mirror of https://github.com/mkerrisk/man-pages
515 lines
14 KiB
Groff
515 lines
14 KiB
Groff
.\" Copyright (C) 2002, 2020 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH SHM_OPEN 3 2021-03-22 "Linux" "Linux Programmer's Manual"
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.SH NAME
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shm_open, shm_unlink \- create/open or unlink POSIX shared memory objects
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.SH SYNOPSIS
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.nf
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.B #include <sys/mman.h>
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.BR "#include <sys/stat.h>" " /* For mode constants */"
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.BR "#include <fcntl.h>" " /* For O_* constants */"
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.PP
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.BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );
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.BI "int shm_unlink(const char *" name );
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.fi
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.PP
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Link with \fI\-lrt\fP.
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.SH DESCRIPTION
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.BR shm_open ()
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creates and opens a new, or opens an existing, POSIX shared memory object.
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A POSIX shared memory object is in effect a handle which can
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be used by unrelated processes to
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.BR mmap (2)
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the same region of shared memory.
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The
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.BR shm_unlink ()
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function performs the converse operation,
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removing an object previously created by
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.BR shm_open ().
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.PP
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The operation of
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.BR shm_open ()
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is analogous to that of
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.BR open (2).
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.I name
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specifies the shared memory object to be created or opened.
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For portable use,
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a shared memory object should be identified by a name of the form
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.IR /somename ;
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that is, a null-terminated string of up to
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.BI NAME_MAX
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(i.e., 255) characters consisting of an initial slash,
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.\" glibc allows the initial slash to be omitted, and makes
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.\" multiple initial slashes equivalent to a single slash.
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.\" This differs from the implementation of POSIX message queues.
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followed by one or more characters, none of which are slashes.
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.\" glibc allows subdirectory components in the name, in which
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.\" case the subdirectory must exist under /dev/shm, and allow the
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.\" required permissions if a user wants to create a shared memory
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.\" object in that subdirectory.
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.PP
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.I oflag
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is a bit mask created by ORing together exactly one of
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.B O_RDONLY
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or
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.B O_RDWR
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and any of the other flags listed here:
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.TP
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.B O_RDONLY
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Open the object for read access.
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A shared memory object opened in this way can be
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.BR mmap (2)ed
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only for read
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.RB ( PROT_READ )
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access.
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.TP
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.B O_RDWR
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Open the object for read-write access.
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.TP
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.B O_CREAT
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Create the shared memory object if it does not exist.
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The user and group ownership of the object are taken
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from the corresponding effective IDs of the calling process,
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.\" In truth it is actually the filesystem IDs on Linux, but these
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.\" are nearly always the same as the effective IDs. (MTK, Jul 05)
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and the object's
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permission bits are set according to the low-order 9 bits of
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.IR mode ,
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except that those bits set in the process file mode
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creation mask (see
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.BR umask (2))
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are cleared for the new object.
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A set of macro constants which can be used to define
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.I mode
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is listed in
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.BR open (2).
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(Symbolic definitions of these constants can be obtained by including
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.IR <sys/stat.h> .)
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.IP
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A new shared memory object initially has zero length\(emthe size of the
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object can be set using
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.BR ftruncate (2).
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The newly allocated bytes of a shared memory
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object are automatically initialized to 0.
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.TP
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.B O_EXCL
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If
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.B O_CREAT
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was also specified, and a shared memory object with the given
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.I name
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already exists, return an error.
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The check for the existence of the object, and its creation if it
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does not exist, are performed atomically.
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.TP
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.B O_TRUNC
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If the shared memory object already exists, truncate it to zero bytes.
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.PP
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Definitions of these flag values can be obtained by including
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.IR <fcntl.h> .
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.PP
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On successful completion
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.BR shm_open ()
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returns a new file descriptor referring to the shared memory object.
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This file descriptor is guaranteed to be the lowest-numbered file descriptor
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not previously opened within the process.
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The
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.B FD_CLOEXEC
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flag (see
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.BR fcntl (2))
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is set for the file descriptor.
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.PP
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The file descriptor is normally used in subsequent calls
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to
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.BR ftruncate (2)
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(for a newly created object) and
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.BR mmap (2).
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After a call to
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.BR mmap (2)
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the file descriptor may be closed without affecting the memory mapping.
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.PP
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The operation
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of
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.BR shm_unlink ()
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is analogous to
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.BR unlink (2):
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it removes a shared memory object name, and, once all processes
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have unmapped the object, deallocates and
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destroys the contents of the associated memory region.
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After a successful
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.BR shm_unlink (),
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attempts to
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.BR shm_open ()
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an object with the same
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.I name
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fail (unless
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.B O_CREAT
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was specified, in which case a new, distinct object is created).
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.SH RETURN VALUE
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On success,
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.BR shm_open ()
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returns a file descriptor (a nonnegative integer).
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On success,
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.BR shm_unlink ()
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returns 0.
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On failure, both functions return \-1 and set
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.I errno
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to indicate the error.
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.SH ERRORS
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.TP
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.B EACCES
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Permission to
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.BR shm_unlink ()
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the shared memory object was denied.
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.TP
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.B EACCES
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Permission was denied to
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.BR shm_open ()
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.I name
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in the specified
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.IR mode ,
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or
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.B O_TRUNC
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was specified and the caller does not have write permission on the object.
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.TP
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.B EEXIST
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Both
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.B O_CREAT
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and
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.B O_EXCL
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were specified to
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.BR shm_open ()
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and the shared memory object specified by
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.I name
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already exists.
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.TP
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.B EINVAL
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The
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.I name
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argument to
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.BR shm_open ()
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was invalid.
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.TP
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.B EMFILE
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The per-process limit on the number of open file descriptors has been reached.
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.TP
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.B ENAMETOOLONG
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The length of
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.I name
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exceeds
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.BR PATH_MAX .
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.TP
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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 ENOENT
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An attempt was made to
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.BR shm_open ()
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a
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.I name
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that did not exist, and
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.B O_CREAT
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was not specified.
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.TP
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.B ENOENT
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An attempt was to made to
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.BR shm_unlink ()
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a
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.I name
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that does not exist.
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.SH VERSIONS
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These functions are provided in glibc 2.2 and later.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.ad l
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.nh
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.TS
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allbox;
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lbx lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR shm_open (),
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.BR shm_unlink ()
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T} Thread safety MT-Safe locale
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.TE
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.hy
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.ad
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.sp 1
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008.
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.PP
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POSIX.1-2001 says that the group ownership of a newly created shared
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memory object is set to either the calling process's effective group ID
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or "a system default group ID".
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POSIX.1-2008 says that the group ownership
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may be set to either the calling process's effective group ID
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or, if the object is visible in the filesystem,
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the group ID of the parent directory.
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.SH NOTES
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POSIX leaves the behavior of the combination of
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.B O_RDONLY
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and
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.B O_TRUNC
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unspecified.
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On Linux, this will successfully truncate an existing
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shared memory object\(emthis may not be so on other UNIX systems.
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.PP
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The POSIX shared memory object implementation on Linux makes use
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of a dedicated
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.BR tmpfs (5)
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filesystem that is normally mounted under
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.IR /dev/shm .
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.SH EXAMPLES
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The programs below employ POSIX shared memory and POSIX unnamed semaphores
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to exchange a piece of data.
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The "bounce" program (which must be run first) raises the case
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of a string that is placed into the shared memory by the "send" program.
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Once the data has been modified, the "send" program then prints
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the contents of the modified shared memory.
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An example execution of the two programs is the following:
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.PP
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.in +4n
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.EX
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$ \fB./pshm_ucase_bounce /myshm &\fP
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[1] 270171
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$ \fB./pshm_ucase_send /myshm hello\fP
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HELLO
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.EE
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.in
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.PP
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Further detail about these programs is provided below.
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.\"
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.SS Program source: pshm_ucase.h
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The following header file is included by both programs below.
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Its primary purpose is to define a structure that will be imposed
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on the memory object that is shared between the two programs.
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.PP
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.in +4n
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.EX
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#include <sys/mman.h>
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#include <fcntl.h>
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#include <semaphore.h>
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#include <sys/stat.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
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} while (0)
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#define BUF_SIZE 1024 /* Maximum size for exchanged string */
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/* Define a structure that will be imposed on the shared
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memory object */
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struct shmbuf {
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sem_t sem1; /* POSIX unnamed semaphore */
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sem_t sem2; /* POSIX unnamed semaphore */
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size_t cnt; /* Number of bytes used in \(aqbuf\(aq */
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char buf[BUF_SIZE]; /* Data being transferred */
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};
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.EE
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.in
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.\"
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.SS Program source: pshm_ucase_bounce.c
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The "bounce" program creates a new shared memory object with the name
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given in its command-line argument and sizes the object to
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match the size of the
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.I shmbuf
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structure defined in the header file.
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It then maps the object into the process's address space,
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and initializes two POSIX semaphores inside the object to 0.
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.PP
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After the "send" program has posted the first of the semaphores,
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the "bounce" program upper cases the data that has been placed
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in the memory by the "send" program and then posts the second semaphore
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to tell the "send" program that it may now access the shared memory.
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.PP
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.in +4n
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.EX
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/* pshm_ucase_bounce.c
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Licensed under GNU General Public License v2 or later.
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*/
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#include <ctype.h>
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#include "pshm_ucase.h"
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int
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main(int argc, char *argv[])
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{
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if (argc != 2) {
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fprintf(stderr, "Usage: %s /shm\-path\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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char *shmpath = argv[1];
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/* Create shared memory object and set its size to the size
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of our structure. */
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int fd = shm_open(shmpath, O_CREAT | O_EXCL | O_RDWR,
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S_IRUSR | S_IWUSR);
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if (fd == \-1)
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errExit("shm_open");
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if (ftruncate(fd, sizeof(struct shmbuf)) == \-1)
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errExit("ftruncate");
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/* Map the object into the caller\(aqs address space. */
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struct shmbuf *shmp = mmap(NULL, sizeof(*shmp),
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PROT_READ | PROT_WRITE,
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MAP_SHARED, fd, 0);
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if (shmp == MAP_FAILED)
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errExit("mmap");
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/* Initialize semaphores as process\-shared, with value 0. */
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if (sem_init(&shmp\->sem1, 1, 0) == \-1)
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errExit("sem_init\-sem1");
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if (sem_init(&shmp\->sem2, 1, 0) == \-1)
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errExit("sem_init\-sem2");
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/* Wait for \(aqsem1\(aq to be posted by peer before touching
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shared memory. */
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if (sem_wait(&shmp\->sem1) == \-1)
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errExit("sem_wait");
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/* Convert data in shared memory into upper case. */
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for (int j = 0; j < shmp\->cnt; j++)
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shmp\->buf[j] = toupper((unsigned char) shmp\->buf[j]);
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/* Post \(aqsem2\(aq to tell the peer that it can now
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access the modified data in shared memory. */
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if (sem_post(&shmp\->sem2) == \-1)
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errExit("sem_post");
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/* Unlink the shared memory object. Even if the peer process
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is still using the object, this is okay. The object will
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be removed only after all open references are closed. */
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shm_unlink(shmpath);
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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.\"
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.SS Program source: pshm_ucase_send.c
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The "send" program takes two command-line arguments:
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the pathname of a shared memory object previously created by the "bounce"
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program and a string that is to be copied into that object.
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.PP
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The program opens the shared memory object
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and maps the object into its address space.
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It then copies the data specified in its second argument
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into the shared memory,
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and posts the first semaphore,
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which tells the "bounce" program that it can now access that data.
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After the "bounce" program posts the second semaphore,
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the "send" program prints the contents of the shared memory
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on standard output.
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.PP
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.in +4n
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.EX
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/* pshm_ucase_send.c
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Licensed under GNU General Public License v2 or later.
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*/
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#include <string.h>
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#include "pshm_ucase.h"
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int
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main(int argc, char *argv[])
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{
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if (argc != 3) {
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fprintf(stderr, "Usage: %s /shm\-path string\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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char *shmpath = argv[1];
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char *string = argv[2];
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size_t len = strlen(string);
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if (len > BUF_SIZE) {
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fprintf(stderr, "String is too long\en");
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exit(EXIT_FAILURE);
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}
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/* Open the existing shared memory object and map it
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into the caller\(aqs address space. */
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int fd = shm_open(shmpath, O_RDWR, 0);
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if (fd == \-1)
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errExit("shm_open");
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struct shmbuf *shmp = mmap(NULL, sizeof(*shmp),
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PROT_READ | PROT_WRITE,
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MAP_SHARED, fd, 0);
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if (shmp == MAP_FAILED)
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errExit("mmap");
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/* Copy data into the shared memory object. */
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shmp\->cnt = len;
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memcpy(&shmp\->buf, string, len);
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/* Tell peer that it can now access shared memory. */
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if (sem_post(&shmp\->sem1) == \-1)
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errExit("sem_post");
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/* Wait until peer says that it has finished accessing
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the shared memory. */
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if (sem_wait(&shmp\->sem2) == \-1)
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errExit("sem_wait");
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/* Write modified data in shared memory to standard output. */
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write(STDOUT_FILENO, &shmp\->buf, len);
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write(STDOUT_FILENO, "\en", 1);
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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|
.SH SEE ALSO
|
|
.BR close (2),
|
|
.BR fchmod (2),
|
|
.BR fchown (2),
|
|
.BR fcntl (2),
|
|
.BR fstat (2),
|
|
.BR ftruncate (2),
|
|
.BR memfd_create (2),
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.BR mmap (2),
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|
.BR open (2),
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|
.BR umask (2),
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|
.BR shm_overview (7)
|