我做了一个仍在开发中的程序。我没有在我的程序中故意声明main。因为我正在开发一个库来制作图形和其他我将在我的程序中使用的算法。用C语言开发这样一个库的目的是解决《算法导论》一书中给出的问题。托马斯H科勒曼这里是代码，如果有人想看的话。

#include<stdio.h>
#include<stdlib.h>
#define GREY 1
#define BLACK 0
#define WHITE 2
typedef struct node *graph;

graph cnode(int data);      //cnode is to create a node for graph
void cgraph(void);
struct node {
    int data, color;
    struct node *LEFT, *RIGHT, *TOP, *DOWN;
};

graph root = NULL;

void cgraph(void)
{
    int n, choice, dir, count;

    choice = 1;
    count = 1;
    graph priv, temp;

    printf("Printf we are making a graph the first is root node\n");
    while (choice == 1) {
        count++;
        if (count == 1) {
            printf("This is going to be root node \n");
            scanf("%d", &n);
            root = cnode(n);
            count--;
            priv = root;
        }       //ending if
        else {
            printf
                ("Enter direction you want to go LEFT 1 RIGHT 2 TOP 3 DOWN 4\n");
            scanf("%d", &dir);
            printf("Enter the data  for graph node\n");
            scanf("%d", &n);
            temp = cnode(n);
            if (dir == 1) {
                priv->LEFT = temp;
            }
            if (dir == 2) {
                priv->RIGHT = temp;
            }
            if (dir == 3) {
                priv->TOP = temp;
            }
            if (dir == 4) {
                priv->DOWN = temp;
            }
            priv = temp;
        }       //ending else
        printf
            ("Enter 1 to continue adding nodes to graph any thing else would take you out\n");
        scanf("%d", &choice);
    }           //ending while
}               //ending main

graph cnode(int data)
{
    graph temp = (graph) malloc(sizeof(graph));

    temp->data = data;
    temp->LEFT = NULL;
    temp->RIGHT = NULL;
    temp->TOP = NULL;
    temp->DOWN = NULL;
    temp->color = -1;
    return temp;
}

当我编译上述程序时，我得到了以下错误.

cc graph.c
/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: ld returned 1 exit status

这个错误意味着什么？为什么要在程序中声明main？