Hex_string to uint8_t msg[] - c++

Question

I want convert the characters in hex string
"0b7c28c9b7290c98d7438e70b3d3f7c848fbd7d1dc194ff83f4f7cc9b1378e98"
to uint8_t msg[] and do not understand how to do it.
It seems simple, but have been unable to figure it out. I want to convert each character to a uint8_t hex value. For example if I have
string result = "0123456789abcdef";
How do I convert the string to:
uint8_t msg[] = "0123456789abcdef";

Answer 1

This func (thanks Converting a hex string to a byte array )
vector<uint8_t> HexToBytes(const string& hex) {
vector<uint8_t> bytes;
for (unsigned int i = 0; i < hex.length(); i += 2) {
string byteString = hex.substr(i, 2);
uint8_t byte = (uint8_t) strtol(byteString.c_str(), nullptr, 16);
bytes.push_back(byte);
}
return bytes;
}
using the above function we get the vector of bytes and call the method data()
I want to thank the community now everything is working correctly. Thanks for the comments, I was already desperate that I could not do such a simple thing.
Special thanks to #johnny-mopp

Answer 2

Edit - Updated to ready bytes not characters
Rather than using .substr() and calling C strtol and casting to uint8_t, you can simply use an istringstream along with std::setbase(16) to read the bytes as unsigned values directly into your vector<uint8_t> msg. See std::setbase.
For instance you can create an istringstream from your string containing the hex characters, and then along with your vector of uint8_t and a temporary unsigned to read directly into before pushing back into your vector you could do, e.g.
std::string result ("0123456789abcdef"); /* input hex string */
std::string s2; /* string for 2-chars */
std::istringstream ss (result); /* stringstream of result */
std::vector<uint8_t> msg; /* vector of uint8_t */
while ((ss >> std::setw(2) >> s2)) { /* read 2-char at a time */
unsigned u; /* tmp unsigned value */
std::istringstream ss2 (s2); /* create 2-char stringstream */
ss2 >> std::setbase(16) >> u; /* convert hex to unsigned */
msg.push_back((uint8_t)u); /* add value as uint8_t */
}
In that way, each 2 characters in result read using std::setw(2) are used to create a 2-character stringstream that is then converted a an unsigned value using std::setbase(16). A complete example would be:
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include <vector>
int main (void) {
std::string result ("0123456789abcdef"); /* input hex string */
std::string s2; /* string for 2-chars */
std::istringstream ss (result); /* stringstream of result */
std::vector<uint8_t> msg; /* vector of uint8_t */
while ((ss >> std::setw(2) >> s2)) { /* read 2-char at a time */
unsigned u; /* tmp unsigned value */
std::istringstream ss2 (s2); /* create 2-char stringstream */
ss2 >> std::setbase(16) >> u; /* convert hex to unsigned */
msg.push_back((uint8_t)u); /* add value as uint8_t */
}
std::cout << "string: " << result << "\nmsg: \n";
for (auto& h : msg) /* for each element of msg, output hex value */
std::cout << "\t" << std::setfill('0') << std::hex << std::setw(2)
<< (uint32_t)h << '\n';;
}
(note the cast required in the output to explicitly tell cout to treat the uint8_t value as an unsigned value rather than a uint8_t value which defaults to an character type by default.
Example Use/Output
$ ./bin/hexstr2uint8_t
string: 0123456789abcdef
msg:
01
23
45
67
89
ab
cd
ef
(note there are 8 uint8_t ("byte") values stored this time instead of 16 character values)
It's just an alternative using the C++ iostream features which avoids the need to cast things around rather than calling strtol directly (which in your case should probably be strtoul to begin with).
Manual Hex Conversion
In your last comment you indicate that using iostream and stringstream for the conversion is slow. You can attempt to optimize a bit by eliminating the stringstream and using a string::iterator to step through the string manually converting each character and forming each uint8_t byte as you go (protecting against a final nibble or 1/2-byte), e.g.
#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
/* simple manual conversion of hexchar to value */
uint8_t c2hex (const char c)
{
uint8_t u = 0;
if ('0' <= c && c <= '9')
u = c - '0';
else if ('a' <= c && c <= 'f')
u = c - 'W';
else if ('A' <= c && c <= 'F')
u = c - '7';
else
std::cerr << "error: invalid hex char '" << c << "'\n";
return u;
}
int main (void) {
std::string s ("0123456789abcdef");
std::vector<uint8_t> msg;
for (std::string::iterator n = s.begin(); n != s.end(); n += 2) {
uint8_t u = c2hex (*n); /* save high-nibble */
if (n + 1 != s.end()) /* if low-nibble available */
u = (u << 4) | c2hex (n[1]); /* shift high left 4 & or */
msg.push_back(u); /* store byte in msg */
}
std::cout << "string: " << s << "\nmsg:\n";
for (auto& h : msg)
std::cout << "\t" << std::setfill('0') << std::hex
<< std::setw(2) << (unsigned)h << '\n';
}
(output is the same as above)
If you can guarantee there will always be an even number of characters in your string (bytes only and no 1/2-byte as the final-odd character), you can further optimize by removing the conditional and simply using:
uint8_t u = c2hex (n[1]) | (c2hex (*n) << 4);
Make sure you are compiling with full optimization, e.g. -O3 (or -Ofast gcc version >= 4.6) on gcc/clang and /Ox with VS.
Give that a try and compare performance, you can additionally dump the differing versions to assembly and see if there are any additional hints there.