Boost C++ Libraries

Compress input and write output.

Synopsis

void
write(
    z_params& zs,
    Flush flush,
    error_code& ec);

Description

This function compresses as much data as possible, and stops when the input buffer becomes empty or the output buffer becomes full. It may introduce some output latency (reading input without producing any output) except when forced to flush.

In each call, one or both of these actions are performed:

Before the call, the application must ensure that at least one of the actions is possible, by providing more input and/or consuming more output, and updating zs.avail_in or zs.avail_out accordingly; zs.avail_out should never be zero before the call. The application can consume the compressed output when it wants, for example when the output buffer is full (zs.avail_out == 0), or after each call of write. If write returns no error with zero zs.avail_out, it must be called again after making room in the output buffer because there might be more output pending.

Normally the parameter flush is set to Flush::none, which allows deflate to decide how much data to accumulate before producing output, in order to maximize compression.

If the parameter flush is set to Flush::sync, all pending output is flushed to the output buffer and the output is aligned on a byte boundary, so that the decompressor can get all input data available so far. In particular zs.avail_in is zero after the call if enough output space has been provided before the call. Flushing may degrade compression for some compression algorithms and so it should be used only when necessary. This completes the current deflate block and follows it with an empty stored block that is three bits plus filler bits to the next byte, followed by the four bytes { 0x00, 0x00 0xff 0xff }.

If flush is set to Flush::partial, all pending output is flushed to the output buffer, but the output is not aligned to a byte boundary. All of the input data so far will be available to the decompressor, as for Z_SYNC_FLUSH. This completes the current deflate block and follows it with an empty fixed codes block that is 10 bits long. This assures that enough bytes are output in order for the decompressor to finish the block before the empty fixed code block.

If flush is set to Flush::block, a deflate block is completed and emitted, as for Flush::sync, but the output is not aligned on a byte boundary, and up to seven bits of the current block are held to be written as the next byte after the next deflate block is completed. In this case, the decompressor may not be provided enough bits at this point in order to complete decompression of the data provided so far to the compressor. It may need to wait for the next block to be emitted. This is for advanced applications that need to control the emission of deflate blocks.

If flush is set to Flush::full, all output is flushed as with Flush::sync, and the compression state is reset so that decompression can restart from this point if previous compressed data has been damaged or if random access is desired. Using Flush::full too often can seriously degrade compression.

If write returns with zs.avail_out == 0, this function must be called again with the same value of the flush parameter and more output space (updated zs.avail_out), until the flush is complete (write returns with non-zero zs.avail_out). In the case of a Flush::fullor Flush::sync, make sure that zs.avail_out is greater than six to avoid repeated flush markers due to zs.avail_out == 0 on return.

If the parameter flush is set to Flush::finish, pending input is processed, pending output is flushed and deflate returns the error error::end_of_stream if there was enough output space; if deflate returns with no error, this function must be called again with Flush::finish and more output space (updated zs.avail_out) but no more input data, until it returns the error error::end_of_stream or another error. After write has returned the error::end_of_stream error, the only possible operations on the stream are to reset or destroy.

Flush::finish can be used immediately after initialization if all the compression is to be done in a single step. In this case, zs.avail_out must be at least value returned by upper_bound (see below). Then write is guaranteed to return the error::end_of_stream error. If not enough output space is provided, deflate will not return error::end_of_stream, and it must be called again as described above.

write returns no error if some progress has been made (more input processed or more output produced), error::end_of_stream if all input has been consumed and all output has been produced (only when flush is set to Flush::finish), error::stream_error if the stream state was inconsistent (for example if zs.next_in or zs.next_out was nullptr), error::need_buffers if no progress is possible (for example zs.avail_in or zs.avail_out was zero). Note that error::need_buffers is not fatal, and write can be called again with more input and more output space to continue compressing.