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From: babil Bampilis on 26 Apr 2010 09:54
Hi all,
if I use ssPrintf to output messages for debugging in the comandline I get an error about converting 'unsigned char *' in 'const char *'. What to do?? any idea?
This is the code:

unsigned char* buf=myData->getBuffer();
ssPrintf(buf);


Thanks a lot!
From: Walter Roberson on 27 Apr 2010 11:08
babil Bampilis wrote:
> Hi all,
> if I use ssPrintf to output messages for debugging in the comandline I
> get an error about converting 'unsigned char *' in 'const char *'. What
> to do?? any idea?
> This is the code:
>
> unsigned char* buf=myData->getBuffer();
> ssPrintf(buf);

This is a frequent problem in C99 code, sometimes called "const
pollution". The only "pure" answer within C is to never use a writable
char variable (or array), and never to use string literals of the form
"string": e.g.,

char foo[] = "hello, world!"; /* will lead to trouble with const */
const char foo[14] - {'h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r',
'l', 'd', 0}; /* const-safe */

It is called "const pollution" because once you fix one individual
routine that has a char* passed in, you have to climb the ladder of
routines that can possibly call that routine, fixing the 'const' in each
of them -- a single 'const' can require fixing up hundreds of routines
if you are mandated to avoid compiler warnings :(

When I say "a problem in C99 code", it isn't that the issue didn't exist
in C89, but the C99 standard libraries had lots of parameters marked
with 'const' where they were not 'const' in C89's definition, so the
problem became acute if you want your code to compile without warnings.

The work-around to this "const pollution" is to type-cast as you pass
your non-const parameters into routines that demand const. Doing so can
be a lot cheaper than fixing hierarchies of routines (some of which
simply cannot be made const because the data is variable!) -- but
type-casting can be a clash with the coding standards that exist at some
places, as some places prohibit type-casting.


Anyhow, here is the revised code for you:

unsigned char* buf=myData->getBuffer();
ssPrintf((const char *) buf);


Warning: if that code really is a printf variant, then you are passing
unvalidated user input into a formatted print routine, which is a
security risk if the user has formatting strings in the data! And
suppose there is a null character in the data? If you wish to copy data
unchanged from input to output, a printf variant should never be
involved: that's what fwrite() and kin are for.
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