//! Interfaces for managing memory-backed files.

use cfg_if::cfg_if;

use std::os::unix::io::RawFd;

use crate::errno::Errno;

use crate::Result;

use std::ffi::CStr;

libc_bitflags!(

    /// Options that change the behavior of [`memfd_create`].

    pub struct MemFdCreateFlag: libc::c_uint {

        /// Set the close-on-exec ([`FD_CLOEXEC`]) flag on the new file descriptor.

        ///

        /// By default, the new file descriptor is set to remain open across an [`execve`]

        /// (the `FD_CLOEXEC` flag is initially disabled). This flag can be used to change

        /// this default. The file offset is set to the beginning of the file (see [`lseek`]).

        ///

        /// See also the description of the `O_CLOEXEC` flag in [`open(2)`].

        ///

        /// [`execve`]: crate::unistd::execve

        /// [`lseek`]: crate::unistd::lseek

        /// [`FD_CLOEXEC`]: crate::fcntl::FdFlag::FD_CLOEXEC

        /// [`open(2)`]: https://man7.org/linux/man-pages/man2/open.2.html

        MFD_CLOEXEC;

        /// Allow sealing operations on this file.

        ///

        /// See also the file sealing notes given in [`memfd_create(2)`].

        ///

        /// [`memfd_create(2)`]: https://man7.org/linux/man-pages/man2/memfd_create.2.html

        MFD_ALLOW_SEALING;

    }

);

/// Creates an anonymous file that lives in memory, and return a file-descriptor to it.

///

/// The file behaves like a regular file, and so can be modified, truncated, memory-mapped, and so on.

/// However, unlike a regular file, it lives in RAM and has a volatile backing storage.

///

/// For more information, see [`memfd_create(2)`].

///

/// [`memfd_create(2)`]: https://man7.org/linux/man-pages/man2/memfd_create.2.html

pub fn memfd_create(name: &CStr, flags: MemFdCreateFlag) -> Result<RawFd> {

    let res = unsafe {

        cfg_if! {

            if #[cfg(all(

                // Android does not have a memfd_create symbol

                not(target_os = "android"),

                any(

                    target_os = "freebsd",

                    // If the OS is Linux, gnu and musl expose a memfd_create symbol but not uclibc

                    target_env = "gnu",

                    target_env = "musl",

                )))]

            {

                libc::memfd_create(name.as_ptr(), flags.bits())

            } else {

                libc::syscall(libc::SYS_memfd_create, name.as_ptr(), flags.bits())

            }

        }

    };

    Errno::result(res).map(|r| r as RawFd)

}