blob: 68e0a0f29be4f8f5108b148438044e8de61d29b2 [file] [log] [blame]
/* gznorm.c -- normalize a gzip stream
* Copyright (C) 2018 Mark Adler
* For conditions of distribution and use, see copyright notice in zlib.h
* Version 1.0 7 Oct 2018 Mark Adler */
// gznorm takes a gzip stream, potentially containing multiple members, and
// converts it to a gzip stream with a single member. In addition the gzip
// header is normalized, removing the file name and time stamp, and setting the
// other header contents (XFL, OS) to fixed values. gznorm does not recompress
// the data, so it is fast, but no advantage is gained from the history that
// could be available across member boundaries.
#include <stdio.h> // fread, fwrite, putc, fflush, ferror, fprintf,
// vsnprintf, stdout, stderr, NULL, FILE
#include <stdlib.h> // malloc, free
#include <string.h> // strerror
#include <errno.h> // errno
#include <stdarg.h> // va_list, va_start, va_end
#include "zlib.h" // inflateInit2, inflate, inflateReset, inflateEnd,
// z_stream, z_off_t, crc32_combine, Z_NULL, Z_BLOCK,
#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)
# define SET_BINARY_MODE(file)
#define local static
// printf to an allocated string. Return the string, or NULL if the printf or
// allocation fails.
local char *aprintf(char *fmt, ...) {
// Get the length of the result of the printf.
va_list args;
va_start(args, fmt);
int len = vsnprintf(NULL, 0, fmt, args);
if (len < 0)
return NULL;
// Allocate the required space and printf to it.
char *str = malloc(len + 1);
if (str == NULL)
return NULL;
va_start(args, fmt);
vsnprintf(str, len + 1, fmt, args);
return str;
// Return with an error, putting an allocated error message in *err. Doing an
// inflateEnd() on an already ended state, or one with state set to Z_NULL, is
// permitted.
#define BYE(...) \
do { \
inflateEnd(&strm); \
*err = aprintf(__VA_ARGS__); \
return 1; \
} while (0)
// Chunk size for buffered reads and for decompression. Twice this many bytes
// will be allocated on the stack by gzip_normalize(). Must fit in an unsigned.
#define CHUNK 16384
// Read a gzip stream from in and write an equivalent normalized gzip stream to
// out. If given no input, an empty gzip stream will be written. If successful,
// 0 is returned, and *err is set to NULL. On error, 1 is returned, where the
// details of the error are returned in *err, a pointer to an allocated string.
// The input may be a stream with multiple gzip members, which is converted to
// a single gzip member on the output. Each gzip member is decompressed at the
// level of deflate blocks. This enables clearing the last-block bit, shifting
// the compressed data to concatenate to the previous member's compressed data,
// which can end at an arbitrary bit boundary, and identifying stored blocks in
// order to resynchronize those to byte boundaries. The deflate compressed data
// is terminated with a 10-bit empty fixed block. If any members on the input
// end with a 10-bit empty fixed block, then that block is excised from the
// stream. This avoids appending empty fixed blocks for every normalization,
// and assures that gzip_normalize applied a second time will not change the
// input. The pad bits after stored block headers and after the final deflate
// block are all forced to zeros.
local int gzip_normalize(FILE *in, FILE *out, char **err) {
// initialize the inflate engine to process a gzip member
z_stream strm;
strm.zalloc = Z_NULL;
strm.zfree = Z_NULL;
strm.opaque = Z_NULL;
strm.avail_in = 0;
strm.next_in = Z_NULL;
if (inflateInit2(&strm, 15 + 16) != Z_OK)
BYE("out of memory");
// State while processing the input gzip stream.
enum { // BETWEEN -> HEAD -> BLOCK -> TAIL -> BETWEEN -> ...
BETWEEN, // between gzip members (must end in this state)
HEAD, // reading a gzip header
BLOCK, // reading deflate blocks
TAIL // reading a gzip trailer
} state = BETWEEN; // current component being processed
unsigned long crc = 0; // accumulated CRC of uncompressed data
unsigned long len = 0; // accumulated length of uncompressed data
unsigned long buf = 0; // deflate stream bit buffer of num bits
int num = 0; // number of bits in buf (at bottom)
// Write a canonical gzip header (no mod time, file name, comment, extra
// block, or extra flags, and OS is marked as unknown).
fwrite("\x1f\x8b\x08\0\0\0\0\0\0\xff", 1, 10, out);
// Process the gzip stream from in until reaching the end of the input,
// encountering invalid input, or experiencing an i/o error.
int more; // true if not at the end of the input
do {
// State inside this loop.
unsigned char *put; // next input buffer location to process
int prev; // number of bits from previous block in
// the bit buffer, or -1 if not at the
// start of a block
unsigned long long memb; // uncompressed length of member
size_t tail; // number of trailer bytes read (0..8)
unsigned long part; // accumulated trailer component
// Get the next chunk of input from in.
unsigned char dat[CHUNK];
strm.avail_in = fread(dat, 1, CHUNK, in);
if (strm.avail_in == 0)
more = strm.avail_in == CHUNK;
strm.next_in = put = dat;
// Run that chunk of input through the inflate engine to exhaustion.
do {
// At this point it is assured that strm.avail_in > 0.
// Inflate until the end of a gzip component (header, deflate
// block, trailer) is reached, or until all of the chunk is
// consumed. The resulting decompressed data is discarded, though
// the total size of the decompressed data in each member is
// tracked, for the calculation of the total CRC.
do {
// inflate and handle any errors
unsigned char scrap[CHUNK];
strm.avail_out = CHUNK;
strm.next_out = scrap;
int ret = inflate(&strm, Z_BLOCK);
if (ret == Z_MEM_ERROR)
BYE("out of memory");
if (ret == Z_DATA_ERROR)
BYE("input invalid: %s", strm.msg);
if (ret != Z_OK && ret != Z_BUF_ERROR && ret != Z_STREAM_END)
BYE("internal error");
// Update the number of uncompressed bytes generated in this
// member. The actual count (not modulo 2^32) is required to
// correctly compute the total CRC.
unsigned got = CHUNK - strm.avail_out;
memb += got;
if (memb < got)
BYE("overflow error");
// Continue to process this chunk until it is consumed, or
// until the end of a component (header, deflate block, or
// trailer) is reached.
} while (strm.avail_out == 0 && (strm.data_type & 0x80) == 0);
// Since strm.avail_in was > 0 for the inflate call, some input was
// just consumed. It is therefore assured that put < strm.next_in.
// Disposition the consumed component or part of a component.
switch (state) {
state = HEAD;
// Fall through to HEAD when some or all of the header is
// processed.
case HEAD:
// Discard the header.
if (strm.data_type & 0x80) {
// End of header reached -- deflate blocks follow.
put = strm.next_in;
prev = num;
memb = 0;
state = BLOCK;
case BLOCK:
// Copy the deflate stream to the output, but with the
// last-block-bit cleared. Re-synchronize stored block
// headers to the output byte boundaries. The bytes at
// put..strm.next_in-1 is the compressed data that has been
// processed and is ready to be copied to the output.
// At this point, it is assured that new compressed data is
// available, i.e., put < strm.next_in. If prev is -1, then
// that compressed data starts in the middle of a deflate
// block. If prev is not -1, then the bits in the bit
// buffer, possibly combined with the bits in *put, contain
// the three-bit header of the new deflate block. In that
// case, prev is the number of bits from the previous block
// that remain in the bit buffer. Since num is the number
// of bits in the bit buffer, we have that num - prev is
// the number of bits from the new block currently in the
// bit buffer.
// If strm.data_type & 0xc0 is 0x80, then the last byte of
// the available compressed data includes the last bits of
// the end of a deflate block. In that case, that last byte
// also has strm.data_type & 0x1f bits of the next deflate
// block, in the range 0..7. If strm.data_type & 0xc0 is
// 0xc0, then the last byte of the compressed data is the
// end of the deflate stream, followed by strm.data_type &
// 0x1f pad bits, also in the range 0..7.
// Set bits to the number of bits not yet consumed from the
// last byte. If we are at the end of the block, bits is
// either the number of bits in the last byte belonging to
// the next block, or the number of pad bits after the
// final block. In either of those cases, bits is in the
// range 0..7.
; // (required due to C syntax oddity)
int bits = strm.data_type & 0x1f;
if (prev != -1) {
// We are at the start of a new block. Clear the last
// block bit, and check for special cases. If it is a
// stored block, then emit the header and pad to the
// next byte boundary. If it is a final, empty fixed
// block, then excise it.
// Some or all of the three header bits for this block
// may already be in the bit buffer. Load any remaining
// header bits into the bit buffer.
if (num - prev < 3) {
buf += (unsigned long)*put++ << num;
num += 8;
// Set last to have a 1 in the position of the last
// block bit in the bit buffer.
unsigned long last = (unsigned long)1 << prev;
if (((buf >> prev) & 7) == 3) {
// This is a final fixed block. Load at least ten
// bits from this block, including the header, into
// the bit buffer. We already have at least three,
// so at most one more byte needs to be loaded.
if (num - prev < 10) {
if (put == strm.next_in)
// Need to go get and process more input.
// We'll end up back here to finish this.
buf += (unsigned long)*put++ << num;
num += 8;
if (((buf >> prev) & 0x3ff) == 3) {
// That final fixed block is empty. Delete it
// to avoid adding an empty block every time a
// gzip stream is normalized.
num = prev;
buf &= last - 1; // zero the pad bits
else if (((buf >> prev) & 6) == 0) {
// This is a stored block. Flush to the next
// byte boundary after the three-bit header.
num = (prev + 10) & ~7;
buf &= last - 1; // zero the pad bits
// Clear the last block bit.
buf &= ~last;
// Write out complete bytes in the bit buffer.
while (num >= 8) {
putc(buf, out);
buf >>= 8;
num -= 8;
// If no more bytes left to process, then we have
// consumed the byte that had bits from the next block.
if (put == strm.next_in)
bits = 0;
// We are done handling the deflate block header. Now copy
// all or almost all of the remaining compressed data that
// has been processed so far. Don't copy one byte at the
// end if it contains bits from the next deflate block or
// pad bits at the end of a deflate block.
// mix is 1 if we are at the end of a deflate block, and if
// some of the bits in the last byte follow this block. mix
// is 0 if we are in the middle of a deflate block, if the
// deflate block ended on a byte boundary, or if all of the
// compressed data processed so far has been consumed.
int mix = (strm.data_type & 0x80) && bits;
// Copy all of the processed compressed data to the output,
// except for the last byte if it contains bits from the
// next deflate block or pad bits at the end of the deflate
// stream. Copy the data after shifting in num bits from
// buf in front of it, leaving num bits from the end of the
// compressed data in buf when done.
unsigned char *end = strm.next_in - mix;
if (put < end) {
if (num)
// Insert num bits from buf before the data being
// copied.
do {
buf += (unsigned)(*put++) << num;
putc(buf, out);
buf >>= 8;
} while (put < end);
else {
// No shifting needed -- write directly.
fwrite(put, 1, end - put, out);
put = end;
// Process the last processed byte if it wasn't written.
if (mix) {
// Load the last byte into the bit buffer.
buf += (unsigned)(*put++) << num;
num += 8;
if (strm.data_type & 0x40) {
// We are at the end of the deflate stream and
// there are bits pad bits. Discard the pad bits
// and write a byte to the output, if available.
// Leave the num bits left over in buf to prepend
// to the next deflate stream.
num -= bits;
if (num >= 8) {
putc(buf, out);
num -= 8;
buf >>= 8;
// Force the pad bits in the bit buffer to zeros.
buf &= ((unsigned long)1 << num) - 1;
// Don't need to set prev here since going to TAIL.
// At the end of an internal deflate block. Leave
// the last byte in the bit buffer to examine on
// the next entry to BLOCK, when more bits from the
// next block will be available.
prev = num - bits; // number of bits in buffer
// from current block
// Don't have a byte left over, so we are in the middle of
// a deflate block, or the deflate block ended on a byte
// boundary. Set prev appropriately for the next entry into
else if (strm.data_type & 0x80)
// The block ended on a byte boundary, so no header
// bits are in the bit buffer.
prev = num;
// In the middle of a deflate block, so no header here.
prev = -1;
// Check for the end of the deflate stream.
if ((strm.data_type & 0xc0) == 0xc0) {
// That ends the deflate stream on the input side, the
// pad bits were discarded, and any remaining bits from
// the last block in the stream are saved in the bit
// buffer to prepend to the next stream. Process the
// gzip trailer next.
tail = 0;
part = 0;
state = TAIL;
case TAIL:
// Accumulate available trailer bytes to update the total
// CRC and the total uncompressed length.
do {
part = (part >> 8) + ((unsigned long)(*put++) << 24);
if (tail == 4) {
// Update the total CRC.
z_off_t len2 = memb;
if (len2 < 0 || (unsigned long long)len2 != memb)
BYE("overflow error");
crc = crc ? crc32_combine(crc, part, len2) : part;
part = 0;
else if (tail == 8) {
// Update the total uncompressed length. (It's ok
// if this sum is done modulo 2^32.)
len += part;
// At the end of a member. Set up to inflate an
// immediately following gzip member. (If we made
// it this far, then the trailer was valid.)
if (inflateReset(&strm) != Z_OK)
BYE("internal error");
state = BETWEEN;
} while (put < strm.next_in);
// Process the input buffer until completely consumed.
} while (strm.avail_in > 0);
// Process input until end of file, invalid input, or i/o error.
} while (more);
// Done with the inflate engine.
// Verify the validity of the input.
if (state != BETWEEN)
BYE("input invalid: incomplete gzip stream");
// Write the remaining deflate stream bits, followed by a terminating
// deflate fixed block.
buf += (unsigned long)3 << num;
putc(buf, out);
putc(buf >> 8, out);
if (num > 6)
putc(0, out);
// Write the gzip trailer, which is the CRC and the uncompressed length
// modulo 2^32, both in little-endian order.
putc(crc, out);
putc(crc >> 8, out);
putc(crc >> 16, out);
putc(crc >> 24, out);
putc(len, out);
putc(len >> 8, out);
putc(len >> 16, out);
putc(len >> 24, out);
// Check for any i/o errors.
if (ferror(in) || ferror(out))
BYE("i/o error: %s", strerror(errno));
// All good!
*err = NULL;
return 0;
// Normalize the gzip stream on stdin, writing the result to stdout.
int main(void) {
// Avoid end-of-line conversions on evil operating systems.
// Normalize from stdin to stdout, returning 1 on error, 0 if ok.
char *err;
int ret = gzip_normalize(stdin, stdout, &err);
if (ret)
fprintf(stderr, "gznorm error: %s\n", err);
return ret;