untar.cpp

untar.cpp

#pragma once

// from https://github.com/libarchive/libarchive/blob/master/contrib/untar.c
/*
 * This file is in the public domain.  Use it as you see fit.
 */

/*
 * "untar" is an extremely simple tar extractor:
 *  * A single C source file, so it should be easy to compile
 *    and run on any system with a C compiler.
 *  * Extremely portable standard C.  The only non-ANSI function
 *    used is mkdir().
 *  * Reads basic ustar tar archives.
 *  * Does not require libarchive or any other special library.
 *
 * To compile: cc -o untar untar.c
 *
 * Usage:  untar <archive>
 *
 * In particular, this program should be sufficient to extract the
 * distribution for libarchive, allowing people to bootstrap
 * libarchive on systems that do not already have a tar program.
 *
 * To unpack libarchive-x.y.z.tar.gz:
 *    * gunzip libarchive-x.y.z.tar.gz
 *    * untar libarchive-x.y.z.tar
 *
 * Written by Tim Kientzle, March 2009.
 *
 * Released into the public domain.
 */

/* These are all highly standard and portable headers. */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <string>
#include <fstream>
#include <streambuf>
/* This is for mkdir(); this may need to be changed for some platforms. */
#include <sys/stat.h>  /* For mkdir() */

#include <experimental/filesystem>

/* Parse an octal number, ignoring leading and trailing nonsense. */
static int
parseoct(const char *p, size_t n)
{
	int i = 0;

	while ((*p < '0' || *p > '7') && n > 0) {
		++p;
		--n;
	}
	while (*p >= '0' && *p <= '7' && n > 0) {
		i *= 8;
		i += *p - '0';
		++p;
		--n;
	}
	return (i);
}

/* Returns true if this is 512 zero bytes. */
static int
is_end_of_archive(const char *p)
{
	int n;
	for (n = 511; n >= 0; --n)
		if (p[n] != '\0')
			return (0);
	return (1);
}

/* Create a directory, including parent directories as necessary. */
static void
create_dir(const char *pathname, int mode)
{
	std::error_code ec;

  std::experimental::filesystem::create_directories(pathname, ec);

	if (ec) fprintf(stderr, "Could not create directory %s\n", pathname);
}

/* Create a file, including parent directory as necessary. */
static FILE *
create_file(char *pathname_in, int mode)
{

  const auto longlink_path = std::experimental::filesystem::path("@LongLink");
  std::string pathname(pathname_in);

  if (
    // if this pathname_in isn't @LongLink...
    longlink_path != std::experimental::filesystem::path(pathname_in)
    // ... and there exists an @LongLink file
    && std::experimental::filesystem::exists(longlink_path)
  ) {

    std::cout << " @LongLink detected" << std::endl;
    std::cout << " Renaming " << pathname_in << " to @LongLink contents";
    std::cout << std::endl;

    // then set the pathname to the contents of @LongLink...
    std::ifstream longlink_stream(longlink_path);
    pathname = std::string(
      std::istreambuf_iterator<char>(longlink_stream),
      std::istreambuf_iterator<char>()
    );
    // ... and delete the @LongLink file
    std::experimental::filesystem::remove(longlink_path);
  }

	FILE *f;
	f = fopen(pathname.c_str(), "wb+");
	if (f == NULL) {
		/* Try creating parent dir and then creating file. */
		create_dir(
      std::experimental::filesystem::path(pathname).parent_path().c_str(),
      0755
    );
		f = fopen(pathname.c_str(), "wb+");
  }

	return (f);
}

/* Verify the tar checksum. */
static int
verify_checksum(const char *p)
{
	int n, u = 0;
	for (n = 0; n < 512; ++n) {
		if (n < 148 || n > 155)
			/* Standard tar checksum adds unsigned bytes. */
			u += ((unsigned char *)p)[n];
		else
			u += 0x20;

	}
	return (u == parseoct(p + 148, 8));
}

/* Extract a tar archive. */
static void
untar(FILE *a, const char *path)
{
	char buff[512];
	FILE *f = NULL;
	size_t bytes_read;
	int filesize;

	printf("Extracting from %s\n", path);
	for (;;) {
		bytes_read = fread(buff, 1, 512, a);
		if (bytes_read < 512) {
			fprintf(stderr,
			    "Short read on %s: expected 512, got %d\n",
			    path, (int)bytes_read);
			return;
		}
		if (is_end_of_archive(buff)) {
			printf("End of %s\n", path);
			return;
		}
		if (!verify_checksum(buff)) {
			fprintf(stderr, "Checksum failure\n");
			return;
		}
		filesize = parseoct(buff + 124, 12);
		switch (buff[156]) {
		case '1':
			printf(" Ignoring hardlink %s\n", buff);
			break;
		case '2':
			printf(" Ignoring symlink %s\n", buff);
			break;
		case '3':
			printf(" Ignoring character device %s\n", buff);
				break;
		case '4':
			printf(" Ignoring block device %s\n", buff);
			break;
		case '5':
			printf(" Extracting dir %s\n", buff);
			create_dir(buff, parseoct(buff + 100, 8));
			filesize = 0;
			break;
		case '6':
			printf(" Ignoring FIFO %s\n", buff);
			break;
		default:
			printf(" Extracting file %s\n", buff);
			f = create_file(buff, parseoct(buff + 100, 8));
			break;
		}
		while (filesize > 0) {
			bytes_read = fread(buff, 1, 512, a);
			if (bytes_read < 512) {
				fprintf(stderr,
				    "Short read on %s: Expected 512, got %d\n",
				    path, (int)bytes_read);
				return;
			}
			if (filesize < 512)
				bytes_read = filesize;
			if (f != NULL) {
				if (fwrite(buff, 1, bytes_read, f)
				    != bytes_read)
				{
					fprintf(stderr, "Failed write\n");
					fclose(f);
					f = NULL;
				}
			}
			filesize -= bytes_read;
		}
		if (f != NULL) {
			fclose(f);
			f = NULL;
		}
	}
}