list_tar.c

// 64-bit only
// sizeof(long): 8 byte

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <sys/mman.h>
#include <fcntl.h>

typedef struct
{
	char name[100];		/*   0 */
	char mode[8];		/* 100 */
	char uid[8];		/* 108 */
	char gid[8];		/* 116 */
	char size[12];		/* 124 */
	char mtime[12];		/* 136 */
	char chksum[8];		/* 148 */
	char typeflag;		/* 156 */
	char linkname[100];	/* 157 */
	char magic[6];		/* 257 */
	char version[2];	/* 263 */
	char uname[32];		/* 265 */
	char gname[32];		/* 297 */
	char devmajor[8];	/* 329 */
	char devminor[8];	/* 337 */
	char prefix[155];	/* 345 */
	char preserve[12];	/* 500 */
				/* 512 */
} tar_hdr_t;

// convert oct string to long int
long parse_tar_oct(const char *oct)
{
	long ret = 0;
	for(int i = 0; i < 11; ++i)
	{
		ret += (oct[i] - '0') * (1 << (3 * (11 - i - 1)));
	}
	return ret;
}

int list_entry(const char *buf, const long sz)
{
	if(sz < 512)
	{
		printf("[-] not a tar (size)\n");
		exit(1);
	}
	for(const char *p = buf; p - buf < sz; /* */)
	{
		tar_hdr_t *info = (tar_hdr_t *)p;
		if(memcmp("ustar", info->magic, 5) != 0) // end of file will pad blocks filled with zero
		{
			printf("[+] end of file\n");
			break;
		}
		long len = parse_tar_oct(info->size);
		printf("[+] [%p] %s\n", p, info->name);
		p += sizeof(tar_hdr_t) + len;
		if(len % 512 != 0) // padding zeros at the end of content file
		{
			p += 512 - len % 512;
		}
	}
}

int main(int argc, char **argv)
{
	if(argc != 2)
	{
		printf("%s <file>\n", argv[0]);
		return 1;
	}

	int fd = open(argv[1], O_RDONLY);
	off_t size = lseek(fd, 0, SEEK_END);

	char *p = (char *)mmap(0, size, PROT_READ, MAP_SHARED, fd, 0);
	printf("[+] [%s] is on [%p]-[%p]\n", argv[1], p, p+size);

	list_entry(p, size);

	munmap(p, size);
	close(fd);
	return 0;
}