.\"	$NetBSD: memfd_create.2,v 1.1 2023/07/10 02:31:54 christos Exp $
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.Dd July 5, 2023
.Dt MEMFD_CREATE 2
.Os
.Sh NAME
.Nm memfd_create
.Nd create anonymous files
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/mman.h
.Ft int
.Fn memfd_create "const char *name" "unsigned int flags"
.Sh DESCRIPTION
The
.Fn memfd_create
system call returns a file descriptor to a file named
.Fa name
backed only by RAM.
Initially, the size of the file is zero.
.Pp
The length of
.Fa name
must not exceed
.Dv NAME_MAX-6
characters in length, to allow for the prefix
.Dq memfd:
to be added.
But since the file descriptor does not live on disk,
.Fa name
does not have to be unique.
.Fa name
is only intended to be used for debugging purposes and commands like
.Xr fstat 1 .
.Pp
Additionally, any of the following may be specified as the
.Fa flags :
.Bl -tag -width MFD_ALLOW_SEALING
.It Dv MFD_CLOEXEC
Set the
.Xr close 2
on
.Xr exec 3
flag.
.It Dv MFD_ALLOW_SEALING
Allow adding seals to the file descriptor using the
.Xr fcntl 2
.Dv F_ADD_SEALS
command.
.El
.Pp
Otherwise, the returned file descriptor behaves the same as a regular file,
including the ability to be mapped by
.Xr mmap 2 .
.Sh RETURN VALUES
If successful, the
.Fn memfd_create
system call returns a non-negative integer.
On failure -1 is returned and
.Fa errno
is set to indicate the error.
.Sh ERRORS
.Fn memfd_create
will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
The argument
.Fa name
is
.Dv NULL
or points to invalid memory.
.It Bq Er EINVAL
The argument
.Fa flags
has any bits set other than
.Dv MFD_CLOEXEC
or
.Dv MFD_ALLOW_SEALING .
.It Bq Er ENAMETOOLONG
The length of
.Fa name
appended with the prefix
.Dq memfd:
would exceed
.Dv NAME_MAX .
.It Bq Er ENFILE
The system file table is full.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr mmap 2 ,
.Xr shmget 2 ,
.Xr shm_open 3
.Sh HISTORY
.Fn memfd_create
is compatible with the Linux system call of the same name that first appeared in
Linux 3.17.