Update the shm_open.2 man page to reflect objective reality.
PR: 215612 Submitted by: rwatson MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D9066
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@ -28,7 +28,7 @@
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 18, 2013
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.Dd January 13, 2017
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.Dt SHM_OPEN 2
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.Os
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.Sh NAME
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@ -171,7 +171,8 @@ and
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.Dv O_TRUNC
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flags may be used in portable programs.
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.Pp
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The result of using
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.Tn POSIX
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specifications state that the result of using
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.Xr open 2 ,
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.Xr read 2 ,
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or
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@ -179,19 +180,43 @@ or
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on a shared memory object, or on the descriptor returned by
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.Fn shm_open ,
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is undefined.
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It is also undefined whether the shared memory object itself, or its
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contents, persist across reboots.
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.Pp
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In FreeBSD,
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However, the
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.Fx
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kernel implementation explicitly includes support for
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.Xr read 2
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and
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.Xr write 2
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on a shared memory object will fail with
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.Er EOPNOTSUPP
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and neither shared memory objects nor their contents persist across reboots.
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.Xr write 2 .
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.Pp
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Neither shared memory objects nor their contents persist across reboots.
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.Pp
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Writes do not extend shared memory objects, so
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.Xr ftruncate 2
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must be called before any data can be written.
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See
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.Sx EXAMPLES .
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.Sh EXAMPLES
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This example fails without the call to
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.Xr ftruncate 2 :
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.Bd -literal -compact
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uint8_t buffer[getpagesize()];
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ssize_t len;
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int fd;
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fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
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if (fd < 0)
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err(EX_OSERR, "%s: shm_open", __func__);
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if (ftruncate(fd, getpagesize()) < 0)
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err(EX_IOERR, "%s: ftruncate", __func__);
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len = pwrite(fd, buffer, getpagesize(), 0);
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if (len < 0)
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err(EX_IOERR, "%s: pwrite", __func__);
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if (len != getpagesize())
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errx(EX_IOERR, "%s: pwrite length mismatch", __func__);
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.Ed
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.Sh ERRORS
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The following errors are defined for
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.Fn shm_open :
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.Fn shm_open
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fails with these error codes for these conditions:
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.Bl -tag -width Er
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.It Bq Er EINVAL
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A flag other than
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@ -235,8 +260,8 @@ are specified and the named shared memory object does exist.
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The required permissions (for reading or reading and writing) are denied.
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.El
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.Pp
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The following errors are defined for
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.Fn shm_unlink :
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.Fn shm_unlink
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fails with these error codes for these conditions:
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.Bl -tag -width Er
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.It Bq Er EFAULT
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The
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