named mmap without Disk IO - c

Question

I wanted to use the POSIX standard mmap as if shmat had an id, they share the same shared memory.
mmap seems to normally use fd=-1 when using MAP_ANON, and in this case it is said to be valid when inheriting from a child process.
I wanted to ensure the same behavior in the spawn method. Then I can use named shared memory, but I wish it didn't do DISK IO at all.
I think MAP_ANON is essential because I want to always guarantee the speed of RAM without leaving any files on the DISK in case of an unexpected shutdown.
So is it reasonable to provide fd!=-1 while being MAP_ANON|MAP_SHARED?
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <errno.h>
int main (int argc, char *argv[])
{
struct stat sb;
char *p;
int fd;
fd = shm_open("test", O_RDWR | O_CREAT, 0600);
if (fd == -1) {
perror("open");
return 1;
}
size_t len = 128;
if (ftruncate(fd, len) == -1) {
perror("ftruncate");
return 1;
}
p = (char*)mmap(0, len, PROT_WRITE|PROT_READ, MAP_ANON|MAP_SHARED, fd, 0);
if (p == MAP_FAILED){
perror("mmap");
return 1;
}
if (close(fd)==-1) {
perror("close");
return 1;
}
for (int i = 0; i < len; i++) {
putchar(p[i]);
}
if (munmap(p, len) == -1) {
perror("munmap");
return 1;
}
if(shm_unlink("test")) {
perror("unlink");
return 1;
}
fprintf(stderr,"\n");
return 0;
}