Printing hex digits 2 bytes at a time - c++

Question

I am looking to convert existing code in C to perform the following: I am attempting to write a hex dump program that prints out address: values printable characters.
Currently, the code for values is printing in the following format:
0003540: 05 04 06 75 6e 73 69 67 6e 65 64 20 63 68 61 72 ...unsigned char
Desired hex output:
0003540: 0504 0675 6e73 6967 6e65 6420 6368 6172 ...unsigned char
Current code printing in pairs:
addr = 0;
while ( ( cnt = ( long )
fread ( buf, sizeof ( unsigned char ), 16, filein ) ) > 0 ) {
b = buf;
/* Print the address in hexadecimal. */
fprintf ( fileout, "%07lx ", addr );
addr = addr + 16;
/* Print 16 data items, in pairs, in hexadecimal. */
cnt2 = 0;
for ( m = 0; m < 16; m++ ) {
cnt2 = cnt2 + 1;
if ( cnt2 <= cnt ) {
fprintf ( fileout, "%02x", *b++ );
}
else {
fprintf ( fileout, " " );
}
fprintf ( fileout, " " );
}
/* Print the printable characters, or a period if unprintable. */
fprintf ( fileout, " " );
cnt2 = 0;
for ( n = 0; n < 16; n++ ) {
cnt2 = cnt2 + 1;
if ( cnt2 <= cnt ) {
if ( ( buf[n] < 32 ) || ( buf[n] > 126 ) ) {
fprintf ( fileout, "%c", '.' );
}
else {
fprintf ( fileout, "%c", buf[n] );
}
}
}
fprintf( fileout, "\n" );
}
How can I alter this code to achieve the AB12 CD34 format?
Thanks!

Answer 1

Use the modulo (remainder) operator % to test if m is divisible by 2. Only write the space when it is:
for ( m = 0; m < 16; m++ ) {
if ( m > 0 && m % 2 == 0 ) {
fprintf ( fileout, " " );
}
fprintf ( fileout, "%02x", *b++ );
}
Edit 3:
for ( m = 0; m < 16; m++ ) {
if ( m > 0 && m % 2 == 0 ) {
fprintf ( fileout, " " ); // space between every second byte
}
if ( m < cnt ) {
fprintf ( fileout, "%02x", *b++ );
} else {
fprintf ( fileout, " " ); // blank if run out of bytes on this line
}
}

Answer 2

I think this can be simplified quite a bit. For example, I'd consider starting with something like this:
#include <stdio.h>
#include <ctype.h>
int main(){
char buffer[17];
size_t bytes;
int i;
while (0 < (bytes = fread(buffer, 1, 16, stdin))) {
for (i = 0; i < bytes / 2; i++) // print out bytes, 2 at a time
printf("%02x%02x ", buffer[i * 2], buffer[i * 2 + 1]);
if (i * 2 < bytes) // take care of (possible) odd byte
printf("%02x ", buffer[i * 2]);
for (; i < 8; i++) // right pad hex bytes
printf(" ");
for (i = 0; i < bytes; i++) // change unprintable to '.'
if (!isprint(buffer[i]))
buffer[i] = '.';
buffer[i] = '\0'; // terminate string
printf("\t%s\n", buffer); // print out characters
}
return 0;
}