get_symbols <- inline::cfunction(
  includes = "
    #define HSIZE   49157 /* The size of the hash table for symbols */
    extern SEXP* R_SymbolTable;
  ",
  body = "
    int symbol_count = 0;
    SEXP s;
    int j;
    for (j = 0; j < HSIZE; j++) {
      for (s = R_SymbolTable[j]; s != R_NilValue; s = CDR(s)) {
        if (CAR(s) != R_NilValue) {
          symbol_count++;
        }
      }
    }


    SEXP result = PROTECT(Rf_allocVector(STRSXP, symbol_count));
    symbol_count = 0;
    for (j = 0; j < HSIZE; j++) {
      for (s = R_SymbolTable[j]; s != R_NilValue; s = CDR(s)) {
        if (CAR(s) != R_NilValue) {
          SET_STRING_ELT(result, symbol_count, PRINTNAME(CAR(s)));
          symbol_count++;
        }
      }
    }

    UNPROTECT(1);
    return result;
  "
)

# Test it out
get_symbols()



# new_symbols() returns a character vector of symbols that have been added since
# the last time it was run.
last_symbols <- get_symbols()
new_symbols <- function() {
  cur_symbols <- get_symbols()
  res <- setdiff(cur_symbols, last_symbols)
  last_symbols <<- cur_symbols
  res
}

# Example

# The first couple times it's run, R might do something that adds symbols, like
# load the compiler package. Run it a bunch of times until it returns
# character(0).
new_symbols()
new_symbols()
new_symbols()
# character(0)


# After R stops loading things, add a new symbol and test if it's detected.
abcdefg <- 1
new_symbols()
#> [1] "abcdefg"