14

I would like to print the following hashed data. How should I do it?

unsigned char hashedChars[32];
SHA256((const unsigned char*)data.c_str(),
       data.length(), 
       hashedChars);
printf("hashedChars: %X\n", hashedChars);  // doesn't seem to work??
Robᵩ
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louis.luo
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2 Answers2

18

The hex format specifier is expecting a single integer value but you're providing instead an array of char. What you need to do is print out the char values individually as hex values. 

printf("hashedChars: ");
for (int i = 0; i < 32; i++) {
  printf("%x", hashedChars[i]);
}
printf("\n");

Since you are using C++ though you should consider using cout instead of printf (it's more idiomatic for C++. 

cout << "hashedChars: ";
for (int i = 0; i < 32; i++) {
  cout << hex << hashedChars[i];
}
cout << endl;
Aritz
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JaredPar
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    @vidit the user gave the questions in terms of `printf` so I decided to answer that way. They clearly have a bit of confusion as to how `printf` works, and I wanted to clear that up – JaredPar May 04 '12 at 15:14
  • Ok Sir. I was just thinking because he tagged C++ – vidit May 04 '12 at 15:16
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    `printf` is valid C++ too! There are times when I use it instead of streams. It has its place. – aardvarkk May 04 '12 at 15:18
  • @vidit i went ahead and added the C++ version as well for completeness – JaredPar May 04 '12 at 15:18
  • @aardvarkk- Yes it is, but I thought we should prefer C++ features compared to the ones carried from C – vidit May 04 '12 at 15:27
  • It's more "would" than "should", otherwise it's like saying things like eg.: `sin` and `isspace` should not be used. – Luis Machuca May 04 '12 at 15:57
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    @JaredPar So.. the `printf` actually worked for me, but `cout` didn't work for me. I am using `xCode` and "hashedChars: " got printed, but not the rest. – louis.luo May 04 '12 at 15:59
  • @Louis_PIG i tried this locally and it's working on my machine. I'm not familiar enough with xCode to diagnose any problems easily. Have you tried printing without the `hex` to see what it does? – JaredPar May 04 '12 at 16:36
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    You'll want to ensure that 2 digits are output for each `char`, otherwise you'll be missing some zeros from the output. For the printf change the format to `%02X`, not sure how to do it for cout. – Mark Ransom May 04 '12 at 16:57
  • @JaredPar as indicated by @louis.luo , the `cout << hex << ...` does not work for me – Sven Williamson Mar 01 '17 at 15:47
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    FWIW, had to cast to `unsigned int`, i.e. `hex << (unsigned int)hashedChars[i]` to get it to work for me. – Tomislav Nakic-Alfirevic Aug 09 '17 at 14:07
  • Does %x actually expect the argument to be of type char and not int or something else? If yes, can you point me to the part of the specification that says that? – Niko O Feb 01 '21 at 08:19
4

In C++

#include <iostream>
#include <iomanip>

unsigned char buf0[] = {4, 85, 250, 206};
for (int i = 0;i < sizeof buf0 / sizeof buf0[0]; i++) {
    std::cout << std::setfill('0') 
              << std::setw(2) 
              << std::uppercase 
              << std::hex << (0xFF & buf0[i]) << " ";
}
eigenfield
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  • Can you please explain the '0xFF & '. – Newbie Apr 25 '21 at 09:12
  • While the `0xFF &` is needed to make it work, however, I must admit that it's not the most idiomatic for this case. The `0xFF &` is promoting the `unsigned char` to become an `int` so that it will result to the call of the proper overloaded method of `std::cout < – eigenfield Apr 26 '21 at 05:32
  • Many thanks. Indeed, after your response, I tried to go deeper into the cout functionality. It seems that the transformation is related to the shift operator not to cout. For more info, please refer to this https://stackoverflow.com/questions/47439616/can-you-bitwise-shift-a-bool-in-c. A shift of bool returns an int. As a matter of proof: bool b = true; bool d = (b << 2); printf(d ? "1\n" : "0\n"); – Newbie Apr 27 '21 at 04:31
  • Your findings with regards to the `< – eigenfield Apr 27 '21 at 06:41
  • you are totally right! I integrated your idea of 'Anding' to my unit test and forgot about your example while commenting. You use no shifts! – Newbie Apr 27 '21 at 11:00