C++ Question: Convert from char[4] to uint32

RsaXVC · **Posted:** Wed 09-20-2006 12:24PM

I'm trying to read data from binary files. I can do this easily, but it only stores data into a char*. I would like to take 4 chars, and convert them into an unsigned integer-they have the same length so it should be possible. 4chars=32bits=1 int.

Any ideas how?

heretic · **Posted:** Wed 09-20-2006 12:49PM

If you are trying to read 1 field in at a time.. try this

Code:

unsigned long i;
char *buffer;

... read into buffer ....

i = *(unsigned long *)buffer;

if you are trying to read multiple fields try something like this

Code:

struct fields
{
  unsigned long field1;
  unsigned long field2;
  unsigned long field3;
} __attribute__ (( packed ));

struct fields * f;
char * buffer;
.. read in buffer ...

f = (struct fields *)buffer;

f->field1 ...
f->field2 ...
f->field3 ...

elyk · **Posted:** Wed 09-20-2006 5:28PM

Don't forget about the byte order, that might give you some problems.

bsiever · **Posted:** Thu 09-21-2006 5:29AM

You sould just use read (and write) to read the binary data directly into
the desired data type. For example, to read do something like:

Code:

#include <iostream>
#include <fstream>
using namespace std;

int main(void)
{
  float test;
  ifstream file ("example.txt", ios::in|ios::binary);
  if (file.is_open())
  {
    file.read((char*) &test, sizeof(float));
    file.close();

    cout << "the float: " << test << endl;
  }
  return 0;
}

The corresponding write is:

Code:

#include <iostream>
#include <fstream>
using namespace std;


int main(void)
{
  float test=123.456;
  ofstream file ("example.txt", ios::out|ios::binary|ios::ate);
  if (file.is_open())
  {
    file.write((char *)&test, sizeof(float));
    file.close();
  }
  return 0;
}

See: http://www.cplusplus.com/doc/tutorial/files.html

If you're only working on one platform you probably don't have to worry
about byte order.

Bill

ritalinkid18 · **Posted:** Thu 09-28-2006 7:41PM

Back and Forth.

Here's an example I wrote to show how to convert both ways.

Code:

#include <stdlib.h>
#include <unistd.h>

int main()
{
   unsigned int number = 0;
   unsigned int * numptr;   

   unsigned char before[2];
   unsigned char *ptr;

   before[0] = 0xee; 
   before[1] = 0xff;

   ptr = before;
   number = *(unsigned int*)ptr;      

   //Note the byte position change!
   printf("Char: %X%X, Int: %X \n", before[1], before[0], number);

   before[0] = 0xAA;  //Just to prove it changes
   before[0] = 0xBB;   
   number = 0x1111;
   
   numptr = &number;

   ptr = numptr;

   before[0] = (unsigned char)ptr[0];
   before[1] = (unsigned char)ptr[1];

   printf("Char: %X%X, Int: %X \n", before[0], before[1], number);

   return 0;
}

RsaXVC · **Posted:** Fri 09-29-2006 8:12AM

Heretic's way seemed the sneakiest and least memory intensive, that and I didn't know you could cast pointer types and not pointers.

so here's what I used:

#include <iostream>
#include <fstream>
using namespace std;
char *buffer2=new char[2]; //(unsigned short)
char *buffer4=new char[4]; //(unsigned int)
char *buffer6=new char[6]; //(unsigned bssid)
char *buffer8=new char[8]; //(unsigned long)
stream inputline("netstumbler.ns1,ios::in|ios::binary);
...blah blah blah...
inputline.read(buffer4,4);
cout<<"Number of access points: "<<*(unsigned int *)buffer4;
...blah blah blah...
inputline.close();

delete <everything>

C++ Question: Convert from char[4] to uint32

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