Re: [pdf-devel] New developer and UUID module request

[Aleksander Morgado]

Hi Albert,

> const char * pdf_uuid_string (pdf_uuid_t uuid, char
> ascii_rep[PDF_UUID_CHAR_LENGTH]);
> 
> is thread-safe and working well. The function returns the same pointer
> it gets as input parameter (ascii_rep). The user is still required to
> initialize the char array, but the size he or she specifies seems to
> have no effect; the following testing code worked fine to me, writing
> PDF_UUID_CHAR_LENGTH chars in stdout:
> 
> pdf_uuid_t t;
> char t_a[1];
> 
> pdf_uuid_t t = pdf_uuid_generate (PDF_UUID_TIME);
> 
> printf("Generated UUID, time 1: %s\n", pdf_uuid_string (t, t_a));
> 

You shouldn't do that. The idea to pass the array specifying its size is
in order to tell the developer the size of the array that needs to be
passed. So, do not try to pass a 1-byte array or a non-allocated array,
when you're kindly asked to pass a PDF_UUID_CHAR_LENGTH-byte array :-)
And when you pass the array like that, you're just passing it as "the
address of the first byte of the buffer".

If you pass a 1-byte, and the function then writes PDF_UUID_CHAR_LENGTH
bytes, you'll be writing out of buffer and corrupting the stack (at
least in this case with the buffer in the stack). 

If the buffer is not allocated at all and you pass it, you'll be passing
a pointer to nowhere known, and that will definitely crash
instantaneously.

Spend some time running your test under valgrind and you'll see it
complaining about invalid reads and writes.

> 
> So, regarding the first case, is gcc re-allocating the char array when
> it reads the function header?

No, there's no gcc magic involved here. The array needs to be fully
allocated either in stack or heap and have PDF_UUID_CHAR_LENGTH bytes.
If the programmer doesn't do so, it's a programmer error.

Either way, what Jose said about using:
  void pdf_uuid_string (pdf_uuid_t, char * ascii_rep);
is equally fine if we specify in the documentation that the ascii_rep
array needs to be preallocated and have at least PDF_UUID_CHAR_LENGTH
bytes.

Cheers,

-- 
Aleksander

signature.asc
Description: This is a digitally signed message part