Print character array as hex in C

Question

I have a 2D array called char **str (allocated by malloc). Lets say str[0] has the string "hello". How would I print that hex?

I tried printf("%d\n", (unsigned char)strtol(str[0], NULL, 16)), but that doesn't print it out in hex.

Any help would be appreciated, thank you!

possible duplicate of Printing hexadecimal characters in C

yelliver
– yelliver

2015-09-20 03:32:45 +00:00
Commented Sep 20, 2015 at 3:32 — yelliver
– yelliver, Commented Sep 20, 2015 at 3:32

R Sahu · Accepted Answer · 2022-04-27 13:53:50Z

8

You are confused about the fuctionality of strtol. If you have a string that represents a number in hex, you can use strtol like you have:

char s[] = "ff2d";
int n = strtol(s, NULL, 16);
printf("Number: %d\n", n);

When you want to print the characters of a string in hex, use %x format specifier for each character of the string.

char s[] = "Hello";
char* cp = s;
for ( ; *cp != '\0'; ++cp )
{
   printf("%02x", *cp);
}

edited Apr 27, 2022 at 13:53

answered Sep 20, 2015 at 3:15

R Sahu

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2 Comments

neby Over a year ago

Thanks! But what does the 2 do in %2x? If I take it out, it still prints the same hex.

R Sahu Over a year ago

It means use a width of 2 to print the number. It should really be %02x so that it prints a leading zero if the number is less than 16.

amdixon · Accepted Answer · 2015-09-20 03:42:14Z

5

Use %x flag to print hexadecimal integer

example.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) 
{
  char *string = "hello", *cursor;
  cursor = string;
  printf("string: %s\nhex: ", string);
  while(*cursor)
  {
    printf("%02x", *cursor);
    ++cursor;
  }
  printf("\n");
  return 0;
}

output

$ ./example 
string: hello
hex: 68656c6c6f

reference

edited Sep 20, 2015 at 3:42

answered Sep 20, 2015 at 3:14

amdixon

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3 Comments

neby Over a year ago

Thank you as well! The links really helped and always nice to see different ways to do the same thing. Also, the same question above what does the 02 do in %02x?

amdixon Over a year ago

02 is just zeropadding so in the event the hexadecimal integer was less than 2 bytes long eg. 9 then it would print 09.. not really required for the alpha characters as these are all in the two byte hexadecimal range

not2qubit Over a year ago

Can you explain the cursor thing? Also, this will give you compiler error unless you change *string to string[].

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