How to convert literal char hex value into the hex value itself? - c

Question

I'm having the literal value that should be stored on an unsigned char[64] array. How can I convert such values to it's hex equivalent?
int main() {
unsigned char arry[1] = { 0xaa }
char* str = "fe"; //I want to store 0xfe on arry[0]
arry[0] = 0xfe; // this works, but I have to type it
arry[0] = 0x + str; //obviously fails
return 0;
}
Any pointers?

Answer 1

arr[0] = strtol(str,NULL,16); // If one entry is big enough to hold it.

Answer 2

For each character c, the value is:
if ('0' <= c && c <= '9') return c - '0';
if ('a' <= c && c <= 'f') return c - 'a' + 10;
if ('A' <= c && c <= 'F') return c - 'A' + 10;
// else error, invalid digit character
Now just iterate over the string from left to right, adding up the digit values, and multiplying the result by 16 each time.
(This is implemented for you by the standard library in the strto*l functions with base 16.)

Answer 3

Use function strtol() to convert a string to a long in a specific base: http://www.cplusplus.com/reference/cstdlib/strtol/
"Parses the C-string str interpreting its content as an integral number of the specified base, which is returned as a long int value. If endptr is not a null pointer, the function also sets the value of endptr to point to the first character after the number."
Example:
#include <stdio.h> /* printf */
#include <stdlib.h> /* strtol */
int main ()
{
char szNumbers[] = "2001 60c0c0 -1101110100110100100000 0x6fffff";
char * pEnd;
long int li1, li2, li3, li4;
li1 = strtol (szNumbers,&pEnd,10);
li2 = strtol (pEnd,&pEnd,16);
li3 = strtol (pEnd,&pEnd,2);
li4 = strtol (pEnd,NULL,0);
printf ("The decimal equivalents are: %ld, %ld, %ld and %ld.\n", li1, li2, li3, li4);
return 0;
}

Answer 4

Put together an arbitrary length solution to and from.
Sadly the String to X is verbose: pesky dealing with non hex string, odd length, too big, etc.
#include <string.h>
#include <stdio.h>
// S assumed to be long enough.
// X is little endian
void BigXToString(const unsigned char *X, size_t Length, char *S) {
size_t i;
for (i = Length; i-- > 0; ) {
sprintf(S, "%02X", X[i]);
S += 2;
}
}
int BigStringToX(const char *S, unsigned char X[], size_t Length) {
size_t i;
size_t ls = strlen(S);
if (ls > (Length * 2)) {
return 1; //fail, too big
}
int flag = ls & 1;
size_t Unused = Length - (ls/2) - flag;
memset(&X[Length - Unused], 0, Unused); // 0 fill unused
char little[3];
little[2] = '\0';
for (i = Length - Unused; i-- > 0;) {
little[0] = *S++;
little[1] = flag ? '\0' : *S++;
flag = 0;
char *endptr;
X[i] = (unsigned char) strtol(little, &endptr, 16);
if (*endptr) return 1; // non-hex found
if (*S == '\0') break;
}
return 0;
}
int main() {
unsigned char X[64];
char S[64 * 2 + 2];
char T[64 * 2 + 2];
strcpy(S, "12345");
BigStringToX(S, X, sizeof(X));
BigXToString(X, sizeof(X), T);
printf("'%s'\n", T);
return 0;
}