zlib: how to dimension avail_out

Question

I would like to deflate a small block of memory (<= 16 KiB) using zlib. The output is stored in a block of memory as well. No disk or database access here.

According to the documentation, I should call deflate() repeatedly until the whole input is deflated. In between, I have to increase the size of the memory block where the output goes.

However, that seems unnecessarily complicated and perhaps even inefficient. As I know the size of the input, can't I predetermine the maximum size needed for the output, and then do everything with just one call to deflate()?

If so, what is the maximum output size? I assume something like: size of input + some bytes overhead

Answer 1

zlib has a function to calculate the maximum size a buffer will deflate to. Your assumption is correct - the returned value is the size of the input buffer + header sizes. After deflation you can realloc the buffer to reclaim the 'wasted' memory.
From zlib.h:

ZEXTERN uLong ZEXPORT deflateBound OF((z_streamp strm, uLong sourceLen));
/*
   deflateBound() returns an upper bound on the compressed size after
   deflation of sourceLen bytes.  It must be called after deflateInit() or
   deflateInit2(), and after deflateSetHeader(), if used.  This would be used
   to allocate an output buffer for deflation in a single pass, and so would be
   called before deflate().
*/