libtar c extract file

Question

Linux

Completely noob question, but I cannot understand it myself. I'm using libtar from http://www.feep.net/libtar/

I've zlib'ed several files and tar'ed the by means of libtar.

TAR *pTar;
snprintf(tar_name,sizeof(tar_name),"%s.tar",BACKUP_TASK.path_to_backup);
tar_open(&pTar, tar_name, NULL, O_WRONLY | O_CREAT, 0777, TAR_GNU);
tar_append_tree(pTar, BACKUP_TASK.path_to_backup, BACKUP_TASK.task_name);
//Append vm list to archive
tar_append_file(pTar, "task.xml", "backup.xml");
//Close it
tar_close(pTar);

Then I need to seek the tar, extract some files

Here is real content of TAR:

 #tar tf /home/backup/ftp_templates.tar 

 ftp_templates/
 ftp_templates/d982d6cc-810a-7582-ddec-2c6b6a9d5530.xva.zlib
 ftp_templates/51979771-7362-2061-9043-3606682fa0f3.xva.zlib
 ftp_templates/f9e5a919-4aa9-a02d-b1ec-3667ecaa4a5b.xva.zlib
 backup.xml

I'm trying to list files inside:

TAR *pTar;
char *tarFilename = "/home/backup/ftp_templates.tar";
tar_open(&pTar, tarFilename, NULL, O_RDONLY | TAR_NOOVERWRITE, 0777, TAR_GNU|TAR_VERBOSE);
while(th_read(pTar)==0)
{
    puts(pTar->th_buf.name);
}
tar_close(pTar);

But it gives only

ftp_templates/
ftp_templates/d982d6cc-810a-7582-ddec-2c6b6a9d5530.xva.zlib

also if I try to extract any file with

tar_extract_file(pTar,"backup.xml");

it creates empty file, 0 in size

Extract all files works well. I just didn't find any example of extracting single file....

tar -xvf /home/backup/ftp_templates.tar backup.xml

This also did well, so tar itself is OK..

Answer 1

If you want to seek for many files, build an index of the tar first:

#include <iostream>
#include <string>
#include <vector>
#include <libtar.h>
#include <fcntl.h>
#include <unistd.h>
using namespace std;

int extract_from_tar(TAR *pTar, const vector<tar_header> &fileList, 
    const vector<long int> &fileInPos, size_t index)
{
    const tar_header &th = fileList[index];
    pTar->th_buf = th;
    lseek (tar_fd(pTar), fileInPos[index], SEEK_SET);

    return tar_extract_regfile(pTar, pTar->th_buf.name);
}

int main()
{
    time_t seed = time( NULL );
    srand( seed );

    vector<tar_header> fileList;
    vector<long int> fileInPos;
    TAR *pTar = nullptr;
    tar_open(&pTar, "test.tar", NULL, O_RDONLY, 0, 0);

    //Build index for quick access
    int i=0;    
    while ((i = th_read(pTar)) == 0)
    {
        if(TH_ISREG(pTar))
        {
            fileList.push_back(pTar->th_buf);
            long fd = tar_fd(pTar);
            long int pos = lseek(fd, 0, SEEK_CUR);

            fileInPos.push_back(pos);
            if (tar_skip_regfile(pTar) != 0) return -1;
        }
    }

    //Extract a random file from archive
    if(fileList.size()>0)
        cout << "extract_from_tar " << extract_from_tar(pTar, fileList, fileInPos, rand()%fileList.size()) << endl;

    tar_close(pTar);
}

Answer 2

Thats how I did it. Works fine but was not too obvious for me

TAR *pTar;  
int i=0;
tar_open(&pTar, xva_filename, 0, O_RDONLY, 0, 0);
while ((i = th_read(pTar)) == 0)
{
    //th_print_long_ls(pTar);
    //th_print(pTar);
    if(strcmp(pTar->th_buf.name,"ova.xml")==0)
    {
        puts(pTar->th_buf.name);
        tar_extract_regfile(pTar,pTar->th_buf.name);
    }


    if (TH_ISREG(pTar) && tar_skip_regfile(pTar) != 0)
    {
        fprintf(stderr, "tar_skip_regfile(): %s\n",
            strerror(errno));
        return;
    }
}