freebsd-nq/lib/libc/sys/shm_open.2

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.Dd June 25, 2021
.Dt SHM_OPEN 2
.Os
.Sh NAME
.Nm memfd_create , shm_open , shm_rename, shm_unlink
.Nd "shared memory object operations"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/mman.h
.In fcntl.h
.Ft int
.Fn memfd_create "const char *name" "unsigned int flags"
.Ft int
.Fn shm_open "const char *path" "int flags" "mode_t mode"
.Ft int
.Fn shm_rename "const char *path_from" "const char *path_to" "int flags"
.Ft int
.Fn shm_unlink "const char *path"
.Sh DESCRIPTION
The
.Fn shm_open
system call opens (or optionally creates) a
POSIX
shared memory object named
.Fa path .
The
.Fa flags
argument contains a subset of the flags used by
.Xr open 2 .
An access mode of either
.Dv O_RDONLY
or
.Dv O_RDWR
must be included in
.Fa flags .
The optional flags
.Dv O_CREAT ,
.Dv O_EXCL ,
and
.Dv O_TRUNC
may also be specified.
.Pp
If
.Dv O_CREAT
is specified,
then a new shared memory object named
.Fa path
will be created if it does not exist.
In this case,
the shared memory object is created with mode
.Fa mode
subject to the process' umask value.
If both the
.Dv O_CREAT
and
.Dv O_EXCL
flags are specified and a shared memory object named
.Fa path
already exists,
then
.Fn shm_open
will fail with
.Er EEXIST .
.Pp
Newly created objects start off with a size of zero.
If an existing shared memory object is opened with
.Dv O_RDWR
and the
.Dv O_TRUNC
flag is specified,
then the shared memory object will be truncated to a size of zero.
The size of the object can be adjusted via
.Xr ftruncate 2
and queried via
.Xr fstat 2 .
.Pp
The new descriptor is set to close during
.Xr execve 2
system calls;
see
.Xr close 2
and
.Xr fcntl 2 .
.Pp
As a
.Fx
extension, the constant
.Dv SHM_ANON
may be used for the
.Fa path
argument to
.Fn shm_open .
In this case, an anonymous, unnamed shared memory object is created.
Since the object has no name,
it cannot be removed via a subsequent call to
.Fn shm_unlink ,
or moved with a call to
.Fn shm_rename .
Instead,
the shared memory object will be garbage collected when the last reference to
the shared memory object is removed.
The shared memory object may be shared with other processes by sharing the
file descriptor via
.Xr fork 2
or
.Xr sendmsg 2 .
Attempting to open an anonymous shared memory object with
.Dv O_RDONLY
will fail with
.Er EINVAL .
All other flags are ignored.
.Pp
The
.Fn shm_rename
system call atomically removes a shared memory object named
.Fa path_from
and relinks it at
.Fa path_to .
If another object is already linked at
.Fa path_to ,
that object will be unlinked, unless one of the following flags are provided:
.Bl -tag -offset indent -width Er
.It Er SHM_RENAME_EXCHANGE
Atomically exchange the shms at
.Fa path_from
and
.Fa path_to .
.It Er SHM_RENAME_NOREPLACE
Return an error if an shm exists at
.Fa path_to ,
rather than unlinking it.
.El
.Fn shm_rename
is also a
.Fx
extension.
.Pp
The
.Fn shm_unlink
system call removes a shared memory object named
.Fa path .
.Pp
The
.Fn memfd_create
function creates an anonymous shared memory object, identical to that created
by
.Fn shm_open
when
.Dv SHM_ANON
is specified.
Newly created objects start off with a size of zero.
The size of the new object must be adjusted via
.Xr ftruncate 2 .
.Pp
The
.Fa name
argument must not be
.Dv NULL ,
but it may be an empty string.
The length of the
.Fa name
argument may not exceed
.Dv NAME_MAX
minus six characters for the prefix
.Dq memfd: ,
which will be prepended.
The
.Fa name
argument is intended solely for debugging purposes and will never be used by the
kernel to identify a memfd.
Names are therefore not required to be unique.
.Pp
The following
.Fa flags
may be specified to
.Fn memfd_create :
.Bl -tag -width MFD_ALLOW_SEALING
.It Dv MFD_CLOEXEC
Set
.Dv FD_CLOEXEC
on the resulting file descriptor.
.It Dv MFD_ALLOW_SEALING
Allow adding seals to the resulting file descriptor using the
.Dv F_ADD_SEALS
.Xr fcntl 2
command.
.It Dv MFD_HUGETLB
This flag is currently unsupported.
.El
.Sh RETURN VALUES
If successful,
.Fn memfd_create
and
.Fn shm_open
both return a non-negative integer,
and
.Fn shm_rename
and
.Fn shm_unlink
return zero.
All functions return -1 on failure, and set
.Va errno
to indicate the error.
.Sh COMPATIBILITY
The
.Fa path ,
.Fa path_from ,
and
.Fa path_to
arguments do not necessarily represent a pathname (although they do in
most other implementations).
Two processes opening the same
.Fa path
are guaranteed to access the same shared memory object if and only if
.Fa path
begins with a slash
.Pq Ql \&/
character.
.Pp
Only the
.Dv O_RDONLY ,
.Dv O_RDWR ,
.Dv O_CREAT ,
.Dv O_EXCL ,
and
.Dv O_TRUNC
flags may be used in portable programs.
.Pp
POSIX
specifications state that the result of using
.Xr open 2 ,
.Xr read 2 ,
or
.Xr write 2
on a shared memory object, or on the descriptor returned by
.Fn shm_open ,
is undefined.
However, the
.Fx
kernel implementation explicitly includes support for
.Xr read 2
and
.Xr write 2 .
.Pp
.Fx
also supports zero-copy transmission of data from shared memory
objects with
.Xr sendfile 2 .
.Pp
Neither shared memory objects nor their contents persist across reboots.
.Pp
Writes do not extend shared memory objects, so
.Xr ftruncate 2
must be called before any data can be written.
See
.Sx EXAMPLES .
.Sh EXAMPLES
This example fails without the call to
.Xr ftruncate 2 :
.Bd -literal -compact

        uint8_t buffer[getpagesize()];
        ssize_t len;
        int fd;

        fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
        if (fd < 0)
                err(EX_OSERR, "%s: shm_open", __func__);
        if (ftruncate(fd, getpagesize()) < 0)
                err(EX_IOERR, "%s: ftruncate", __func__);
        len = pwrite(fd, buffer, getpagesize(), 0);
        if (len < 0)
                err(EX_IOERR, "%s: pwrite", __func__);
        if (len != getpagesize())
                errx(EX_IOERR, "%s: pwrite length mismatch", __func__);
.Ed
.Sh ERRORS
.Fn memfd_create
fails with these error codes for these conditions:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa name
argument was NULL.
.It Bq Er EINVAL
The
.Fa name
argument was too long.
.Pp
An invalid or unsupported flag was included in
.Fa flags .
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er ENOSYS
In
.Fa memfd_create ,
.Dv MFD_HUGETLB
was specified in
.Fa flags ,
and this system does not support forced hugetlb mappings.
.El
.Pp
.Fn shm_open
fails with these error codes for these conditions:
.Bl -tag -width Er
.It Bq Er EINVAL
A flag other than
.Dv O_RDONLY ,
.Dv O_RDWR ,
.Dv O_CREAT ,
.Dv O_EXCL ,
or
.Dv O_TRUNC
was included in
.Fa flags .
.It Bq Er EMFILE
The process has already reached its limit for open file descriptors.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er EINVAL
.Dv O_RDONLY
was specified while creating an anonymous shared memory object via
.Dv SHM_ANON .
.It Bq Er EFAULT
The
.Fa path
argument points outside the process' allocated address space.
.It Bq Er ENAMETOOLONG
The entire pathname exceeds 1023 characters.
.It Bq Er EINVAL
The
.Fa path
does not begin with a slash
.Pq Ql \&/
character.
.It Bq Er ENOENT
.Dv O_CREAT
is not specified and the named shared memory object does not exist.
.It Bq Er EEXIST
.Dv O_CREAT
and
.Dv O_EXCL
are specified and the named shared memory object does exist.
.It Bq Er EACCES
The required permissions (for reading or reading and writing) are denied.
.El
.Pp
The following errors are defined for
.Fn shm_rename :
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa path_from
or
.Fa path_to
argument points outside the process' allocated address space.
.It Bq Er ENAMETOOLONG
The entire pathname exceeds 1023 characters.
.It Bq Er ENOENT
The shared memory object at
.Fa path_from
does not exist.
.It Bq Er EACCES
The required permissions are denied.
.It Bq Er EEXIST
An shm exists at
.Fa path_to ,
and the
.Dv SHM_RENAME_NOREPLACE
flag was provided.
.El
.Pp
.Fn shm_unlink
fails with these error codes for these conditions:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa path
argument points outside the process' allocated address space.
.It Bq Er ENAMETOOLONG
The entire pathname exceeds 1023 characters.
.It Bq Er ENOENT
The named shared memory object does not exist.
.It Bq Er EACCES
The required permissions are denied.
.Fn shm_unlink
requires write permission to the shared memory object.
.El
.Sh SEE ALSO
.Xr posixshmcontrol 1 ,
.Xr close 2 ,
.Xr fstat 2 ,
.Xr ftruncate 2 ,
.Xr mmap 2 ,
.Xr munmap 2 ,
.Xr sendfile 2
.Sh STANDARDS
The
.Fn memfd_create
function is expected to be compatible with the Linux system call of the same
name.
.Pp
The
.Fn shm_open
and
.Fn shm_unlink
functions are believed to conform to
.St -p1003.1b-93 .
.Sh HISTORY
The
.Fn memfd_create
function appeared in
.Fx 13.0 .
.Pp
The
.Fn shm_open
and
.Fn shm_unlink
functions first appeared in
.Fx 4.3 .
The functions were reimplemented as system calls using shared memory objects
directly rather than files in
.Fx 8.0 .
.Pp
.Fn shm_rename
first appeared in
.Fx 13.0
as a
.Fx
extension.
.Sh AUTHORS
.An Garrett A. Wollman Aq Mt wollman@FreeBSD.org
(C library support and this manual page)
.Pp
.An Matthew Dillon Aq Mt dillon@FreeBSD.org
.Pq Dv MAP_NOSYNC
.Pp
.An Matthew Bryan Aq Mt matthew.bryan@isilon.com
.Pq Dv shm_rename implementation