want send sonar range result. first byte s, distance integer follows, add terminator. attempt..

code: [select]

void rfpacketout(char commandchar, int data)
{
 char* packet;
 char* buffer;
 packet[0] = commandchar;
 itoa(data, buffer, 10);
 strcat(packet, buffer);
 strcat(packet, "\n");
 vw_send((uint8_t *)packet, strlen(packet));
 serial.println(packet);
}

but if try rfpacketout('a', 134); serial , w/l output both blank.
i don`t think know enough strings/characters/pointers etc know whats going on :-/

code: [select]

char* packet;
 char* buffer;
 packet[0] = commandchar

"packet" doesn't point - haven't initialised it, declared it, "packet

• " (or "*packet") anywhere - 1 of main dangers of c programming.
  
  same goes "buffer".
  
  
  code: [select]
  
typedef struct {
 byte command;
 char buffer [11];   //don't forget account terminator.
} packet_t;
..
..
packet_t packet;

 packet.command = commandchar;
 itoa(data, packet.buffer, 10);

 vw_send((uint8_t *)&packet, sizeof(packet));

  
  might 1 solution start playing with.

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