Problem with zlib deflation header
(too old to reply)
Farhan Khan
2018-10-04 03:58:25 UTC
Hi all,

I am trying to use zlib to deflate (compress?) data from a textfile..
It seems to work when I compress a file, but I am trying to prepend
the zlib compressed file with custom header. Both the file and header
should be compressed. However, when I add the header, the length of
the compressed (deflated) file is much shorter than expected and comes
out as an invalid zlib compressed object.

The code works great, until I add the header block of code between the
XXX comments below.

The "FILE *source" variable is a sample file, I typically use
/etc/passwd and the "char *header" is "blob 2172\0".
Without the header block, the output is 904 bytes and deflatable
(decompressable), but with the header it comes out to only 30 bytes.
It also comes out as an invalid zlib object with the header block of

Any ideas where I am making a mistake, specifically why the output is
invalid and shorter *with* the header?

Code below:
zcompress_and_header(FILE *source, char *header)
int ret, flush;
z_stream strm;
unsigned int have;
unsigned char in[Z_CHUNK];
unsigned char out[Z_CHUNK];

FILE *dest = stdout; // This is a temporary test

strm.zalloc = Z_NULL;
strm.zfree = Z_NULL;
strm.opaque = Z_NULL;
ret = deflateInit(&strm, Z_BEST_SPEED);
//ret = deflateInit2(&strm, Z_BEST_SPEED, Z_DEFLATED, 15 | 16, 8,

if (ret != Z_OK)
return ret;

/* XXX Beginning of writing the header */

strm.next_in = (unsigned char *) header;
strm.avail_in = strlen(header) + 1;

do {
strm.avail_out = Z_CHUNK;
strm.next_out = out;
if (deflate (& strm, Z_FINISH) < 0) {
fprintf(stderr, "returned a bad status of.\n");
have = Z_CHUNK - strm.avail_out;
fwrite(out, 1, have, stdout);
} while(strm.avail_out == 0);

/* XXX End of writing the header */

do {
strm.avail_in = fread(in, 1, Z_CHUNK, source);
if (ferror(source)) {
return Z_ERRNO;

flush = feof(source) ? Z_FINISH : Z_NO_FLUSH;
strm.next_in = in;

do {
strm.avail_out = Z_CHUNK;
strm.next_out = out;
ret = deflate(&strm, flush);
have = Z_CHUNK - strm.avail_out;
if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
return Z_ERRNO;
} while(strm.avail_out == 0);

} while (flush != Z_FINISH);

} // End of function

Farhan Khan
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