mmap_file.c

#include<stdio.h>
#include<stdlib.h>
#include<sys/mman.h>
#include<string.h>
#include<assert.h>

#define GLOBAL_FILE 0x4000000
void start_file(const char *fname) {
  void *fp = mmap((void*)GLOBAL_FILE, sizeof(FILE), PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE,0,0);
  FILE *f = fopen(fname, "w");
  assert(*(int*)fp!=-1);
  memcpy(fp, f, sizeof(FILE));
}

void write_file(const char *what) {
  FILE *f = (FILE*)(void*)GLOBAL_FILE;
  fwrite(what,1,strlen(what),f);
}
int  main() {
	start_file("file.txt");
	write_file("content");
	return 0;

}