How to Convert Hex Strings to Byte Arrays in C ?

Converting Hex Strings to Byte Arrays

When working with binary data, it's often necessary to convert hexadecimal strings into byte arrays. The hex string represents the raw binary data in its hexadecimal format. Here's how you can accomplish this conversion in C using a variable-length hex string.

To convert the hex string "01A1" into a byte array, we'll use the following approach:

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 function iterates through the hex string, extracting pairs of characters at a time and converting them to their corresponding bytes using the strtol() function. The resulting byte array is then returned.

Once you have converted the hex string to a byte array, you can store it in a variable of type char* or std::vector as required by your specific application. By writing the byte array to a file and using the hexdump -C command, you can verify that the data corresponds to the original hex string.

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