Every byte in a file, as represented in an int, takes on a value of 0..255. fgetc doesn't return a char, it returns an int, which means it returns a value of -1

Every byte in a file, as represented in an int, takes on a value of 0..255. fgetc doesn't return a char, it returns an int, which means it returns a value of -1..255 (i.e., 257 possible values). If you try to represent the return value of fgetc as a char, two of those values get the same representation, namely 255 and -1. The difference between ARM and x86 is that on ARM (unsigned char), the -1-as-255 is represented as 255 when cast back to an int for comparison, whereas on x86, the 255 when cast back to an int is -1, so it would return the same value as EOF (although it would also do so had the value in the file originally been -1). Representing binary data as unsigned char (as opposed to char or signed char) is the norm however.