How to extract all data from bzip2 archive using C?

Question

I have a concatenated file consisting of a number of archives bzip2. I also know the sizes of individual fragments bzip2in this file.

I would like to unzip the stream bzip2from a separate bzip2 data block and write the output to standard output.

First I use fseekto move the file cursor to the desired archive byte, and then read the "size" of the file's point in the call BZ2_bzRead:

int headerSize = 1234;
int firstChunkSize = 123456;
FILE *fp = fopen("pathToConcatenatedFile", "r+b");
char *bzBuf = malloc(sizeof(char) * firstChunkSize);
int bzError, bzNBuf;
BZFILE *bzFp = BZ2_bzReadOpen(&bzError, *fp, 0, 0, NULL, 0);

# move cursor past header of known size, to the first bzip2 "chunk"
fseek(*fp, headerSize, SEEK_SET); 

while (bzError != BZ_STREAM_END) {
    # read the first chunk of known size, decompress it
    bzNBuf = BZ2_bzRead(&bzError, bzFp, bzBuf, firstChunkSize);
    fprintf(stdout, bzBuf);
}

BZ2_bzReadClose(&bzError, bzFp);
free(bzBuf);
fclose(fp);

The problem is that when I compare the output of the instruction fprintfwith the output from bzip2the command line, I get two different answers.

In particular, I get less result from this code than from running bzip2on the command line.

, , , bzip2.

, bzip2 , , , C . , .

bzip2 libbzip2, - , ? .

Answer 1

:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <bzlib.h>

int
bunzip_one(FILE *f) {
  int bzError;
  BZFILE *bzf;
  char buf[4096];

  bzf = BZ2_bzReadOpen(&bzError, f, 0, 0, NULL, 0);
  if (bzError != BZ_OK) {
    fprintf(stderr, "E: BZ2_bzReadOpen: %d\n", bzError);
    return -1;
  }

  while (bzError == BZ_OK) {
    int nread = BZ2_bzRead(&bzError, bzf, buf, sizeof buf);
    if (bzError == BZ_OK || bzError == BZ_STREAM_END) {
      size_t nwritten = fwrite(buf, 1, nread, stdout);
      if (nwritten != (size_t) nread) {
        fprintf(stderr, "E: short write\n");
        return -1;
      }
    }
  }

  if (bzError != BZ_STREAM_END) {
    fprintf(stderr, "E: bzip error after read: %d\n", bzError);
    return -1;
  }

  BZ2_bzReadClose(&bzError, bzf);
  return 0;
}

int
bunzip_many(const char *fname) {
  FILE *f;

  f = fopen(fname, "rb");
  if (f == NULL) {
    perror(fname);
    return -1;
  }

  fseek(f, 0, SEEK_SET);
  if (bunzip_one(f) == -1)
    return -1;

  fseek(f, 42, SEEK_SET); /* hello.bz2 is 42 bytes long in my case */
  if (bunzip_one(f) == -1)
    return -1;

  fclose(f);
  return 0;
}

int
main(int argc, char **argv) {
  if (argc < 2) {
    fprintf(stderr, "usage: bunz <fname>\n");
    return EXIT_FAILURE;
  }
  return bunzip_many(argv[1]) != 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}

• . , , bzError BZ_OK BZ_STREAM_END, . , bzError undefined.
• , 50 . . .
• . (f, bzf) .

, :

$ echo hello > hello
$ echo world > world
$ bzip2 hello
$ bzip2 world
$ cat hello.bz2 world.bz2 > helloworld.bz2
$ gcc -W -Wall -Os -o bunz bunz.c -lbz2
$ ls -l *.bz2
-rw-r--r-- 1 roland None 42 Oct 12 09:26 hello.bz2
-rw-r--r-- 1 roland None 86 Oct 12 09:36 helloworld.bz2
-rw-r--r-- 1 roland None 44 Oct 12 09:26 world.bz2
$ ./bunz.exe helloworld.bz2 
hello
world