Sayyiduna Abu Waqid al-Laythi (may Allah be pleased with him) says that a man said: O Messenger of Allah! Hunger afflicts us in a certain land; when is carrion lawful for us? He (peace and blessings be upon him) said: "When you find no food in the morning or evening, nor vegetables, then it is up to you."
Abu Ubayd said that Abu Ubaydah said: (Tahtafi'u) comes from al-hafa', which is the fresh white root of the papyrus plant, and it is eaten; that is, as long as you cannot pull up this very plant and eat it. As for "as long as you have no morning or evening drink", it means: you may take from it (carrion) a morning meal or an evening meal, but do not combine both from carrion.
Ibn Awn said: I saw with al-Hasan (may Allah have mercy on him) the letters of Samurah (may Allah be pleased with him) to his sons, (stating) that in a state of compulsion or necessity a morning or an evening drink suffices.
The Shaykh said: Carrion is lawful when lawful food by which one can sustain life is not available in the morning and evening.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19638