main.c: contains the fucntion main().
minmax.h: it is in the folder lib, and contains the signatures of the functions min() and max()

minmax.c: the implementation of the functions min() and max()


You can run the program with both files using the following command:
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gcc -Wall -std=c99 -o run_main main.c minmax.c
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Or you can simply run "make", and it will run the command written in the makefile.