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.\" ========================================================================
.\"
.IX Title "CMS_compress 3"
.TH CMS_compress 3 2019-12-20 1.0.2u OpenSSL
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
CMS_compress \- create a CMS CompressedData structure
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/cms.h>
\&
\& CMS_ContentInfo *CMS_compress(BIO *in, int comp_nid, unsigned int flags);
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\fBCMS_compress()\fR creates and returns a CMS CompressedData structure. \fBcomp_nid\fR
is the compression algorithm to use or \fBNID_undef\fR to use the default
algorithm (zlib compression). \fBin\fR is the content to be compressed.
\&\fBflags\fR is an optional set of flags.
.SH NOTES
.IX Header "NOTES"
The only currently supported compression algorithm is zlib using the NID
NID_zlib_compression.
.PP
If zlib support is not compiled into OpenSSL then \fBCMS_compress()\fR will return
an error.
.PP
If the \fBCMS_TEXT\fR flag is set MIME headers for type \fBtext/plain\fR are
prepended to the data.
.PP
Normally the supplied content is translated into MIME canonical format (as
required by the S/MIME specifications) if \fBCMS_BINARY\fR 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 \fBCMS_BINARY\fR is set then
\&\fBCMS_TEXT\fR is ignored.
.PP
If the \fBCMS_STREAM\fR flag is set a partial \fBCMS_ContentInfo\fR structure is
returned suitable for streaming I/O: no data is read from the BIO \fBin\fR.
.PP
The compressed data is included in the CMS_ContentInfo structure, unless
\&\fBCMS_DETACHED\fR is set in which case it is omitted. This is rarely used in
practice and is not supported by \fBSMIME_write_CMS()\fR.
.SH NOTES
.IX Header "NOTES"
If the flag \fBCMS_STREAM\fR is set the returned \fBCMS_ContentInfo\fR structure is
\&\fBnot\fR complete and outputting its contents via a function that does not
properly finalize the \fBCMS_ContentInfo\fR structure will give unpredictable
results.
.PP
Several functions including \fBSMIME_write_CMS()\fR, \fBi2d_CMS_bio_stream()\fR,
\&\fBPEM_write_bio_CMS_stream()\fR finalize the structure. Alternatively finalization
can be performed by obtaining the streaming ASN1 \fBBIO\fR directly using
\&\fBBIO_new_CMS()\fR.
.PP
Additional compression parameters such as the zlib compression level cannot
currently be set.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBCMS_compress()\fR returns either a CMS_ContentInfo structure or NULL if an error
occurred. The error can be obtained from \fBERR_get_error\fR\|(3).
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBERR_get_error\fR\|(3), \fBCMS_uncompress\fR\|(3)
.SH HISTORY
.IX Header "HISTORY"
\&\fBCMS_compress()\fR was added to OpenSSL 0.9.8
The \fBCMS_STREAM\fR flag was first supported in OpenSSL 1.0.0.