Another best practice: Please do not use logging as an excuse to avoid implementing proper error propagation in libraries or utility code.

Bad:

  void something_failable(void *data) {
    if (!is_valid(data)) {
      log_err("invalid data"); 
    }
    
    /* ... */
  }

Less bad:

  int something_failable(xxx_t *ctx, void *data) {
    if (!is_valid(data)) {
      return xxx_err(ctx, "invalid data"); 
    }
    
    /* ... */
  }