How to Convert a Hex String to a Byte Array or Character Array in C ?

Converting a Hex String to a Byte Array

In the realm of data handling, it often becomes necessary to convert hex strings into byte arrays to facilitate efficient storage and processing. This process involves translating each pair of hexadecimal digits into their corresponding binary representation.

Converting Hex String to Character Array

If you wish to convert a hex string into a character array, you can leverage the following code:

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

  for (unsigned int i = 0; i < hex.length(); i += 2) {
    std::string byteString = hex.substr(i, 2);
    char byte = (char) strtol(byteString.c_str(), NULL, 16);
    bytes.push_back(byte);
  }

  return bytes;
}

This implementation capitalizes on the intrinsic strtol() function that proficiently transforms text into bytes. Notably, it's capable of handling any even-length hex string.

Example:

std::string hex = "01A1";
auto bytes = HexToBytes(hex);

This code converts the hex string "01A1" into a character array named bytes containing the binary data. Subsequently, you can write this data to a file and examine it using hexdump -C to validate its accuracy.

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