=pod

=head1 NAME

CMS_compress - create a CMS CompressedData structure

=head1 SYNOPSIS

 #include <openssl/cms.h>

 CMS_ContentInfo *CMS_compress(BIO *in, int comp_nid, unsigned int flags);

=head1 DESCRIPTION

CMS_compress() creates and returns a CMS CompressedData structure. B<comp_nid>
is the compression algorithm to use or B<NID_undef> to use the default
algorithm (zlib compression). B<in> is the content to be compressed.
B<flags> is an optional set of flags.

=head1 NOTES

The only currently supported compression algorithm is zlib using the NID
NID_zlib_compression.

If zlib support is not compiled into OpenSSL then CMS_compress() will return
an error.

If the B<CMS_TEXT> flag is set MIME headers for type B<text/plain> are
prepended to the data.

Normally the supplied content is translated into MIME canonical format (as
required by the S/MIME specifications) if B<CMS_BINARY> is set no translation
occurs. This option should be used if the supplied data is in binary format
otherwise the translation will corrupt it. If B<CMS_BINARY> is set then
B<CMS_TEXT> is ignored.

If the B<CMS_STREAM> flag is set a partial B<CMS_ContentInfo> structure is
returned suitable for streaming I/O: no data is read from the BIO B<in>.

The compressed data is included in the CMS_ContentInfo structure, unless
B<CMS_DETACHED> is set in which case it is omitted. This is rarely used in
practice and is not supported by SMIME_write_CMS().

=head1 NOTES

If the flag B<CMS_STREAM> is set the returned B<CMS_ContentInfo> structure is
B<not> complete and outputting its contents via a function that does not
properly finalize the B<CMS_ContentInfo> structure will give unpredictable
results.

Several functions including SMIME_write_CMS(), i2d_CMS_bio_stream(),
PEM_write_bio_CMS_stream() finalize the structure. Alternatively finalization
can be performed by obtaining the streaming ASN1 B<BIO> directly using
BIO_new_CMS().

Additional compression parameters such as the zlib compression level cannot
currently be set.

=head1 RETURN VALUES

CMS_compress() returns either a CMS_ContentInfo structure or NULL if an error
occurred. The error can be obtained from ERR_get_error(3).

=head1 SEE ALSO

L<ERR_get_error(3)|ERR_get_error(3)>, L<CMS_uncompress(3)|CMS_uncompress(3)>

=head1 HISTORY

CMS_compress() was added to OpenSSL 0.9.8
The B<CMS_STREAM> flag was first supported in OpenSSL 0.9.9.

=cut
