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=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 1.0.0.
=cut
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