Finding unprintable characters and printing out their hex form in C - c

Question

I currently have a finite state machine which analyzes a long string, separates the long string by white space, and analyzes each token to either octal, hex, float, error, etc.
Here is a brief overview of how I analyze each token:
enum state mystate = start_state;
while (current_index <= end_index - 1) { // iterate through whole token
switch (mystate) {
case 0:
// analyze first character and move to appropriate state
// cases 1-5 represent the valid states, if error set mystate = 6
case 6: // this is the error state
current_index = end_index - 1; // end loop
break;
}
current_index++;
}
At the end of this loop, I analyze what state my token fell under, for example if the token didn't fit into any category and it went to state 6 (the error state):
if (mystate == 6) {
// token is char pointer to string token
fprintf(stdout, "Error: \" %s \" is invalid\n", token);
}
Now, I am supposed to print out unprintable characters from 0x20 and under, such as start-of-text, start-of-header, etc. in their hex form, such as [0x02] and [0x01]. I found a good list of the ASCII unprintable characters from 0x20 and under here: http://www.theasciicode.com.ar/ascii-control-characters/start-of-header-ascii-code-1.html
Firstly, I am confused how to even type the unprintable characters into the command line. How does one type an unprintable character as a command line argument for my program to analyze?
After that hurdle, I know that the unprintable characters will fall into state 6, my error state. So I have to modify my error state if statement slightly. Here is my thought process of how to do so in pseudo code:
if (mystate == 6) {
if (token is equal to unprintable character) {
// print hex form, use 0x%x for formatting
} else {
// still error, but not unprintable so just have original error statement
fprintf(stdout, "Error: \" %s \" is invalid\n", token);
}
}
Another thought I had was:
if (mystate == 6) {
if (the token's hex value is between 0x01 and 0x20) {
// print hex form, use 0x%x for formatting
} else {
// still error, but not unprintable so just have original error statement
fprintf(stdout, "Error: \" %s \" is invalid\n", token);
}
}

Answer 1

With a sane libc you would use
#include <ctype.h>
...
if (!isprint((int)ch) {
unsigned x = ch;
printf ("[0x%02x]", 0xff&(int)ch);
}
...
to find non-printable ascii characters, assumed that char ch is your current input character.
To use them in a command line you could use printf(1) from the command line.
printf '\x02'|xxd
0000000: 02
There you see the STX character. BTW. There is an excellent manual page about ascii (ascii(7))!
So as a complete command line:
YOUR_Program "`printf '\x02\x03\x18\x19'`"
(The xxd was just to show what comes out of printf, as it is non-printable). xxd is just a hexdump utility, similar to od.
Note: When you really want unprintable input, it is more convenient to take the input either from a file, or from stdin. That simplifies your program call:
printf '\x02\x03\x18\x19'|YOUR_Program

Answer 2

One piece of your puzzle is printing in hex.
Printf("%02x", 7);
This prints the two digit hex value 07.
Another piece is detecting non printable.
If (c < 20).
This translates as if the character has any value less than a space.
You might research the isprint function as there are some unprintable characters that are greater than space.
Good luck. Welcome to c.