man-pages/man3/shm_open.3

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.TH SHM_OPEN 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
shm_open, shm_unlink \- create/open or unlink POSIX shared memory objects
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.BR "#include <sys/stat.h>" "        /* For mode constants */"
.BR "#include <fcntl.h>" "           /* For O_* constants */"
.PP
.BI "int shm_open(const char *" name ", int " oflag ", mode_t " mode );
.BI "int shm_unlink(const char *" name );
.fi
.PP
Link with \fI\-lrt\fP.
.SH DESCRIPTION
.BR shm_open ()
creates and opens a new, or opens an existing, POSIX shared memory object.
A POSIX shared memory object is in effect a handle which can
be used by unrelated processes to
.BR mmap (2)
the same region of shared memory.
The
.BR shm_unlink ()
function performs the converse operation,
removing an object previously created by
.BR shm_open ().
.PP
The operation of
.BR shm_open ()
is analogous to that of
.BR open (2).
.I name
specifies the shared memory object to be created or opened.
For portable use,
a shared memory object should be identified by a name of the form
.IR /somename ;
that is, a null-terminated string of up to
.BI NAME_MAX
(i.e., 255) characters consisting of an initial slash,
.\" glibc allows the initial slash to be omitted, and makes
.\" multiple initial slashes equivalent to a single slash.
.\" This differs from the implementation of POSIX message queues.
followed by one or more characters, none of which are slashes.
.\" glibc allows subdirectory components in the name, in which
.\" case the subdirectory must exist under /dev/shm, and allow the
.\" required permissions if a user wants to create a shared memory
.\" object in that subdirectory.
.PP
.I oflag
is a bit mask created by ORing together exactly one of
.B O_RDONLY
or
.B O_RDWR
and any of the other flags listed here:
.TP
.B O_RDONLY
Open the object for read access.
A shared memory object opened in this way can be
.BR mmap (2)ed
only for read
.RB ( PROT_READ )
access.
.TP
.B O_RDWR
Open the object for read-write access.
.TP
.B O_CREAT
Create the shared memory object if it does not exist.
The user and group ownership of the object are taken
from the corresponding effective IDs of the calling process,
.\" In truth it is actually the filesystem IDs on Linux, but these
.\" are nearly always the same as the effective IDs.  (MTK, Jul 05)
and the object's
permission bits are set according to the low-order 9 bits of
.IR mode ,
except that those bits set in the process file mode
creation mask (see
.BR umask (2))
are cleared for the new object.
A set of macro constants which can be used to define
.I mode
is listed in
.BR open (2).
(Symbolic definitions of these constants can be obtained by including
.IR <sys/stat.h> .)
.IP
A new shared memory object initially has zero length\(emthe size of the
object can be set using
.BR ftruncate (2).
The newly allocated bytes of a shared memory
object are automatically initialized to 0.
.TP
.B O_EXCL
If
.B O_CREAT
was also specified, and a shared memory object with the given
.I name
already exists, return an error.
The check for the existence of the object, and its creation if it
does not exist, are performed atomically.
.TP
.B O_TRUNC
If the shared memory object already exists, truncate it to zero bytes.
.PP
Definitions of these flag values can be obtained by including
.IR <fcntl.h> .
.PP
On successful completion
.BR shm_open ()
returns a new file descriptor referring to the shared memory object.
This file descriptor is guaranteed to be the lowest-numbered file descriptor
not previously opened within the process.
The
.B FD_CLOEXEC
flag (see
.BR fcntl (2))
is set for the file descriptor.
.PP
The file descriptor is normally used in subsequent calls
to
.BR ftruncate (2)
(for a newly created object) and
.BR mmap (2).
After a call to
.BR mmap (2)
the file descriptor may be closed without affecting the memory mapping.
.PP
The operation
of
.BR shm_unlink ()
is analogous to
.BR unlink (2):
it removes a shared memory object name, and, once all processes
have unmapped the object, deallocates and
destroys the contents of the associated memory region.
After a successful
.BR shm_unlink (),
attempts to
.BR shm_open ()
an object with the same
.I name
fail (unless
.B O_CREAT
was specified, in which case a new, distinct object is created).
.SH RETURN VALUE
On success,
.BR shm_open ()
returns a file descriptor (a nonnegative integer).
On success,
.BR shm_unlink ()
returns 0.
On failure, both functions return \-1 and set
.I errno
to indicate the error.
.SH ERRORS
.TP
.B EACCES
Permission to
.BR shm_unlink ()
the shared memory object was denied.
.TP
.B EACCES
Permission was denied to
.BR shm_open ()
.I name
in the specified
.IR mode ,
or
.B O_TRUNC
was specified and the caller does not have write permission on the object.
.TP
.B EEXIST
Both
.B O_CREAT
and
.B O_EXCL
were specified to
.BR shm_open ()
and the shared memory object specified by
.I name
already exists.
.TP
.B EINVAL
The
.I name
argument to
.BR shm_open ()
was invalid.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENAMETOOLONG
The length of
.I name
exceeds
.BR PATH_MAX .
.TP
.B ENFILE
The system-wide limit on the total number of open files has been reached.
.TP
.B ENOENT
An attempt was made to
.BR shm_open ()
a
.I name
that did not exist, and
.B O_CREAT
was not specified.
.TP
.B ENOENT
An attempt was to made to
.BR shm_unlink ()
a
.I name
that does not exist.
.SH VERSIONS
These functions are provided in glibc 2.2 and later.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR shm_open (),
.BR shm_unlink ()
T}	Thread safety	MT-Safe locale
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.PP
POSIX.1-2001 says that the group ownership of a newly created shared
memory object is set to either the calling process's effective group ID
or "a system default group ID".
POSIX.1-2008 says that the group ownership
may be set to either the calling process's effective group ID
or, if the object is visible in the filesystem,
the group ID of the parent directory.
.SH NOTES
POSIX leaves the behavior of the combination of
.B O_RDONLY
and
.B O_TRUNC
unspecified.
On Linux, this will successfully truncate an existing
shared memory object\(emthis may not be so on other UNIX systems.
.PP
The POSIX shared memory object implementation on Linux makes use
of a dedicated
.BR tmpfs (5)
filesystem that is normally mounted under
.IR /dev/shm .
.SH EXAMPLES
The programs below employ POSIX shared memory and POSIX unnamed semaphores
to exchange a piece of data.
The "bounce" program (which must be run first) raises the case
of a string that is placed into the shared memory by the "send" program.
Once the data has been modified, the "send" program then prints
the contents of the modified shared memory.
An example execution of the two programs is the following:
.PP
.in +4n
.EX
$ \fB./pshm_ucase_bounce /myshm &\fP
[1] 270171
$ \fB./pshm_ucase_send /myshm hello\fP
HELLO
.EE
.in
.PP
Further detail about these programs is provided below.
.\"
.SS Program source: pshm_ucase.h
The following header file is included by both programs below.
Its primary purpose is to define a structure that will be imposed
on the memory object that is shared between the two programs.
.PP
.in +4n
.EX
#include <sys/mman.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
                        } while (0)

#define BUF_SIZE 1024   /* Maximum size for exchanged string */

/* Define a structure that will be imposed on the shared
   memory object */

struct shmbuf {
    sem_t  sem1;            /* POSIX unnamed semaphore */
    sem_t  sem2;            /* POSIX unnamed semaphore */
    size_t cnt;             /* Number of bytes used in \(aqbuf\(aq */
    char   buf[BUF_SIZE];   /* Data being transferred */
};
.EE
.in
.\"
.SS Program source: pshm_ucase_bounce.c
The "bounce" program creates a new shared memory object with the name
given in its command-line argument and sizes the object to
match the size of the
.I shmbuf
structure defined in the header file.
It then maps the object into the process's address space,
and initializes two POSIX semaphores inside the object to 0.
.PP
After the "send" program has posted the first of the semaphores,
the "bounce" program upper cases the data that has been placed
in the memory by the "send" program and then posts the second semaphore
to tell the "send" program that it may now access the shared memory.
.PP
.in +4n
.EX
/* pshm_ucase_bounce.c

   Licensed under GNU General Public License v2 or later.
*/
#include <ctype.h>
#include "pshm_ucase.h"

int
main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s /shm\-path\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    char *shmpath = argv[1];

    /* Create shared memory object and set its size to the size
       of our structure. */

    int fd = shm_open(shmpath, O_CREAT | O_EXCL | O_RDWR,
                      S_IRUSR | S_IWUSR);
    if (fd == \-1)
        errExit("shm_open");

    if (ftruncate(fd, sizeof(struct shmbuf)) == \-1)
        errExit("ftruncate");

    /* Map the object into the caller\(aqs address space. */

    struct shmbuf *shmp = mmap(NULL, sizeof(*shmp),
                               PROT_READ | PROT_WRITE,
                               MAP_SHARED, fd, 0);
    if (shmp == MAP_FAILED)
        errExit("mmap");

    /* Initialize semaphores as process\-shared, with value 0. */

    if (sem_init(&shmp\->sem1, 1, 0) == \-1)
        errExit("sem_init\-sem1");
    if (sem_init(&shmp\->sem2, 1, 0) == \-1)
        errExit("sem_init\-sem2");

    /* Wait for \(aqsem1\(aq to be posted by peer before touching
       shared memory. */

    if (sem_wait(&shmp\->sem1) == \-1)
        errExit("sem_wait");

    /* Convert data in shared memory into upper case. */

    for (int j = 0; j < shmp\->cnt; j++)
        shmp\->buf[j] = toupper((unsigned char) shmp\->buf[j]);

    /* Post \(aqsem2\(aq to tell the peer that it can now
       access the modified data in shared memory. */

    if (sem_post(&shmp\->sem2) == \-1)
        errExit("sem_post");

    /* Unlink the shared memory object. Even if the peer process
       is still using the object, this is okay. The object will
       be removed only after all open references are closed. */

    shm_unlink(shmpath);

    exit(EXIT_SUCCESS);
}
.EE
.in
.\"
.SS Program source: pshm_ucase_send.c
The "send" program takes two command-line arguments:
the pathname of a shared memory object previously created by the "bounce"
program and a string that is to be copied into that object.
.PP
The program opens the shared memory object
and maps the object into its address space.
It then copies the data specified in its second argument
into the shared memory,
and posts the first semaphore,
which tells the "bounce" program that it can now access that data.
After the "bounce" program posts the second semaphore,
the "send" program prints the contents of the shared memory
on standard output.
.PP
.in +4n
.EX
/* pshm_ucase_send.c

   Licensed under GNU General Public License v2 or later.
*/
#include <string.h>
#include "pshm_ucase.h"

int
main(int argc, char *argv[])
{
    if (argc != 3) {
        fprintf(stderr, "Usage: %s /shm\-path string\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    char *shmpath = argv[1];
    char *string = argv[2];
    size_t len = strlen(string);

    if (len > BUF_SIZE) {
        fprintf(stderr, "String is too long\en");
        exit(EXIT_FAILURE);
    }

    /* Open the existing shared memory object and map it
       into the caller\(aqs address space. */

    int fd = shm_open(shmpath, O_RDWR, 0);
    if (fd == \-1)
        errExit("shm_open");

    struct shmbuf *shmp = mmap(NULL, sizeof(*shmp),
                               PROT_READ | PROT_WRITE,
                               MAP_SHARED, fd, 0);
    if (shmp == MAP_FAILED)
        errExit("mmap");

    /* Copy data into the shared memory object. */

    shmp\->cnt = len;
    memcpy(&shmp\->buf, string, len);

    /* Tell peer that it can now access shared memory. */

    if (sem_post(&shmp\->sem1) == \-1)
        errExit("sem_post");

    /* Wait until peer says that it has finished accessing
       the shared memory. */

    if (sem_wait(&shmp\->sem2) == \-1)
        errExit("sem_wait");

    /* Write modified data in shared memory to standard output. */

    write(STDOUT_FILENO, &shmp\->buf, len);
    write(STDOUT_FILENO, "\en", 1);

    exit(EXIT_SUCCESS);
}
.EE
.in
.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),
.BR mmap (2),
.BR open (2),
.BR umask (2),
.BR shm_overview (7)