Help!

[davegibelli]

I am trying to use printf to output an unsigned integer from a array of bytes but I cannot figure out how?

e.g.
char ptr[4]={0x00, 0x00, 0x00, 0xff);

printf("%u",*ptr);

but I cannot get the answer 255

 

[xwb]

printf ("%u", (unsigned int)*ptr);

*ptr just gives you a char
(unsigned int)*ptr takes however many bytes an unsigned int takes and puts it into a char. Note that if you do this on a 16 bit machine, it will only take the first 2 bytes. If you do it on a 64 bit machine, it may take 4 or you might get a crash if it takes 8.

 

[davegibelli]

I still get the answer 0 instead of 255, I am using a 32bit machine...

 

[xwb]

Sorry it should have been

printf ("%u", (unsigned int)ptr);

 

[davegibelli]

Nope, that gives a different number each time the prog is run!

 

[xwb]

What does the program look like? There may be a bug somewhere else.

 

[Salem]

1. You would need to cast the pointer to an unsigned int pointer, then dereference it.
[tt]printf ("%u", *((unsigned int*)ptr) );[/tt]

2. This is a really bad idea, as int probably has stricter alignment requirements than char. It may work for you, but then again you may just get a "bus error".

3. If you want 255 from 00 00 00 FF, then you would need to be on a big endian machine. Consider FF 00 00 00 if you get a consistently large number like 4278190080.


--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[ArkM]

If (and only if ptr is an array, you may use this awkward (but portable construct. See also Salem's notes about little/big-endian(s)...

#define Min(x,y) ((x)<(y)?(x):(y))
unsigned junk = 0;
...
printf("%u\n",
  (memcpy(&u,ptr,Min(sizeof junk,sizeof ptr)),u)
);