## zlib Usage Example

We often get questions about how the `deflate()` and `inflate()` functions should be used. Users wonder when they should provide more input, when they should use more output, what to do with a `Z_BUF_ERROR`, how to make sure the process terminates properly, and so on. So for those who have read `zlib.h` (a few times), and would like further edification, below is an annotated example in C of simple routines to compress and decompress from an input file to an output file using `deflate()` and `inflate()` respectively. The annotations are interspersed between lines of the code. So please read between the lines. We hope this helps explain some of the intricacies of zlib.

Without further ado, here is the program `zpipe.c`:

    /* zpipe.c: example of proper use of zlib's inflate() and deflate()
       Not copyrighted -- provided to the public domain
       Version 1.4  11 December 2005  Mark Adler */

    /* Version history:
       1.0  30 Oct 2004  First version
       1.1   8 Nov 2004  Add void casting for unused return values
                         Use switch statement for inflate() return values
       1.2   9 Nov 2004  Add assertions to document zlib guarantees
       1.3   6 Apr 2005  Remove incorrect assertion in inf()
       1.4  11 Dec 2005  Add hack to avoid MSDOS end-of-line conversions
                         Avoid some compiler warnings for input and output buffers
     */

We now include the header files for the required definitions. From `stdio.h` we use `fopen()`, `fread()`, `fwrite()`, `feof()`, `ferror()`, and `fclose()` for file i/o, and `fputs()` for error messages. From `string.h` we use `strcmp()` for command line argument processing. From `assert.h` we use the `assert()` macro. From `zlib.h` we use the basic compression functions `deflateInit()`, `deflate()`, and `deflateEnd()`, and the basic decompression functions `inflateInit()`, `inflate()`, and `inflateEnd()`.

    #include <stdio.h>
    #include <string.h>
    #include <assert.h>
    #include "zlib.h"

This is an ugly hack required to avoid corruption of the input and output data on Windows/MS-DOS systems. Without this, those systems would assume that the input and output files are text, and try to convert the end-of-line characters from one standard to another. That would corrupt binary data, and in particular would render the compressed data unusable. This sets the input and output to binary which suppresses the end-of-line conversions. `SET_BINARY_MODE()` will be used later on `stdin` and `stdout`, at the beginning of `main()`.

    #if defined(MSDOS) || defined(OS2) || defined(WIN32) || defined(__CYGWIN__)
    #  include <fcntl.h>
    #  include <io.h>
    #  define SET_BINARY_MODE(file) setmode(fileno(file), O_BINARY)
    #else
    #  define SET_BINARY_MODE(file)
    #endif

`CHUNK` is simply the buffer size for feeding data to and pulling data from the zlib routines. Larger buffer sizes would be more efficient, especially for `inflate()`. If the memory is available, buffers sizes on the order of 128K or 256K bytes should be used.

    #define CHUNK 16384

The `def()` routine compresses data from an input file to an output file. The output data will be in the zlib format, which is different from the gzip or zip formats. The zlib format has a very small header of only two bytes to identify it as a zlib stream and to provide decoding information, and a four-byte trailer with a fast check value to verify the integrity of the uncompressed data after decoding.

    /* Compress from file source to file dest until EOF on source.
       def() returns Z_OK on success, Z_MEM_ERROR if memory could not be
       allocated for processing, Z_STREAM_ERROR if an invalid compression
       level is supplied, Z_VERSION_ERROR if the version of zlib.h and the
       version of the library linked do not match, or Z_ERRNO if there is
       an error reading or writing the files. */
    int def(FILE *source, FILE *dest, int level)
    {

Here are the local variables for `def()`. `ret` will be used for zlib return codes. `flush` will keep track of the current flushing state for `deflate()`, which is either no flushing, or flush to completion after the end of the input file is reached. `have` is the amount of data returned from `deflate()`. The `strm` structure is used to pass information to and from the zlib routines, and to maintain the `deflate()` state. `in` and `out` are the input and output buffers for `deflate()`.

        int ret, flush;
        unsigned have;
        z_stream strm;
        unsigned char in[CHUNK];
        unsigned char out[CHUNK];

The first thing we do is to initialize the zlib state for compression using `deflateInit()`. This must be done before the first use of `deflate()`. The `zalloc`, `zfree`, and `opaque` fields in the `strm` structure must be initialized before calling `deflateInit()`. Here they are set to the zlib constant `Z_NULL` to request that zlib use the default memory allocation routines. An application may also choose to provide custom memory allocation routines here. `deflateInit()` will allocate on the order of 256K bytes for the internal state. (See zlib Technical Details.)

`deflateInit()` is called with a pointer to the structure to be initialized and the compression level, which is an integer in the range of -1 to 9. Lower compression levels result in faster execution, but less compression. Higher levels result in greater compression, but slower execution. The zlib constant Z_DEFAULT_COMPRESSION, equal to -1, provides a good compromise between compression and speed and is equivalent to level 6. Level 0 actually does no compression at all, and in fact expands the data slightly to produce the zlib format (it is not a byte-for-byte copy of the input). More advanced applications of zlib may use `deflateInit2()` here instead. Such an application may want to reduce how much memory will be used, at some price in compression. Or it may need to request a gzip header and trailer instead of a zlib header and trailer, or raw encoding with no header or trailer at all.

We must check the return value of `deflateInit()` against the zlib constant `Z_OK` to make sure that it was able to allocate memory for the internal state, and that the provided arguments were valid. `deflateInit()` will also check that the version of zlib that the `zlib.h` file came from matches the version of zlib actually linked with the program. This is especially important for environments in which zlib is a shared library.

Note that an application can initialize multiple, independent zlib streams, which can operate in parallel. The state information maintained in the structure allows the zlib routines to be reentrant.

        /* allocate deflate state */
        strm.zalloc = Z_NULL;
        strm.zfree = Z_NULL;
        strm.opaque = Z_NULL;
        ret = deflateInit(&strm, level);
        if (ret != Z_OK)
            return ret;

With the pleasantries out of the way, now we can get down to business. The outer `do`-loop reads all of the input file and exits at the bottom of the loop once end-of-file is reached. This loop contains the only call of `deflate()`. So we must make sure that all of the input data has been processed and that all of the output data has been generated and consumed before we fall out of the loop at the bottom.

        /* compress until end of file */
        do {

We start off by reading data from the input file. The number of bytes read is put directly into `avail_in`, and a pointer to those bytes is put into `next_in`. We also check to see if end-of-file on the input has been reached using feof(). If we are at the end of file, then `flush` is set to the zlib constant `Z_FINISH`, which is later passed to `deflate()` to indicate that this is the last chunk of input data to compress. If we are not yet at the end of the input, then the zlib constant `Z_NO_FLUSH` will be passed to `deflate` to indicate that we are still in the middle of the uncompressed data.

If there is an error in reading from the input file, the process is aborted with `deflateEnd()` being called to free the allocated zlib state before returning the error. We wouldn't want a memory leak, now would we? `deflateEnd()` can be called at any time after the state has been initialized. Once that's done, `deflateInit()` (or `deflateInit2()`) would have to be called to start a new compression process. There is no point here in checking the `deflateEnd()` return code. The deallocation can't fail.

            strm.avail_in = fread(in, 1, CHUNK, source);
            if (ferror(source)) {
                (void)deflateEnd(&strm);
                return Z_ERRNO;
            }
            flush = feof(source) ? Z_FINISH : Z_NO_FLUSH;
            strm.next_in = in;

The inner `do`-loop passes our chunk of input data to `deflate()`, and then keeps calling `deflate()` until it is done producing output. Once there is no more new output, `deflate()` is guaranteed to have consumed all of the input, i.e., `avail_in` will be zero.

            /* run deflate() on input until output buffer not full, finish
               compression if all of source has been read in */
            do {

Output space is provided to `deflate()` by setting `avail_out` to the number of available output bytes and `next_out` to a pointer to that space.

                strm.avail_out = CHUNK;
                strm.next_out = out;

Now we call the compression engine itself, `deflate()`. It takes as many of the `avail_in` bytes at `next_in` as it can process, and writes as many as `avail_out` bytes to `next_out`. Those counters and pointers are then updated past the input data consumed and the output data written. It is the amount of output space available that may limit how much input is consumed. Hence the inner loop to make sure that all of the input is consumed by providing more output space each time. Since `avail_in` and `next_in` are updated by `deflate()`, we don't have to mess with those between `deflate()` calls until it's all used up.

The parameters to `deflate()` are a pointer to the `strm` structure containing the input and output information and the internal compression engine state, and a parameter indicating whether and how to flush data to the output. Normally `deflate` will consume several K bytes of input data before producing any output (except for the header), in order to accumulate statistics on the data for optimum compression. It will then put out a burst of compressed data, and proceed to consume more input before the next burst. Eventually, `deflate()` must be told to terminate the stream, complete the compression with provided input data, and write out the trailer check value. `deflate()` will continue to compress normally as long as the flush parameter is `Z_NO_FLUSH`. Once the `Z_FINISH` parameter is provided, `deflate()` will begin to complete the compressed output stream. However depending on how much output space is provided, `deflate()` may have to be called several times until it has provided the complete compressed stream, even after it has consumed all of the input. The flush parameter must continue to be `Z_FINISH` for those subsequent calls.

There are other values of the flush parameter that are used in more advanced applications. You can force `deflate()` to produce a burst of output that encodes all of the input data provided so far, even if it wouldn't have otherwise, for example to control data latency on a link with compressed data. You can also ask that `deflate()` do that as well as erase any history up to that point so that what follows can be decompressed independently, for example for random access applications. Both requests will degrade compression by an amount depending on how often such requests are made.

`deflate()` has a return value that can indicate errors, yet we do not check it here. Why not? Well, it turns out that `deflate()` can do no wrong here. Let's go through `deflate()`'s return values and dispense with them one by one. The possible values are `Z_OK`, `Z_STREAM_END`, `Z_STREAM_ERROR`, or `Z_BUF_ERROR`. `Z_OK` is, well, ok. `Z_STREAM_END` is also ok and will be returned for the last call of `deflate()`. This is already guaranteed by calling `deflate()` with `Z_FINISH` until it has no more output. `Z_STREAM_ERROR` is only possible if the stream is not initialized properly, but we did initialize it properly. There is no harm in checking for `Z_STREAM_ERROR` here, for example to check for the possibility that some other part of the application inadvertently clobbered the memory containing the zlib state. `Z_BUF_ERROR` will be explained further below, but suffice it to say that this is simply an indication that `deflate()` could not consume more input or produce more output. `deflate()` can be called again with more output space or more available input, which it will be in this code.

                ret = deflate(&strm, flush);    /* no bad return value */
                assert(ret != Z_STREAM_ERROR);  /* state not clobbered */

Now we compute how much output `deflate()` provided on the last call, which is the difference between how much space was provided before the call, and how much output space is still available after the call. Then that data, if any, is written to the output file. We can then reuse the output buffer for the next call of `deflate()`. Again if there is a file i/o error, we call `deflateEnd()` before returning to avoid a memory leak.

                have = CHUNK - strm.avail_out;
                if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
                    (void)deflateEnd(&strm);
                    return Z_ERRNO;
                }

The inner `do`-loop is repeated until the last `deflate()` call fails to fill the provided output buffer. Then we know that `deflate()` has done as much as it can with the provided input, and that all of that input has been consumed. We can then fall out of this loop and reuse the input buffer.

The way we tell that `deflate()` has no more output is by seeing that it did not fill the output buffer, leaving `avail_out` greater than zero. However suppose that `deflate()` has no more output, but just so happened to exactly fill the output buffer! `avail_out` is zero, and we can't tell that `deflate()` has done all it can. As far as we know, `deflate()` has more output for us. So we call it again. But now `deflate()` produces no output at all, and `avail_out` remains unchanged as `CHUNK`. That `deflate()` call wasn't able to do anything, either consume input or produce output, and so it returns `Z_BUF_ERROR`. (See, I told you I'd cover this later.) However this is not a problem at all. Now we finally have the desired indication that `deflate()` is really done, and so we drop out of the inner loop to provide more input to `deflate()`.

With `flush` set to `Z_FINISH`, this final set of `deflate()` calls will complete the output stream. Once that is done, subsequent calls of `deflate()` would return `Z_STREAM_ERROR` if the flush parameter is not `Z_FINISH`, and do no more processing until the state is reinitialized.

Some applications of zlib have two loops that call `deflate()` instead of the single inner loop we have here. The first loop would call without flushing and feed all of the data to `deflate()`. The second loop would call `deflate()` with no more data and the `Z_FINISH` parameter to complete the process. As you can see from this example, that can be avoided by simply keeping track of the current flush state.

            } while (strm.avail_out == 0);
            assert(strm.avail_in == 0);     /* all input will be used */

Now we check to see if we have already processed all of the input file. That information was saved in the `flush` variable, so we see if that was set to `Z_FINISH`. If so, then we're done and we fall out of the outer loop. We're guaranteed to get `Z_STREAM_END` from the last `deflate()` call, since we ran it until the last chunk of input was consumed and all of the output was generated.

            /* done when last data in file processed */
        } while (flush != Z_FINISH);
        assert(ret == Z_STREAM_END);        /* stream will be complete */

The process is complete, but we still need to deallocate the state to avoid a memory leak (or rather more like a memory hemorrhage if you didn't do this). Then finally we can return with a happy return value.

        /* clean up and return */
        (void)deflateEnd(&strm);
        return Z_OK;
    }

Now we do the same thing for decompression in the `inf()` routine. `inf()` decompresses what is hopefully a valid zlib stream from the input file and writes the uncompressed data to the output file. Much of the discussion above for `def()` applies to `inf()` as well, so the discussion here will focus on the differences between the two.

    /* Decompress from file source to file dest until stream ends or EOF.
       inf() returns Z_OK on success, Z_MEM_ERROR if memory could not be
       allocated for processing, Z_DATA_ERROR if the deflate data is
       invalid or incomplete, Z_VERSION_ERROR if the version of zlib.h and
       the version of the library linked do not match, or Z_ERRNO if there
       is an error reading or writing the files. */
    int inf(FILE *source, FILE *dest)
    {

The local variables have the same functionality as they do for `def()`. The only difference is that there is no `flush` variable, since `inflate()` can tell from the zlib stream itself when the stream is complete.

        int ret;
        unsigned have;
        z_stream strm;
        unsigned char in[CHUNK];
        unsigned char out[CHUNK];

The initialization of the state is the same, except that there is no compression level, of course, and two more elements of the structure are initialized. `avail_in` and `next_in` must be initialized before calling `inflateInit()`. This is because the application has the option to provide the start of the zlib stream in order for `inflateInit()` to have access to information about the compression method to aid in memory allocation. In the current implementation of zlib (up through versions 1.2.x), the method-dependent memory allocations are deferred to the first call of `inflate()` anyway. However those fields must be initialized since later versions of zlib that provide more compression methods may take advantage of this interface. In any case, no decompression is performed by `inflateInit()`, so the `avail_out` and `next_out` fields do not need to be initialized before calling.

Here `avail_in` is set to zero and `next_in` is set to `Z_NULL` to indicate that no input data is being provided.

        /* allocate inflate state */
        strm.zalloc = Z_NULL;
        strm.zfree = Z_NULL;
        strm.opaque = Z_NULL;
        strm.avail_in = 0;
        strm.next_in = Z_NULL;
        ret = inflateInit(&strm);
        if (ret != Z_OK)
            return ret;

The outer `do`-loop decompresses input until `inflate()` indicates that it has reached the end of the compressed data and has produced all of the uncompressed output. This is in contrast to `def()` which processes all of the input file. If end-of-file is reached before the compressed data self-terminates, then the compressed data is incomplete and an error is returned.

        /* decompress until deflate stream ends or end of file */
        do {

We read input data and set the `strm` structure accordingly. If we've reached the end of the input file, then we leave the outer loop and report an error, since the compressed data is incomplete. Note that we may read more data than is eventually consumed by `inflate()`, if the input file continues past the zlib stream. For applications where zlib streams are embedded in other data, this routine would need to be modified to return the unused data, or at least indicate how much of the input data was not used, so the application would know where to pick up after the zlib stream.

            strm.avail_in = fread(in, 1, CHUNK, source);
            if (ferror(source)) {
                (void)inflateEnd(&strm);
                return Z_ERRNO;
            }
            if (strm.avail_in == 0)
                break;
            strm.next_in = in;

The inner `do`-loop has the same function it did in `def()`, which is to keep calling `inflate()` until has generated all of the output it can with the provided input.

            /* run inflate() on input until output buffer not full */
            do {

Just like in `def()`, the same output space is provided for each call of `inflate()`.

                strm.avail_out = CHUNK;
                strm.next_out = out;

Now we run the decompression engine itself. There is no need to adjust the flush parameter, since the zlib format is self-terminating. The main difference here is that there are return values that we need to pay attention to. `Z_DATA_ERROR` indicates that `inflate()` detected an error in the zlib compressed data format, which means that either the data is not a zlib stream to begin with, or that the data was corrupted somewhere along the way since it was compressed. The other error to be processed is `Z_MEM_ERROR`, which can occur since memory allocation is deferred until `inflate()` needs it, unlike `deflate()`, whose memory is allocated at the start by `deflateInit()`.

Advanced applications may use `deflateSetDictionary()` to prime `deflate()` with a set of likely data to improve the first 32K or so of compression. This is noted in the zlib header, so `inflate()` requests that that dictionary be provided before it can start to decompress. Without the dictionary, correct decompression is not possible. For this routine, we have no idea what the dictionary is, so the `Z_NEED_DICT` indication is converted to a `Z_DATA_ERROR`.

`inflate()` can also return `Z_STREAM_ERROR`, which should not be possible here, but could be checked for as noted above for `def()`. `Z_BUF_ERROR` does not need to be checked for here, for the same reasons noted for `def()`. `Z_STREAM_END` will be checked for later.

                ret = inflate(&strm, Z_NO_FLUSH);
                assert(ret != Z_STREAM_ERROR);  /* state not clobbered */
                switch (ret) {
                case Z_NEED_DICT:
                    ret = Z_DATA_ERROR;     /* and fall through */
                case Z_DATA_ERROR:
                case Z_MEM_ERROR:
                    (void)inflateEnd(&strm);
                    return ret;
                }

The output of `inflate()` is handled identically to that of `deflate()`.

                have = CHUNK - strm.avail_out;
                if (fwrite(out, 1, have, dest) != have || ferror(dest)) {
                    (void)inflateEnd(&strm);
                    return Z_ERRNO;
                }

The inner `do`-loop ends when `inflate()` has no more output as indicated by not filling the output buffer, just as for `deflate()`. In this case, we cannot assert that `strm.avail_in` will be zero, since the deflate stream may end before the file does.

            } while (strm.avail_out == 0);

The outer `do`-loop ends when `inflate()` reports that it has reached the end of the input zlib stream, has completed the decompression and integrity check, and has provided all of the output. This is indicated by the `inflate()` return value `Z_STREAM_END`. The inner loop is guaranteed to leave `ret` equal to `Z_STREAM_END` if the last chunk of the input file read contained the end of the zlib stream. So if the return value is not `Z_STREAM_END`, the loop continues to read more input.

            /* done when inflate() says it's done */
        } while (ret != Z_STREAM_END);

At this point, decompression successfully completed, or we broke out of the loop due to no more data being available from the input file. If the last `inflate()` return value is not `Z_STREAM_END`, then the zlib stream was incomplete and a data error is returned. Otherwise, we return with a happy return value. Of course, `inflateEnd()` is called first to avoid a memory leak.

        /* clean up and return */
        (void)inflateEnd(&strm);
        return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR;
    }

That ends the routines that directly use zlib. The following routines make this a command-line program by running data through the above routines from `stdin` to `stdout`, and handling any errors reported by `def()` or `inf()`.

`zerr()` is used to interpret the possible error codes from `def()` and `inf()`, as detailed in their comments above, and print out an error message. Note that these are only a subset of the possible return values from `deflate()` and `inflate()`.

    /* report a zlib or i/o error */
    void zerr(int ret)
    {
        fputs("zpipe: ", stderr);
        switch (ret) {
        case Z_ERRNO:
            if (ferror(stdin))
                fputs("error reading stdin\n", stderr);
            if (ferror(stdout))
                fputs("error writing stdout\n", stderr);
            break;
        case Z_STREAM_ERROR:
            fputs("invalid compression level\n", stderr);
            break;
        case Z_DATA_ERROR:
            fputs("invalid or incomplete deflate data\n", stderr);
            break;
        case Z_MEM_ERROR:
            fputs("out of memory\n", stderr);
            break;
        case Z_VERSION_ERROR:
            fputs("zlib version mismatch!\n", stderr);
        }
    }

Here is the `main()` routine used to test `def()` and `inf()`. The `zpipe` command is simply a compression pipe from `stdin` to `stdout`, if no arguments are given, or it is a decompression pipe if `zpipe -d` is used. If any other arguments are provided, no compression or decompression is performed. Instead a usage message is displayed. Examples are `zpipe < foo.txt > foo.txt.z` to compress, and `zpipe -d < foo.txt.z > foo.txt` to decompress.

    /* compress or decompress from stdin to stdout */
    int main(int argc, char **argv)
    {
        int ret;

        /* avoid end-of-line conversions */
        SET_BINARY_MODE(stdin);
        SET_BINARY_MODE(stdout);

        /* do compression if no arguments */
        if (argc == 1) {
            ret = def(stdin, stdout, Z_DEFAULT_COMPRESSION);
            if (ret != Z_OK)
                zerr(ret);
            return ret;
        }

        /* do decompression if -d specified */
        else if (argc == 2 && strcmp(argv[1], "-d") == 0) {
            ret = inf(stdin, stdout);
            if (ret != Z_OK)
                zerr(ret);
            return ret;
        }

        /* otherwise, report usage */
        else {
            fputs("zpipe usage: zpipe [-d] < source > dest\n", stderr);
            return 1;
        }
    }

Last modified 24 January 2023
Copyright © 2004-2023 Mark Adler
Creative Commons Attribution-NoDerivatives 4.0 International License.