Weird problems with C extension and bytea as input type

Hi,

I have a weird problem with my custom functions (written in C,C++)
that use bytea as input type (cstring works fine). The functions will
work as expected if they are the only function that uses the bytea
column in a query; as soon as there is a second function using the
same column, the C function will return one of the following: an empty
cstring, a substring of the bytea or the correct cstring. Based on
these symptoms I assume there is something fundamental that I do wrong
(or that is missing) with handling the bytea pointer.

In one specific example, the bytea contains a binary file format that
the function converts into a string format. I convert the bytea to a
C++ string with string(VARDATA(b), VARSIZE(b)-VARHDRSZ).

bytea *b = PG_GETARG_BYTEA_P(0);
char *ism;

ism = function(b);

PG_RETURN_CSTRING(ism);