Displaying an integer to hex in C - c

Question

I have this part of code in C class:
int i;
for (i=0;i<val;i++)
mdl_print ("%u ", rec->payload[i]);
mdl_print ("\n");
Variable rec->payload is uint8_t type. I would print it in hexadecimal notation.
How can I do? Thanks.

Answer 1

Quick and easy:
print("%x", (int)(rec->payload[i]));
To display in the format 0x05 then use:
print("0x%02x", ...)
instead.

Answer 2

If mdl_print() works like the standard C function printf(), try something like the following:
printf( "%02x", (int) rec->payload[i] );
The basic printf formatting code for writing in hexadecimal is %x. "02" means to pad the number with '0' until it's two characters wide, which is how you'd normally print an int8.
Many custom output functions follow the format of printf in this way, since it's very familiar to C programmers. You can read more about printf on its man page: http://linux.die.net/man/3/printf

Answer 3

Use "%x", so:
mdl_print ("%x ", rec->payload[i]);
This will give you a hex number that is the "length necessary". If you want a fixed length, use "%02x" for two digits padded with zeros.

Answer 4

Note that providing a uint8_t or int where an unsigned int is expected in printf is undefined behaviour, as stated by section 7.21.6.1p9 of n1570.pdf: "If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined." According to 7.21.6.1p8, %x tells printf to expect an unsigned int. Make sure you explicitly cast your uint8_t to (unsigned int), or use the PRIx8 macro found in <stdint.h>.
#include <assert.h>
#include <stdint.h>
#include <stdio.h>
int main(void) {
uint8_t foo = 42;
/* These two printfs are equivalent: */
printf("Hex for %02u: %02x\n", (unsigned int) foo, (unsigned int) foo);
printf("Hex for %02" PRIu8 ": %02" PRIx8 "\n", foo, foo);
return 0;
}

Answer 5

First your title is so wrong... Casting integer to hex in C casting implies a type change, hex is a number system not a type. You're "displaying an integer in hex" not "casting". If you don't understand the difference please ask for clarification.
As far as the displaying goes, it depends what you want the result to look like. One short cut that I typically use is:
mdl_print("%#x ", rec->payload[i]);
Which displays values as:
0xA6
The caveat on this syntax is that it doesn't work with 0, you don't get 0x0 just 0. So an alternative would be:
mdl_print("0x%x ", rec->payload[i]);
Which will display the value as 0xA6 or 0x0 or whatever it maybe. Of course if you don't want the 0x part you can always just do:
mdl_print("%x ", rec->payload[i]);
Your max value (in hex) with a unsigned 8-bit number is 0xFF so if you want them all to display the same "width" you can use the size specifier too:
mdl_print("%02x ", rec->payload[i]); // or mdl_print("0x%02x, rec->payload[i]);
// or whatever you picked