/*
 * "untar" is a minimal tar extractor.  It should work with
 * most ustar tar archives.  It is intended to be sufficient
 * to extract the distribution for a full-featured tar program.
 *
 * This is a single-file C program that compiles into a standalone
 * executable.
 *
 * To compile: cc -o untar untar.c
 *
 * Usage:  untar <archive>
 *
 * 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>

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

/* 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') {
		++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 = 512; n > 0; --n)
		if (p[n] != '\0')
			return (0);
	return (1);
}

/* Create a directory, including parent directories as necessary. */
static void
create_dir(char *pathname, int mode)
{
	char *p;
	int r;

	/* Strip trailing '/' */
	if (pathname[strlen(pathname) - 1] == '/')
		pathname[strlen(pathname) - 1] = '\0';

	/* Try creating the directory. */
	r = mkdir(pathname, mode);

	if (r != 0) {
		/* On failure, try creating parent directory. */
		p = strrchr(pathname, '/');
		if (p != NULL) {
			*p = '\0';
			create_dir(pathname, 0755);
			*p = '/';
			r = mkdir(pathname, mode);
		}
	}
	if (r != 0)
		fprintf(stderr, "Could not create directory %s\n", pathname);
}

/* Create a file, including parent directory as necessary. */
static FILE *
create_file(char *pathname, int mode)
{
	FILE *f;
	f = fopen(pathname, "w+");
	if (f == NULL) {
		/* Try creating parent dir and then creating file. */
		char *p = strrchr(pathname, '/');
		if (p != NULL) {
			*p = '\0';
			create_dir(pathname, 0755);
			*p = '/';
			f = fopen(pathname, "w+");
		}
	}
	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 int
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, 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));
			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, 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;
		}
	}
}

int
main(int argc, char **argv)
{
	FILE *a;

	++argv; /* Skip program name */
	for ( ;*argv != NULL; ++argv) {
		a = fopen(*argv, "r");
		if (a == NULL)
			fprintf(stderr, "Unable to open %s\n", *argv);
		else {
			untar(a, *argv);
			fclose(a);
		}
	}
	return (0);
}