How to Efficiently Convert a Hex String to a Byte Array in C ?

Hex String Conversion to Byte Array

Converting a variable length hex string to a byte array is a common task in programming, enabling the representation of binary data in a human-readable format. Here's how to achieve this efficiently:

To convert a hex string like "01A1" into a byte array, we can leverage the built-in strtol() function to perform the conversion. The following implementation creates a std::vector of characters to store the byte array:

std::vector<char> HexToBytes(const std::string& hex) {
  std::vector<char> bytes;

  // Loop through the hex string in pairs of characters
  for (unsigned int i = 0; i < hex.length(); i += 2) {
    // Extract the current pair of characters
    std::string byteString = hex.substr(i, 2);

    // Convert the pair to a char using strtol()
    char byte = (char)strtol(byteString.c_str(), NULL, 16);

    // Append the char to the byte array
    bytes.push_back(byte);
  }

  // Return the byte array
  return bytes;
}

This approach works for any even-length hex string. The strtol() function takes a pointer to the character array byteString.c_str() and the base (16 for hexadecimal).

By using this HexToBytes() function, you can easily convert a hex string to a byte array, allowing you to work with binary data in a convenient and flexible manner.

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