Cast uint8_t to hex string (2 digits) - c

Question

I'm currently using the following to print uint8_t to hex:
for(int j = 0; j < len; j++) {
printf("%02X ", bytes[j]);
}
Is it possible to do this without a for-loop and simply assign the result to a variable?

Answer 1

Here's one simple way:
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
bool to_hex(char* dest, size_t dest_len, const uint8_t* values, size_t val_len) {
if(dest_len < (val_len*2+1)) /* check that dest is large enough */
return false;
*dest = '\0'; /* in case val_len==0 */
while(val_len--) {
/* sprintf directly to where dest points */
sprintf(dest, "%02X", *values);
dest += 2;
++values;
}
return true;
}
int main() {
uint8_t values[256];
char buf[sizeof(values)*2+1]; /* one extra for \0 */
for(size_t i=0; i<256; ++i)
values[i] = i;
if(to_hex(buf, sizeof(buf), values, sizeof(values)))
printf("%s\n", buf);
}
Output:
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
For completeness, here's a self contained version using a lookup table instead of sprintf:
bool to_hex(char* dest, size_t dest_len, const uint8_t* values, size_t val_len) {
static const char hex_table[] = "0123456789ABCDEF";
if(dest_len < (val_len*2+1)) /* check that dest is large enough */
return false;
while(val_len--) {
/* shift down the top nibble and pick a char from the hex_table */
*dest++ = hex_table[*values >> 4];
/* extract the bottom nibble and pick a char from the hex_table */
*dest++ = hex_table[*values++ & 0xF];
}
*dest = 0;
return true;
}

Answer 2

Personally, I'd go for the sprintf approach, but if you badly do not want to use one of the library api, you could use something like this:
char HexLookUp[] = "0123456789abcdef";
void bytes2hex (unsigned char *src, char *out, int len)
{
while(len--)
{
*out++ = HexLookUp[*src >> 4];
*out++ = HexLookUp[*src & 0x0F];
src++;
}
*out = 0;
}
Test code:
int main(void)
{
unsigned char bytes[] = {0x00, 0x01, 0x02, 0x03, 0xDE, 0xAD, 0xBE, 0xEF, 0xCC};
char buffer[sizeof(bytes)*2 + 1];
bytes2hex(bytes, buffer, sizeof(bytes));
printf("%s\n",buffer);
return 0;
}
Output:
00010203deadbeefcc