Problems casting a uint8_t array to a struct

Question

I'm having problems casting an array of uint8_t to a struct in C using a Silicon Labs MCU (EFM32GG11B120F2048GQ64). The byte array is the received data from a UART, and I want to cast it to a struct representing the message structure.

When casting the array to the struct, report_type (uint8_t) is byte [0] in the array, as it should. But start_time (uint32_t) is not the value of bytes [1..4] as I would think, but rather bytes [4..7]. Bytes [1..3] are not used.

I tried adding a second uint8_t member report_type2, effectively moving start_time down one position. report_type2 gets the value of byte [1], but start_time still gets the value of bytes [4..7].

Anybody knows what's going on here and why this is?

Byte array:

05 29 68 58 3C 0E 10 02 44 0D D5 C3 3F 0F 5C 29 00 05 69 75 00 00 0A B8 07 25 A7 11 00 B5 01 02 4C 1D 00 1C 0A FF FF 00 00 00 0A 01 B9 06 00 09 07 25 A7 11 00 B5 01 02 4C 1D 00 1C 0A FF FF 00 00 00 0A 01 B9 06 00 09 07 25 A7 11 00 B5 01 02 4C 1D 00 1C

Struct:

typedef struct
{
   uint8_t report_type;
   uint32_t start_time;
   uint16_t period_length;
   uint8_t status_alarm_code;
   float_union_t total;
   float_union_t average;
   subperiod_t subperiod_1;
   subperiod_t subperiod_2;
   subperiod_t subperiod_3;
   subperiod_t subperiod_4;
   subperiod_t subperiod_5;
   subperiod_t subperiod_6;
   subperiod_t subperiod_7;
   subperiod_t subperiod_8;
   subperiod_t subperiod_9;
   subperiod_t subperiod_10;
}report_t;

Casting array to struct:

report_t *report = (report_t *)packet->data;

Answer 1

You cannot do dirty casts like this in C. There are multiple problems:

• The data you copy from might not be aligned.
• The struct might have padding bytes to compensate for internal alignment that are not present in the data you copy from.
• Dereferencing this struct pointer after the cast leads to a strict pointer aliasing violation, meaning it's undefined behavior bug which could result in incorrect machine code getting generated.

---

The safest and most portabile solution is write a function that does serialization/deserialization of the struct data. That is, a function writing to each member manually. This is slightly slower due to the overhead code but the most portable.

Less portable but probably faster is to memcpy the data. If you do this, you must be absolutely sure that the struct has no padding. Which you aren't, the struct you posted is very poorly designed by someone who has never heard of alignment, since it has padding gaps all over it. If you re-design the struct to ensure there's no padding, you could use memcpy or similar methods.

(Union type punning is yet another option.)

Answer 2

Struct elements may not be assumed contiguous for data alignment reasons.

It seems your compiler assumes alignment to 32 bits.

Depending on your compiler, there might be a way to make it pack the struct elements, exact mehtod can be found in the compiler manual.