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Memory allocation
Task schedulers
This can provide cooperative multitasking for simple tasks, avoiding the need of an RTOS.
Small C libraries
http://neptune.billgatliff.com/newlib.html
http://www.musl-libc.org/faq.html
http://www.embecosm.com/appnotes/ean9/ean9-howto-newlib-1.0.html
https://github.com/nothings/single_file_libs
Printf
http://elm-chan.org/fsw/strf/xprintf.html
http://www.menie.org/georges/embedded/printf.html