The chain returned by this function behaves like a standard filter BIO. It
supports non blocking I/O. Content is processed and streamed on the fly and not
all held in memory at once: so it is possible to encode very large structures.
After all content has been written through the chain BIO_flush() must be called
to finalise the structure.
The CMS_STREAM flag must be included in the corresponding flags
parameter of the cms creation function.
If an application wishes to write additional data to out BIOs should be
removed from the chain using BIO_pop() and freed with BIO_free() until out
is reached. If no additional data needs to be written BIO_free_all() can be
called to free up the whole chain.
Any content written through the filter is used verbatim: no canonical
translation is performed.
It is possible to chain multiple BIOs to, for example, create a triple wrapped
signed, enveloped, signed structure. In this case it is the applications
responsibility to set the inner content type of any outer CMS_ContentInfo
Large numbers of small writes through the chain should be avoided as this will
produce an output consisting of lots of OCTET STRING structures. Prepending
a BIO_f_buffer() buffering BIO will prevent this.