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

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.Dd January 13, 2017
.Dt SHM_OPEN 2
.Os
.Sh NAME
.Nm shm_open , 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 shm_open "const char *path" "int flags" "mode_t mode"
.Ft int
.Fn shm_unlink "const char *path"
.Sh DESCRIPTION
The
.Fn shm_open
system call opens (or optionally creates) a
.Tn 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 FreeBSD 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 .
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_unlink
system call removes a shared memory object named
.Fa path .
.Sh RETURN VALUES
If successful,
.Fn shm_open
returns a non-negative integer,
and
.Fn shm_unlink
returns zero.
Both functions return -1 on failure, and set
.Va errno
to indicate the error.
.Sh COMPATIBILITY
The
.Fa path
argument does not necessarily represent a pathname (although it does 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
.Tn 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
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 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 exceeded 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 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
.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 exceeded 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 close 2 ,
.Xr fstat 2 ,
.Xr ftruncate 2 ,
.Xr mmap 2 ,
.Xr munmap 2
.Sh STANDARDS
The
.Fn shm_open
and
.Fn shm_unlink
functions are believed to conform to
.St -p1003.1b-93 .
.Sh HISTORY
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 .
.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