slip.h

// Copyright (c) Cesanta Software Limited
// All rights reserved
// SPDX-License-Identifier: MIT

// Usage example (Arduino):
//      char buf[100];
//      struct slip slip = {.buf = buf, .size = sizeof(buf) - 1};
//      ...
//      unsigned char c = Serial.read();
//      size_t len = slip_recv(c, &slip);
//      if (len > 0) {
//              buf[len] = '\0';
//              Serial.print("Received SLIP frame: ");
//              Serial.println(buf);
//      } else {
//              Serial.write(c);
//      }

// https://datatracker.ietf.org/doc/html/rfc1055
enum { END = 192, ESC = 219, ESC_END = 220, ESC_ESC = 221 };

// SLIP state machine
struct slip {
  unsigned char *buf;  // Buffer for the frame mode
  size_t size;         // Buffer size
  size_t len;          // Number of currently buffered bytes
  int mode;            // Operation mode. 0 - serial, 1 - frame
  unsigned char prev;  // Previously read character
};

// Encapsulate buf,len in a SLIP frame and send using function fn
static inline void slip_send(const void *buf, size_t len,
                             void (*fn)(unsigned char, void *), void *arg) {
  const unsigned char *p = buf;
  size_t i;
  fn(END, arg);
  for (i = 0; i < len; i++) {
    if (p[i] == END) {
      fn(ESC, arg);
      fn(ESC_END, arg);
    } else if (p[i] == ESC) {
      fn(ESC, arg);
      fn(ESC_ESC, arg);
    } else {
      fn(p[i], arg);
    }
  }
  fn(END, arg);
}

// Process incoming byte `c`.
// In serial mode, do nothing, return 1.
// In frame mode, append a byte to the `buf` and increment `len`.
// Return size of the buffered packet when switching to serial mode, or 0
static inline size_t slip_recv(unsigned char c, struct slip *slip) {
  size_t res = 0;
  if (slip->mode) {
    if (slip->prev == ESC && c == ESC_END) {
      slip->buf[slip->len++] = END;
    } else if (slip->prev == ESC && c == ESC_ESC) {
      slip->buf[slip->len++] = ESC;
    } else if (c == END) {
      res = slip->len;
    } else if (c != ESC) {
      slip->buf[slip->len++] = c;
    }
    if (slip->len >= slip->size) slip->len = 0;  // Overflow. Reset silently
  }
  slip->prev = c;
  // The "END" character flips the mode
  if (c == END) slip->len = 0, slip->mode = !slip->mode;
  return res;
}