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enum {
  WAIT_MAGIC0,
  WAIT_MAGIC1,
  WAIT_HEADER,
  WAIT_PAYLOAD,
  DONE,
} state;
 
struct packet {
  uint8_t id;
  uint8_t length;
  uint8_t seq_num;
  uint8_t payload[231];
}
 
// return the size of the consumed data.
size_t parse_packet(uint8_t* data, size_t length, state *state, packet *packet) {
  size_t idx = 0;
  while (idx < length) {
    switch (*state) {
      WAIT_MAGIC0:
        if (data[idx] == 0xab) {
          *state = WAIT_MAGIC1;
        }
        idx++;
        break;
 
      WAIT_MAGIC1:
        if (data[idx] == 0xba) {
          *state = WAIT_HEADER;
        } else {
          *state = WAIT_MAGIC0;
        }
        idx++;
        break;
 
      WAIT_HEADER:
        if (idx + 3 >= length) {
          return idx;
        }
 
        memcpy(packet, data + idx, 3);
        idx += 3;
        *state = WAIT_PAYLOAD;
        break;
 
      WAIT_PAYLOAD:
        if (idx + packet->length >= length) {
          return idx;
        }
 
        memcpy(packet->payload, data + idx, packet->length);
        idx += packet->length;
        *state = DONE;
        return idx;
    }
  }
 
  return idx;
}
 
 
int main() {
  uint8_t buf[MAX_PACKET_LEN * 3];
  size_t buf_size = 0;
 
  state state = WAIT_MAGIC0;
  packet packet;
 
  while (1) {
    size_t rcv_len = uart_read(); // recieve buffer and store to (buf + buf_size)
    buf_size += rcv_len;
 
    size_t consumed_len = parse_packet(buf, buf_size, &state, &packet);
    if (state == DONE) {
      // you get a packet.
    } else {
      // remove the consumed buffers.
    }
 
  }
}
 

prog.c:10:3: error: unknown type name ‘uint8_t’
   uint8_t id;
   ^~~~~~~
prog.c:11:3: error: unknown type name ‘uint8_t’
   uint8_t length;
   ^~~~~~~
prog.c:12:3: error: unknown type name ‘uint8_t’
   uint8_t seq_num;
   ^~~~~~~
prog.c:13:3: error: unknown type name ‘uint8_t’
   uint8_t payload[231];
   ^~~~~~~
prog.c:17:8: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘parse_packet’
 size_t parse_packet(uint8_t* data, size_t length, state *state, packet *packet) {
        ^~~~~~~~~~~~
prog.c: In function ‘main’:
prog.c:64:3: error: unknown type name ‘uint8_t’
   uint8_t buf[MAX_PACKET_LEN * 3];
   ^~~~~~~
prog.c:64:15: error: ‘MAX_PACKET_LEN’ undeclared (first use in this function)
   uint8_t buf[MAX_PACKET_LEN * 3];
               ^~~~~~~~~~~~~~
prog.c:64:15: note: each undeclared identifier is reported only once for each function it appears in
prog.c:65:3: error: unknown type name ‘size_t’
   size_t buf_size = 0;
   ^~~~~~
prog.c:65:3: note: ‘size_t’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
prog.c:1:1:
+#include <stddef.h>
 enum {
prog.c:65:3:
   size_t buf_size = 0;
   ^~~~~~
prog.c:67:3: warning: statement with no effect [-Wunused-value]
   state state = WAIT_MAGIC0;
   ^~~~~
prog.c:67:8: error: expected ‘;’ before ‘state’
   state state = WAIT_MAGIC0;
        ^~~~~~
        ;
prog.c:68:3: error: unknown type name ‘packet’; use ‘struct’ keyword to refer to the type
   packet packet;
   ^~~~~~
   struct 
prog.c:71:5: error: unknown type name ‘size_t’
     size_t rcv_len = uart_read(); // recieve buffer and store to (buf + buf_size)
     ^~~~~~
prog.c:71:5: note: ‘size_t’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
prog.c:71:22: warning: implicit declaration of function ‘uart_read’ [-Wimplicit-function-declaration]
     size_t rcv_len = uart_read(); // recieve buffer and store to (buf + buf_size)
                      ^~~~~~~~~
prog.c:74:5: error: unknown type name ‘size_t’
     size_t consumed_len = parse_packet(buf, buf_size, &state, &packet);
     ^~~~~~
prog.c:74:5: note: ‘size_t’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
prog.c:74:27: warning: implicit declaration of function ‘parse_packet’ [-Wimplicit-function-declaration]
     size_t consumed_len = parse_packet(buf, buf_size, &state, &packet);
                           ^~~~~~~~~~~~
prog.c:74:12: warning: unused variable ‘consumed_len’ [-Wunused-variable]
     size_t consumed_len = parse_packet(buf, buf_size, &state, &packet);
            ^~~~~~~~~~~~
prog.c:64:11: warning: unused variable ‘buf’ [-Wunused-variable]
   uint8_t buf[MAX_PACKET_LEN * 3];
           ^~~

Standard output is empty