diff --git a/main.c b/main.c
index 85f23d5..4ca9bad 100644
--- a/main.c
+++ b/main.c
@@ -54,11 +54,26 @@ enum {
 #define IS_FLOAT(c) ((c >= '0' && c <= '9') || c == '.')
 #define IS_ALPHA(c) ((c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z'))
 
+typedef struct Function {
+	const char *name;
+	real (*func)(real *args);
+	size_t n_args;
+} Function;
+
+#define FUNCTIONS_CAP 256
+Function functions[FUNCTIONS_CAP];
+size_t functions_size = 0;
+
 void push_tok(Tok t) {
 	if (toks_size+1 < TOKS_CAP)
 		toks[toks_size++] = t;
 }
 
+void add_func(const char *name, real (*func)(real *args), size_t n_args) {
+	if (functions_size+1 < FUNCTIONS_CAP)
+		functions[functions_size++] = (Function){.name = name, .func = func, .n_args = n_args};
+}
+
 void tokenize(char *expr) {
 	push_tok((Tok){.kind = TokOp, .Char = '('});
 
@@ -222,22 +237,23 @@ real eval(Tok *t) {
 			t -= 2;
 
 			real outer_res;
-			if (strcmp(t[1].Str, "sqrt") == 0) {
-				if (arg_results_size != 1) {
-					fprintf(stderr, "Error: function sqrt() requires exactly 1 argument\n");
-					exit(1);
+			bool func_found = false;
+			for (size_t i = 0; i < functions_size; i++) {
+				if (strcmp(t[1].Str, functions[i].name) == 0) {
+					func_found = true;
+					if (arg_results_size != functions[i].n_args) {
+						const char *plural = functions[i].n_args == 1 ? "" : "s";
+						fprintf(stderr, "Error: function %s() requires exactly 1 argument%s\n", functions[i].name, plural);
+						exit(1);
+					}
+					outer_res = functions[i].func(arg_results);
 				}
-				outer_res = sqrt(arg_results[0]);
-			} else if (strcmp(t[1].Str, "pow") == 0) {
-				if (arg_results_size != 2) {
-					fprintf(stderr, "Error: function pow() requires exactly 2 arguments\n");
-					exit(1);
-				}
-				outer_res = pow(arg_results[0], arg_results[1]);
-			} else {
-				fprintf(stderr, "Error: unknown function name: %s\n", t[1].Str);
+			}
+			if (!func_found) {
+				fprintf(stderr, "Error: unknown function: %s()\n", t[1].Str);
 				exit(1);
 			}
+
 			t[1].kind = TokNum;
 			t[1].Num = outer_res;
 		}
@@ -289,11 +305,24 @@ void cleanup() {
 	}
 }
 
+real fn_sqrt(real *args)  { return sqrt(args[0]); }
+real fn_pow(real *args)   { return pow(args[0], args[1]); }
+real fn_mod(real *args)   { return fmod(args[0], args[1]); }
+real fn_round(real *args) { return round(args[0]); }
+real fn_floor(real *args) { return floor(args[0]); }
+real fn_ceil(real *args)  { return ceil(args[0]); }
+
 int main(int argc, char **argv) {
 	if (argc != 2 || strcmp(argv[1], "-h") == 0 || strcmp(argv[1], "--help") == 0) {
 		fprintf(stderr, "Usage: ./exp \"<expression>\"\n");
 		exit(1);
 	}
+	add_func("sqrt", fn_sqrt, 1);
+	add_func("pow", fn_pow, 2);
+	add_func("mod", fn_mod, 2);
+	add_func("round", fn_round, 1);
+	add_func("floor", fn_floor, 1);
+	add_func("ceil", fn_ceil, 1);
 	tokenize(argv[1]);
 	print_toks();
 	real res = eval(toks);