is_utf8 is a program and a c library to check if a given file (or stdin) contains only valid utf-8 sequences.

To compile (The program and the lib) :

$ make

To run with a string, note how bash can help to forge string from hexadecimal:

$ echo $'\xe9' | ./is_utf8 -
Encoding error on line 1, character 0
\xE9\x0A
^ After a first byte between E1 and EC, expecting two following bytes.

Or to simply test a file, note that 0 is returned and nothing is printed is the file is correctly encoded:

$ ./is_utf8 main.c
$ ./is_utf8 README.md
$ ./is_utf8 test.sh

$ ./is_utf8 is_utf8
is_utf8: Encoding error on line 1, character 25
\x08@
^ After a first byte between C2 and DF, expecting a 2nd byte between 80 and BF

$ ./is_utf8 /dev/urandom
Encoding error on line 1, character 2
rO\xBFfI\xE2\xDBz}gM~
  ^ Expecting bytes in the following ranges: 00..7F C2..F4

Which prints nothing it it's OK (and returns 0), of prints a usefull error message if an invalid byte sequence is found.

The lib and the program will return 0 (success) if the sequence is a valid utf-8 string. In case an invalid UTF-8 sequence is found they will return the position of the invalid byte and a verbose message about why it's invalid.