Question

Let me explain what I’m trying to realize:

I have a encrypted tar file. I can decrypt it in memory, but obviously I can’t write the decrypted data back to hard disk as a real file. The decrypted data is structured as a char* buffer in memory; how can I untar it in memory?

I can’t find answer with libtar library.
I also tried to untar it with execlp("tar", "tar", "-xvO", (void*)0).
But it didn’t work as I thought.

Anyone can give me a hint of the best solution? Thanks!

Answer 1

I made an example, how to read file contents from an in-memory tar. The is_file_in_tar() function returns the length and the starting position of the named file if it is stored in the tar:

#include <stdio.h> 
#include <fcntl.h> 
#include <string.h> 
#include <sys/mman.h> 

struct tar {
  char name[100];   char _unused[24];
  char size[12];    char _padding[376];
} *tar;

int is_file_in_tar( struct tar *tar, char *name, char **start, int *length ){
  for( ; tar->name[0]; tar+=1+(*length+511)/512 ){
    sscanf( tar->size, "%o", length);
    if( !strcmp(tar->name,name) ){ *start = (char*)(tar+1); return 1; }
  }
  return 0;
}

int main(){
  int fd=open( "libtar-1.2.11.tar", O_RDONLY );
  tar=mmap(NULL, 808960, PROT_READ, MAP_PRIVATE, fd, 0);

  char *start; int length; char name[]="libtar-1.2.11/TODO";
  if( is_file_in_tar(tar,name,&start,&length) ) printf("%.*s",length,start);
}