Part Number: EK-TM4C123GXL
Hey folks,
I want to write a little MQTT ibrary for GSM module (SIM868) and I encountert a very strange behavior while doing so. For paket transmitting I wanted to create packed structs in which I save all the information needed for a MQTT-packet (fixed header, variable header and payload). Therefor its important that every Byte is used and no Bytes are leaved empty. I am using Code Composer Studio 11 and TI-RTOS. Since now I dont really use any of the RTOS features, I have just created a task which should set up the connection and send a message to my server. The code I am using for creating the Structs is the following:
Deklaration:
* ***** Typdef Structs *****
*
* All Structs needed to represent MQTT-control-packets
*
* */
/* Fixed Header Struct */
#pragma pack(push,1)
typedef struct {
char flags : 4;
char paket_type : 4;
char remaining_length : 8;
}fh_struct;
#pragma pack(pop)
/* ===== Connect Paket ===== */
/* Variable-Header-Struct for Connect */
#pragma pack(push,1)
typedef struct {
char length_protokol_name_msb;
char length_protokol_name_lsb;
uint32_t protokol_name; // M Q T T
char generation; // 4
char flags;
char keepalive_msb;
char keepalive_lsb;
}vh_Connect_struct;
#pragma pack(pop)
/* Payload-Struct for Connect */
#pragma pack(push,1)
typedef struct {
char client_id_length_msb;
char client_id_length_lsb;
uint32_t client_id;
}pl_Connect_struct;
#pragma pack(pop)
/* MQTT overall construct for Connect */
#pragma pack(push,1)
typedef struct {
fh_struct fh_Connect;
vh_Connect_struct vh_Connect;
pl_Connect_struct pl_Connect;
char escape;
}mqtt_paket_Connect_struct;
#pragma pack(pop)
/* ===== Publish Paket ===== */
/* Topic Struct */
#pragma pack(push,1)
typedef struct {
char quantity1;
char quantity2;
uint16_t id;
}topic_struct;
#pragma pack(pop)
/* Variable-Header-Struct for Publish */
#pragma pack(push,1)
typedef struct {
char topiclength_msb;
char topiclength_lsb;
topic_struct topic;
char paket_id_msb;
char paket_id_lsb;
}vh_publish_struct;
#pragma pack(pop)
/* Payload-Struct for Publish*/
#pragma pack(push,1)
typedef struct {
char message_length_msb;
char message_length_lsb;
float message;
}pl_Publish_struct;
#pragma pack(pop)
/* MQTT overall construct for Publish */
#pragma pack(push,1)
typedef struct {
fh_struct fixed_header;
vh_publish_struct vh_Publish;
pl_Publish_struct payload;
char escape;
}mqtt_paket_Publish_struct;
#pragma pack(pop)
initialization:
mqtt_paket_Connect_struct mqtt_init_connect (void){
conpak.fh_Connect.paket_type=1;
conpak.fh_Connect.flags=0;
conpak.fh_Connect.remaining_length=15;
conpak.vh_Connect.length_protokol_name_msb=0;
conpak.vh_Connect.length_protokol_name_lsb=4;
conpak.vh_Connect.protokol_name=1414811981; //MQTT
conpak.vh_Connect.generation=4;
conpak.vh_Connect.flags=2;
conpak.vh_Connect.keepalive_msb=0;
conpak.vh_Connect.keepalive_lsb=128;
conpak.pl_Connect.client_id_length_msb=0;
conpak.pl_Connect.client_id_length_lsb=4;
conpak.pl_Connect.client_id=827151175; //GSM1
conpak.escape=26;
return(conpak);
}
mqtt_paket_Publish_struct mqtt_init_publish (void){
pubpak.fixed_header.paket_type=3;
pubpak.fixed_header.flags=0;
pubpak.fixed_header.remaining_length=14;
pubpak.vh_Publish.topiclength_msb=0;
pubpak.vh_Publish.topiclength_lsb=4;
pubpak.vh_Publish.topic.quantity1=116; //t
pubpak.vh_Publish.topic.quantity2=112; //p -> Temperatur
pubpak.vh_Publish.topic.id=2415; //Dummie ID
pubpak.vh_Publish.paket_id_msb=0;
pubpak.vh_Publish.paket_id_lsb=1;
pubpak.payload.message_length_msb=0;
pubpak.payload.message_length_lsb=4;
pubpak.payload.message=24.3;
pubpak.escape=26;
return(pubpak);
}
Im relativly new to this and I especially used #pragma pack for the first time. My structs are globaly defined as the following:
static mqtt_paket_Connect_struct conpak; static mqtt_paket_Publish_struct pubpak;
What I realised after implementing my structs like this is, that in memory my structs are layet out like the following:
You see that conpak (the mqtt_paket_Connet_struct ist layed out like it should with 19 Bytes which is correct (Fixed Header (2 Bytes) + Variable Header (10 Bytes) + Payload =(6 Bytes) + Escape Char = 19 Bytes). I just dont get why my pubpak (the mqtt_paket_publish_struct) is 32 Bytes when it should be 17 Bytes (Fixed Header (2 Bytes) + Variable Header (8 Bytes) + Payload (6 Bytes) + Escape Char = 17 Bytes). I used the same kind of pragma pack operations in both cases so I just dont understand why it works for my connect paket but not for my publish paket. What I also found weird was, that sizeof(pubpak) gives me 17 as value nevertheless.
Any help is highly aprreciatet .
