"fd not available for reading or writing" when memory-mapping a file

Question

I am trying to write data to a memory-mapped file in Rust but it won't memory map the specified file as it states the given fd is not available.

I can see it on the filesystem so it does exist with correct privileges. I suspect this is either a bug or I am not using the new IO API in the correct way.

mmap err = fd not available for reading or writing

Here's the code

use std::fs::File;
use std::os::MemoryMap;
use std::os::unix::prelude::AsRawFd;
use std::os::MapOption::{MapFd, MapWritable, MapReadable};

fn main() {
    let f = File::create("test.dat").unwrap();
    f.set_len(n as u64);

    let fd = f.as_raw_fd();

    let mmap = MemoryMap::new(n, &[MapReadable, MapWritable, MapFd(fd)]);

    match mmap {
        Ok(_) => println!("mmap success"),
        Err(ref err) => println!("mmap err = {}", err),
    }
}

Potentially useful information in this older question: [How to create and write to memory mapped files](http://stackoverflow.com/q/28516996/155423) — Shepmaster, Mar 14 '15 at 15:25

Shepmaster · Accepted Answer · 2017-12-24T16:14:35.003

Files created with File::create are in write-only mode, but you are attempting to map the file for both reading and writing. Use OpenOptions to get a file with both modes:

#![feature(os, std_misc)]

use std::fs::OpenOptions;
use std::os::MemoryMap;
use std::os::unix::prelude::AsRawFd;
use std::os::MapOption::{MapFd, MapReadable, MapWritable};

fn main() {
    let n = 100;

    let f = OpenOptions::new()
        .read(true)
        .write(true)
        .truncate(true)
        .create(true)
        .open("test.dat")
        .unwrap();

    f.set_len(n as u64).unwrap();

    let fd = f.as_raw_fd();

    let mmap = MemoryMap::new(n, &[MapReadable, MapWritable, MapFd(fd)]);

    match mmap {
        Ok(_) => println!("mmap success"),
        Err(err) => println!("mmap err = {}", err),
    }
}

I figured this out by

Grepping the code for "fd not available for reading or writing", which leads to this line, which aligns to ErrFdNotAvail (could also have changed mmap err = {} to mmap err = {:?}).
Searching for that enum variant leads to this line, which maps the underlying libc::EACCES error.
Checked out the man page for mmap to see what EACCES says:

The flag PROT_READ was specified as part of the prot argument and fd was not open for reading. The flags MAP_SHARED and PROT_WRITE were specified as part of the flags and prot argument and fd was not open for writing.

"fd not available for reading or writing" when memory-mapping a file

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