What if there was no one to offer food to monastics?

Even when the Dhamma was at its peak (the time of Gautama Buddha ~ 2500 years ago), not everyone became monks and nuns. What would be the odds of all humans practicing like monks and nuns in this day and age?

You have to travel with your begging bowl until you find someone offer foods. or source of food.(eg. fruit tree not belong to anyone).

I provided an answer before, but reflecting on this question further it occurred to me that it is not a problem even if there is no food. In Buddha's time there was a time when the monks could not get their supply of food from the residents. King Agnidatta forbade his subjects from giving alms to the monks. But a group of horse dealers was passing through that place carrying barley for horses. The monks survived for four months eating this food given to the horses.

Once when Mara closed peoples' hearts against giving food to him, Lord Buddha spoke the following famous verse from the Dhammapada showing his power to obtain spiritual food from meditation:

Happy, indeed, we live, We who possess nothing, Feeders on joy shall we be, Even as gods of the radiant realm. — Dhp 200

Remember well the Supreme Buddha’s words… “Dear Bhikkhus, ones who would not realize this Dhamma is like this earth (in amount). Ones who realize the Nibbāna is like this small amount of soil on my finger nail.” For true disciples who truly follow the Noble Eight Fold Path, food is of no issue as even the Devas / angels conspire to help them. Also read the article http://www.accesstoinsight.org/lib/authors/khantipalo/wheel073.html if time permits. It is very informative in this regard. May all beings, wheresoever they be, taste of this truth.

What if there was no one to offer food to monastics?

If such a scenario occured their might be held a world wide Buddhist Counsil in order to establish some new rules for monastics, e.g. that they could now grow their own food in the right amount and with all precautions taken so that they would do the smallest amount of harm possible to living beings and to Nature.

The other option is that nothing would be done. It would just be seen for what it is, i.e. an absence of food. The dying process is not much different than the living process. It's still a combination of seeing, hearing, smelling, tasting, touching and thinking.

Is there a canonical reference saying monastics can only eat if given food?

There is this rule in the "Food Chapter" in the Bhikkhu Pāṭimokkha - The Bhikkhus' Code of Discipline:

"40. Should any bhikkhu take into his mouth an edible that has not been given — except for water and tooth-cleaning sticks — it is to be confessed".

The rule says that it is an offence to eat food that is not given. There might be other rules and canonical references too. I just do not have knowledge about them.

What about forest monastics who go of four extended periods alone?

I once listened to a dhamma talk by Ajahn Brahmali. He told that when Ajahn Brahmavamso went to into a 6 month solitude retreat the other monks brought Ajahn Brahm his food. They would place it outside his hut and then when they were gone he would come out and take it.

A monk is supposed to go on alms round (begging) for his meal unless he is (1) invited to a meal, (2) the meal is brought to the monastery, or (3) the meal is cooked in the monastery. He is not allowed to cook food, store food overnight, or engage in agriculture to support himself. Thus mendicancy is one of the cornerstones of a Buddhist monk’s lifestyle. This can be seen in a Buddhist country (e.g. Thailand) where a monk has the freedom and support to practise totally in conformity with the Buddha’s teachings. There we see not only forest monks going on alms round but also town and city monks begging for food every day. Since a beggar must not be a chooser, as the saying goes, vegetarianism is incompatible with the Buddhist monk’s lifestyle. –which was probably another reason why the Buddha rejected Devadatta’s request. However the Buddha also said that if a monk does not get sufficient or nutritious food, he should depart from that place.

Although this question is labeled Theravada, historically this did occur with Chinese Mahayana Buddhism. Because historically the Confucian Chinese government frown upon the practice of begging, Chinese monks were by imperial decree not allowed to go out for alms. Hence, they must either lived off donations of food, or grow their own in the monastery. Hence in Chan Buddhism, monks grow their own food, and the process of farming, cooking, serving food, and eating are taken as practices of mindfulness.

This is also the reason why Chinese Buddhism is strictly vegetarian, because growing their own food means that it would not be acceptable to raise animals as food because that would violate the no killing precept. And because they perform hard labor throughout the day, often in very cold weather condition, the vinaya rules of no eating afternoon is removed. Dinner is instead considered to be 'medicine' for keeping themselves healthy.

There are several monastic orders that currently allow monks to cook their own food, but usually there is a lay community that donates the cash or food.

I don't know of anyone who starved merely because there was no one to offer food. The Vinaya is real but so is reality. I think most will find an empty stomach is more real than the Vinaya.

In any case the making an honest living, if absolutely necessary isn't a parajika offense - a monastic may prove to his/her peers that it was essential to earn money, and if the peers agree, there is no penalty. Even if it is found to be an honest mistake, it won't be treated as harshly as if one had broken a parajika vow.

The four transgressions which incur a Parajika, the penalty of automatic disrobal, are as follows:

1. Engaging in sexual intercourse with another being of either sex.

2. Stealing something of value (which includes smuggling, cheating or deliberately avoiding payment of a tax).

3. Purposely killing a human being or encouraging him or her to commit suicide (this includes inciting another to murder somebody and it also includes convincing a woman to have an abortion.

4. Boasting that one has realised a high spiritual attainment, knowing that one is lying. For example, claiming to be enlightened, to be Maitreya Buddha, to have entered Jhana (deep meditation-ecstasy) or that one can read minds when one knows that one hasn't reached any of these states.

source: http://www.budsas.org/ebud/ebsut019.htm