buffer - 是否可以让 zlib 从同一个内存缓冲区读取和写入？

Question

我有一个要压缩到位的字符缓冲区。现在我已经设置好了，所以有两个缓冲区，zlib 的 deflate 从输入缓冲区读取并写入输出缓冲区。然后我必须更改输入缓冲区指针以指向输出缓冲区并释放旧的输入缓冲区。这似乎是不必要的分配量。由于 zlib 正在压缩，next_out 指针应该总是落后于 next_in 指针。无论如何，我找不到足够的文档来验证这一点，并希望有人对此有一些经验。谢谢你的时间！

Answer 1

可以做到，但要小心。下面的例程做到了。并非所有数据都是可压缩的，因此您必须处理输出数据赶上输入数据的情况。它需要大量不可压缩的数据，但它可能会发生（参见代码中的注释），在这种情况下，您必须分配一个缓冲区来临时保存剩余的输入。

/* Compress buf[0..len-1] in place into buf[0..*max-1].  *max must be greater
   than or equal to len.  Return Z_OK on success, Z_BUF_ERROR if *max is not
   enough output space, Z_MEM_ERROR if there is not enough memory, or
   Z_STREAM_ERROR if *strm is corrupted (e.g. if it wasn't initialized or if it
   was inadvertently written over).  If Z_OK is returned, *max is set to the
   actual size of the output.  If Z_BUF_ERROR is returned, then *max is
   unchanged and buf[] is filled with *max bytes of uncompressed data (which is
   not all of it, but as much as would fit).

   Incompressible data will require more output space than len, so max should
   be sufficiently greater than len to handle that case in order to avoid a
   Z_BUF_ERROR. To assure that there is enough output space, max should be
   greater than or equal to the result of deflateBound(strm, len).

   strm is a deflate stream structure that has already been successfully
   initialized by deflateInit() or deflateInit2().  That structure can be
   reused across multiple calls to deflate_inplace().  This avoids unnecessary
   memory allocations and deallocations from the repeated use of deflateInit()
   and deflateEnd(). */
int deflate_inplace(z_stream *strm, unsigned char *buf, unsigned len,
                    unsigned *max)
{
    int ret;                    /* return code from deflate functions */
    unsigned have;              /* number of bytes in temp[] */
    unsigned char *hold;        /* allocated buffer to hold input data */
    unsigned char temp[11];     /* must be large enough to hold zlib or gzip
                                   header (if any) and one more byte -- 11
                                   works for the worst case here, but if gzip
                                   encoding is used and a deflateSetHeader()
                                   call is inserted in this code after the
                                   deflateReset(), then the 11 needs to be
                                   increased to accomodate the resulting gzip
                                   header size plus one */

    /* initialize deflate stream and point to the input data */
    ret = deflateReset(strm);
    if (ret != Z_OK)
        return ret;
    strm->next_in = buf;
    strm->avail_in = len;

    /* kick start the process with a temporary output buffer -- this allows
       deflate to consume a large chunk of input data in order to make room for
       output data there */
    if (*max < len)
        *max = len;
    strm->next_out = temp;
    strm->avail_out = sizeof(temp) > *max ? *max : sizeof(temp);
    ret = deflate(strm, Z_FINISH);
    if (ret == Z_STREAM_ERROR)
        return ret;

    /* if we can, copy the temporary output data to the consumed portion of the
       input buffer, and then continue to write up to the start of the consumed
       input for as long as possible */
    have = strm->next_out - temp;
    if (have <= (strm->avail_in ? len - strm->avail_in : *max)) {
        memcpy(buf, temp, have);
        strm->next_out = buf + have;
        have = 0;
        while (ret == Z_OK) {
            strm->avail_out = strm->avail_in ? strm->next_in - strm->next_out :
                                               (buf + *max) - strm->next_out;
            ret = deflate(strm, Z_FINISH);
        }
        if (ret != Z_BUF_ERROR || strm->avail_in == 0) {
            *max = strm->next_out - buf;
            return ret == Z_STREAM_END ? Z_OK : ret;
        }
    }

    /* the output caught up with the input due to insufficiently compressible
       data -- copy the remaining input data into an allocated buffer and
       complete the compression from there to the now empty input buffer (this
       will only occur for long incompressible streams, more than ~20 MB for
       the default deflate memLevel of 8, or when *max is too small and less
       than the length of the header plus one byte) */
    hold = strm->zalloc(strm->opaque, strm->avail_in, 1);
    if (hold == Z_NULL)
        return Z_MEM_ERROR;
    memcpy(hold, strm->next_in, strm->avail_in);
    strm->next_in = hold;
    if (have) {
        memcpy(buf, temp, have);
        strm->next_out = buf + have;
    }
    strm->avail_out = (buf + *max) - strm->next_out;
    ret = deflate(strm, Z_FINISH);
    strm->zfree(strm->opaque, hold);
    *max = strm->next_out - buf;
    return ret == Z_OK ? Z_BUF_ERROR : (ret == Z_STREAM_END ? Z_OK : ret);
}