Getting a hexadecimal number into a program via the command line

Question

I can do this:

int main(int argc, char** argv) {
  unsigned char cTest = 0xff;
  return 0;
}

But what's the right way to get a hexadecimal number into the program via the command line?

unsigned char cTest = argv[1];

doesn't do the trick. That produces a initialization makes integer from pointer without a cast warning.

Answer 1

You could use strtoul which would walk through the characters in the string and convert them, taking into account the radix (16 in this context) that you pass in:-

char *terminatedAt;
if (argc != 2)
    return 1;

unsigned long value = strtoul( argv[1], &terminatedAt, 16);

if (*terminatedAt != '\0')
    return 2;

if (value > UCHAR_MAX)
    return 3;

unsigned char byte = (unsigned char)value;
printf( "value entered was: %d", byte);

As covered in the other examples, there are shorter ways, but none of them allow you to cleanly error check the processing (what happens if someone passes FFF and you've only got an unsiged char to put it into?

e.g. with sscanf:

int val;
sscanf(argv[1], &val)
printf("%d\n", val); 

Answer 2

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
  printf("%ld\n", strtol(argv[1], NULL, 16));

  return 0;
}

Example usage:

$ ./hex ff
255

Answer 3

atoi, atol, strtoi, strtol

all in stdlib.h

Answer 4

unsigned char cTest = argv[1];

is wrong, because argv[1] is of type char *. If argv[1] contains something like "0xff" and you want to assign the integer value corresponding to that to an unsigned char, the easiest way would be probably to use strtoul() to first convert it to an unsigned long, and then check to see if the converted value is less than or equal to UCHAR_MAX. If yes, you can just assign to cTest.

strtoul()'s third parameter is a base, which can be 0 to denote C-style number parsing (octal and hexadecimal literals are allowed). If you only want to allow base 16, pass that as the third argument to strtoul(). If you want to allow any base (so you can parse 0xff, 0377, 255, etc.), use 0.

UCHAR_MAX is defined in <limits.h>.

Answer 5

The traditional way to do this kind of thing in C is with scanf(). It's exactly the inverse of printf(), reading the given format out of the file (or terminal) and into the variables you list, rather than writing them into it.

In your case, you'd use sscanf as you've already got it in a string rather than a stream.