Output 64 bit integer with hexdump

Question

I currently work on some binary data. In order to check and debug the data previously generated by my application I use hexdump, facing the obstacle of hexdump not appearing to be able to extract a 64-bit integer field. Given the following minimal example:

#include <iostream>
#include <fstream>
#include <cstdint>

int main(int argc, char** argv){
    std::ofstream os("tmp.bin", std::ios::out | std::ios::binary);
    uint64_t x = 7;
    os.write((char*)&x,sizeof(uint64_t));
    os.close();
    return 0;
}

I perform a simple hexdump on my system:

hexdump tmp.bin

> 0000000: 0007 0000 0000 0000
> 0000008:

Now trying to extract the unsigned int of 64 bit width yields:

hexdump -e '/8 "%u"' tmp.bin

> hexdump: bad byte count for conversion character u

According to the well written hexdump-manual by David Mair it should be possible to do it, but I have no success.

What am I missing ?

Answer 1

Our final workaround reads as follows:

x=`hexdump -n 8 -e '2/4 "%08X " "\n"' {FILENAME} | awk 'BEGIN{printf "0x"}{print $2$1}'`
echo $(($x))

Explaination for each part:

• Extract the eight bytes of the 64-bit integer value from file {FILENAME} as two four byte chunks printed as hexadecimal encoded values.

  hexdump -n 8 -e '2/4 "%08X " "\n"' {FILENAME}
  

• Reverses the byte order of the two chunks and prints it as a single eight byte chunk representing the binary value. Prepend 0x for the following processing.

  awk 'BEGIN{printf "0x"}{print $2$1}
  

• Save the hexadecimal representation into x for bash to evaluate.

  x=`....`
  

• Let the bourne shell interpret and output the hexadecimal encoded value of the 64-bit integer variable (here the previously prepended 0x is needed).

  echo $(($x))
  

Answer 2

One can use sed also. The following matches 8 byte hex integers and swaps them. Again this only works for unsigned integers.

hexdump ... | sed 's/0x\([0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]\)\([0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]\)/0x\2\1/'

AFAICT one should be able to write much more clearly

hexdump ... | sed 's/0x\([0-9a-f]{8,8}\)\([0-9a-f]{8,8}\)/0x\2\1/'

typically with some command line option such as -E to enable extended regular expressions, but at least on Mac OS X 10.10 this doesn't work.