Error: ... undeclared (first use in this function)

I am trying to compile this program but it gives me compile error, even though I have added functions.h

Code of p2_e4 (where the main is):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "stack_fp.h"
#include "node.h"
#include "graph.h"
#include "functions.h"

int stack_node_conections(Stack *s, Stack *s_enteros, Graph *g) { 

    int nNodes;
    int numTotal;
    int *ids;

    int i=0, j=0;

    if (!s || !s_enteros|| !g){
        return -1;
    }

    nNodes = graph_getNnodes(g);
    ids = graph_getNodeIds(g);

    numTotal = 2 * graph_getNedges(g);

    for (i=nNodes-1; i >= 0; i--) {

        for (j=nNodes-1; j >= 0; j--) {

            if (graph_areConnected(g, ids[i], ids[j]) == TRUE) {

                stack_push(s, graph_getNode(g, ids[i]));
                stack_push(s_enteros, &ids[i]);
                stack_push(s, graph_getNode(g, ids[j]));
                stack_push(s_enteros, &ids[j]);
            } 
            else 
                continue;
        }
    }
    return numTotal;
}

int main(int argc, char** argv){
    Stack *s;
    Stack *s_enteros;
    Graph *g;

    if(argc<2){
        printf("ERROR. <ejecutable> <archivo>\n");
        return -1;
    }

    g = graph_ini();
    if(!g){
        return -1;
    }
    g = read_graph_from_file(argv[1]);
    s = stack_ini(destroy_node_function, copy_node_function, print_node_function);
    if(!s){
        graph_destroy(g);
        return-1;
    }
    s_enteros = stack_ini(destroy_intp_function, copy_intp_function, print_intp_function);
    if(!s_enteros){
        stack_destroy(s);
        graph_destroy(g);
        return -1;
    }

    stack_node_conections(s, s_enteros, g);
    printf("Pila de enteros:\n");
    stack_print(stdout, s_enteros);
    printf("Pila de nodos:\n");
    stack_print(stdout, s);

    stack_destroy(s_enteros);
    stack_destroy(s);
    graph_destroy(g);
    return 0;
}

Function.h code:

#ifdef FUNCTIONS_H
#define FUNCTIONS_H

void destroy_intp_function(void* e);

void * copy_intp_function(const void* e);

int print_intp_function(FILE * f, const void* e);

void destroy_node_function(void* e);

void * copy_node_function(const void* e);

int print_node_function(FILE * f, const void* e);

#endif