Food sharing and mutual aid among the homeless
I spent over 5 weeks hanging out with homeless people in Kalamazoo, MI by a nearby homeless shelter. I engaged with them nearly every minute of free time I had after work and on weekends. What I observed regarding food was actually an abundance. When someone releases a study indicating that someone doesn't get "enough" to eat, that is extremely subjective. I used to eat once a day myself out of pure choice and habit. The homeless may eat only once or twice a day, but they likely have many options throughout the day if they are in the right location such as in Kalamazoo. No one went hungry near the shelter, yet many chose not to eat. They also chose where to eat since there were often many locations offering food. In fact, some of them declined when I offered to take them out to eat. 

They do not hoard food for lack of anywhere to put it. Thus, they are often willing to share this food but usually do not even need to share since they can grab their own. From my experiences, I do not recall anyone giving out of their own loss given their full knowledge that more food would come tomorrow. I've seen more sharing with cigarettes and dimes, but this is clearly not a need and is, in general, a means of befriending people. Theft was very common as I was regularly told stories of how their clothes or shoes were stolen. Thus, befriending people was a rather good idea and made it less likely to have your things stolen or to get into fights. Perhaps other homeless communities are not as well-off as the one in Kalamazoo, but my own experience leads me to believe that it's not the same as watching people band together in a mutual experience of hardship. Most of the people I saw seemed deficient in the mental arena. They seemed to be stuck in high school relationship mentality with elementary school cognitive prowess. Only a handful of people seemed more capable and simply mooched the system. Perhaps you need someone's input who has been with competent yet unfortunate homeless people, but I thought I'd share my own experience.