zlib inflate, where the file/buffer contains multiple streams.

zlib.inflate.multi-stream.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <zlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <inttypes.h>

/*
	i didn't find any code - in the documentation or otherwise - that
	showed how to inflate a file or buffer that might contain multiple
	streams/files.

	more specifically, i'm looking for a way to decompress a buffer
	that's already in memory, so gzFile, et al, weren't what i was
	looking for.

	in order to decompress multiple streams in the same compressed buffer,
	you must move the pointer sent to inflate(), byt strm.total_in bytes
	after each read. you see this handled below with in_pos and in_byt.
*/

#define INPUT_FILE "filename.gz"

int
unz(z_stream   *strm,
    const void *src, unsigned long src_byt,
          void *dst, unsigned long dst_byt)
{
	strm->total_in  = strm->avail_in  = src_byt;
	strm->total_out = strm->avail_out = dst_byt;
	strm->next_in   = (Bytef*)src;
	strm->next_out  = (Bytef*)dst;
	strm->zalloc    = Z_NULL;
	strm->zfree     = Z_NULL;
	strm->opaque    = Z_NULL;

	int ret = -1;

	//  15 = window bits
	// +32 = detect if using gzip or zlib
	ret = inflateInit2(strm, (15 + 32));
	if (ret == Z_OK)
		ret = inflate(strm, Z_FINISH);

	inflateEnd(strm);
	return ret;
}

int
main()
{
	const char *fnm = INPUT_FILE;
	struct stat fp_stat;
	if (0 == stat(fnm, &fp_stat))
		printf("bytes: %d\n", (int)fp_stat.st_size);

	printf("allocating: zip_buf\n");fflush(stdout);
	void          *zip_buf = calloc(1, fp_stat.st_size);
	unsigned long  zip_byt = (int)fp_stat.st_size;

	printf("allocating: unz_buf\n");fflush(stdout);
	unsigned long  unz_byt = (1024UL * 1024UL * 8UL);	// 8mb is fine here
	void          *unz_buf = calloc(1, zip_byt);

	printf("reading file to memory\n");fflush(stdout);
	FILE *fp = fopen(fnm, "rb");
	fseek(fp, 0, SEEK_SET);
	fread(zip_buf, fp_stat.st_size, 1, fp);
	fclose(fp);

	printf("unzipping\n");fflush(stdout);
	z_stream        strm = {0};
	int              ret = 0;
	int              cnt = 0;
	void         *in_pos = zip_buf;
	unsigned long in_byt = zip_byt;
	while (Z_STREAM_END == unz(&strm, in_pos, in_byt, unz_buf, unz_byt))
	{
		// printf("unz_buf:\n%.*s\n", (int)strm.total_out, (char*)unz_buf);
		// fflush(stdout);
		in_pos += strm.total_in;
		in_byt -= strm.total_in;
		printf("read: %ld\n", strm.total_in);
		printf("left: %ld\n", in_byt);
		printf("totl: %d\n",  ++cnt);
		printf("\n");
		fflush(stdout);
	}

	return 0;
}