How to get file descriptor of buffer in memory?

Question

If I have a buffer which contains the data of a file, how can I get a file descriptor from it? This is a question derived from how to untar file in memory

Answer 1

I wrote a simple example how to make filedescriptor to a memory area:

#include 
#include  
#include  

char buff[]="qwer\nasdf\n";

int main(){
  int p[2]; pipe(p);

  if( !fork() ){
    for( int buffsize=strlen(buff), len=0; buffsize>len; )
      len+=write( p[1], buff+len, buffsize-len );
    return 0;
  }

  close(p[1]);
  FILE *f = fdopen( p[0], "r" );
  char buff[100];
  while( fgets(buff,100,f) ){
    printf("from child: '%s'\n", buff );
  }
  puts("");
}