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memfd_create.2: Various edits; add some FIXMEs
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" starting from a version by Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
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.\"
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.\" FIXME What is _SW_3_PARA?
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.\"
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.\" This program is free software; you can redistribute it and/or modify
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.\" it under the terms of the GNU General Public License as published by
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.\" the Free Software Foundation; either version 2 of the License, or
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@ -26,27 +29,46 @@ memfd_create \- create an anonymous file
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.BI "int memfd_create(const char *" name ", unsigned int " flags ");"
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.SH DESCRIPTION
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.BR memfd_create ()
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creates an anonymous file and returns a file-descriptor to it.
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The file behaves like regular files, thus can be modified,
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truncated, memory-mapped and more.
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However, unlike regular files it lives in main memory and has a volatile
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backing storage.
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creates an anonymous file and returns a file descriptor that refers to it.
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The file behaves like a regular file, and so can be modified,
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truncated, memory-mapped and so on.
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However, unlike a regular file,
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it lives in RAM and has a volatile backing storage.
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.\" FIXME In the following sentence I changed "released" to
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.\" "destroyed". Okay?
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Once all references to the file are dropped, it is automatically released.
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Anonymous memory is used for all backing pages of the file.
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Therefore, they are subject to the same restrictions as other anonymous
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memory allocations like
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.\" FIXME In the following sentence I changed "they" to
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.\" "files created by memfd_create()". Okay?
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Therefore, files created by
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.BR memfd_create ()
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are subject to the same restrictions as other anonymous
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.\" FIXME Can you give some examples on some of the restrictions please.
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memory allocations such as those allocated using
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.BR mmap (2)
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with
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.BR MAP_ANON .
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with the
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.BR MAP_ANONYMOUS
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flag.
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The initial size of the file is set to 0.
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Following the call, the file size should be set using
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.BR ftruncate (2).
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The name supplied in
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.I name
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is used as internal file-name and will occur as such in
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is used as an internal filename and will appear
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.\" FIXME What does "internal" in the previous line mean?
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as the target of the corresponding symbolic link in the directory
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.\" FIXME I added the previous line. Is it correct?
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.IR /proc/self/fd/ .
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The name is always prefixed with
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.BR memfd:
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and serves only debugging purposes.
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.\" FIXME In the next line, I added "as displayed in that
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The displayed name is always prefixed with
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.IR memfd:
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and serves only for debugging purposes.
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Names do not affect the behavior of the memfd,
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.\" FIXME The term "memfd" appears here without having previously been
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.\" defined. Would the correct definition "the memfd" be
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.\" "the file descriptor created by memfd_create"?
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and as such multiple files can have the same name without any side effects.
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The following values may be bitwise ORed in
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@ -63,9 +85,6 @@ See the description of the
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flag in
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.BR open (2)
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for reasons why this may be useful.
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Furthermore,
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.B O_LARGEFILE
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is always set for memfds.
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.TP
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.BR MFD_ALLOW_SEALING
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Allow sealing operations (see
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@ -77,22 +96,34 @@ and
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on this file.
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The initial set of seals is empty.
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If this flag is not set, the initial set of seals will be
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.B F_SEAL_SEAL
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and as such no other seals can be set on the file.
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.BR F_SEAL_SEAL ,
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meaning that no other seals can be set on the file.
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.\" FIXME Why is the MFD_ALLOW_SEALING behavior not simply the default?
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.\" Is it worth adding some text explaining this?
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.PP
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Unused bits must be cleared to 0.
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Unused bits in
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.I flags
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must be 0.
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As its return value,
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.BR memfd_create ()
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returns a new file descriptor that can be used to refer to the file.
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A copy of the file descriptor created by
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.BR memfd_create ()
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is inherited by the child produced by
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.BR fork (2).
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The duplicate file descriptor is associated with the same file.
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File descriptors created by
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.BR memfd_create ()
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are preserved across
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This file descriptor is opened for both reading and writing
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.RB ( O_RDWR )
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and
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.B O_LARGEFILE
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is set for the descriptor.
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With respect to
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.BR fork (2)
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and
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.BR execve (2),
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the usual semantics apply for the file descriptor created by
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.BR memfd_create ().
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A copy of the file descriptor is inherited by the child produced by
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.BR fork (2)
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and refers to the same file.
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The file descriptor is preserved across
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.BR execve (2),
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unless the close-on-exec flag has been set.
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.SH RETURN VALUE
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@ -104,6 +135,11 @@ On error, \-1 is returned and
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is set to indicate the error.
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.SH ERRORS
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.TP
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.B EFAULT
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The address in
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.IR name
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points to invalid memory.
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.TP
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.B EINVAL
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An unsupported value was specified in one of the arguments.
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.TP
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@ -113,18 +149,19 @@ The per-process limit on open file descriptors has been reached.
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.B ENFILE
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The system-wide limit on the total number of open files has been reached.
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.TP
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.B EFAULT
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The name given in
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.IR name
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points to invalid memory.
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.TP
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.B ENOMEM
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There was insufficient memory to create a new anonymous file.
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.SH VERSIONS
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to-be-defined
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.SH CONFORMING TO
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The
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.BR memfd_create ()
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is Linux-specific.
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system call first appeared in Linux 3.17.
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.SH CONFORMING TO
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The
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.BR memfd_create ()
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system call is Linux-specific.
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.\" FIXME Do we have any nice example program that could go in the man page?
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.SH SEE ALSO
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.BR fcntl (2),
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.\" FIXME Why the reference to shmget(2) in particular (and not,
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.\" e.g., shm_open(3), mmap(2))?
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.BR shmget (2)
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