How to get file descriptor of buffer in memory?

Not possible in plain C. In plain C all file access happens via FILE * handles and these can only be created with fopen() and freopen() and in both cases must refer to a file path. As C tries to be as portable as possible, it limits I/O to the absolute bare minimum that probably all systems can support in some way.

If you have POSIX API available (e.g. Linux, macOS, iOS, FreeBSD, most other UNIX systems), you can use fmemopen():

char dataInMemory[] = "This is some data in memory";
FILE * fileDescriptor = fmemopen(dataInMemory, sizeof(dataInMemory), "r");

This is a true file handle that can be used with all C file API. It should also allow seeking, something not possible if you work with pipes as pipes support no seeking (you can emulate forward seeking but there is no way to ever seek backwards).

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

#include <unistd.h>
#include <stdio.h> 
#include <string.h> 

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("");
}