shmop.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright 2020 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu)
.\"          with material from Luigi P. Bai (lpb@softint.com)
.\" Portions Copyright 1993 Luigi P. Bai
.\" Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified, 5 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified, 19 Sep 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added SHM_REMAP flag description
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Changed wording and placement of sentence regarding attachment
.\"		of segments marked for destruction
.\"
.TH SHMOP 2 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
shmat, shmdt \- System V shared memory operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/shm.h>
.PP
.BI "void *shmat(int " shmid ", const void *" shmaddr ", int " shmflg );
.BI "int shmdt(const void *" shmaddr );
.fi
.SH DESCRIPTION
.SS shmat()
.BR shmat ()
attaches the System\ V shared memory segment identified by
.I shmid
to the address space of the calling process.
The attaching address is specified by
.I shmaddr
with one of the following criteria:
.IP \(bu 2
If
.I shmaddr
is NULL,
the system chooses a suitable (unused) page-aligned address to attach
the segment.
.IP \(bu
If
.I shmaddr
isn't NULL
and
.B SHM_RND
is specified in
.IR shmflg ,
the attach occurs at the address equal to
.I shmaddr
rounded down to the nearest multiple of
.BR SHMLBA .
.IP \(bu
Otherwise,
.I shmaddr
must be a page-aligned address at which the attach occurs.
.PP
In addition to
.BR SHM_RND ,
the following flags may be specified in the
.I shmflg
bit-mask argument:
.TP
.BR SHM_EXEC " (Linux-specific; since Linux 2.6.9)"
Allow the contents of the segment to be executed.
The caller must have execute permission on the segment.
.TP
.BR SHM_RDONLY
Attach the segment for read-only access.
The process must have read permission for the segment.
If this flag is not specified,
the segment is attached for read and write access,
and the process must have read and write permission for the segment.
There is no notion of a write-only shared memory segment.
.TP
.BR SHM_REMAP " (Linux-specific)"
This flag specifies
that the mapping of the segment should replace
any existing mapping in the range starting at
.I shmaddr
and continuing for the size of the segment.
(Normally, an
.B EINVAL
error would result if a mapping already exists in this address range.)
In this case,
.I shmaddr
must not be NULL.
.PP
The
.BR brk (2)
value of the calling process is not altered by the attach.
The segment will automatically be detached at process exit.
The same segment may be attached as a read and as a read-write
one, and more than once, in the process's address space.
.PP
A successful
.BR shmat ()
call updates the members of the
.I shmid_ds
structure (see
.BR shmctl (2))
associated with the shared memory segment as follows:
.IP \(bu 2
.I shm_atime
is set to the current time.
.IP \(bu
.I shm_lpid
is set to the process-ID of the calling process.
.IP \(bu
.I shm_nattch
is incremented by one.
.\"
.SS shmdt()
.BR shmdt ()
detaches the shared memory segment located at the address specified by
.I shmaddr
from the address space of the calling process.
The to-be-detached segment must be currently
attached with
.I shmaddr
equal to the value returned by the attaching
.BR shmat ()
call.
.PP
On a successful
.BR shmdt ()
call, the system updates the members of the
.I shmid_ds
structure associated with the shared memory segment as follows:
.IP \(bu 2
.I shm_dtime
is set to the current time.
.IP \(bu
.I shm_lpid
is set to the process-ID of the calling process.
.IP \(bu
.I shm_nattch
is decremented by one.
If it becomes 0 and the segment is marked for deletion,
the segment is deleted.
.SH RETURN VALUE
On success,
.BR shmat ()
returns the address of the attached shared memory segment; on error,
.I (void\ *)\ \-1
is returned, and
.I errno
is set to indicate the error.
.PP
On success,
.BR shmdt ()
returns 0; on error \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.BR shmat ()
can fail with one of the following errors:
.TP
.B EACCES
The calling process does not have the required permissions for
the requested attach type, and does not have the
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EIDRM
\fIshmid\fP points to a removed identifier.
.TP
.B EINVAL
Invalid
.I shmid
value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not
specified) or invalid
.I shmaddr
value, or can't attach segment at
.IR shmaddr ,
or
.B SHM_REMAP
was specified and
.I shmaddr
was NULL.
.TP
.B ENOMEM
Could not allocate memory for the descriptor or for the page tables.
.PP
.BR shmdt ()
.I errno
can fail with one of the following errors:
.TP
.B EINVAL
There is no shared memory segment attached at
.IR shmaddr ;
or,
.\" The following since 2.6.17-rc1:
.I shmaddr
is not aligned on a page boundary.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
.\" SVr4 documents an additional error condition EMFILE.
.PP
In SVID 3 (or perhaps earlier),
the type of the \fIshmaddr\fP argument was changed from
.I "char\ *"
into
.IR "const void\ *" ,
and the returned type of
.BR shmat ()
from
.I "char\ *"
into
.IR "void\ *" .
.SH NOTES
After a
.BR fork (2),
the child inherits the attached shared memory segments.
.PP
After an
.BR execve (2),
all attached shared memory segments are detached from the process.
.PP
Upon
.BR _exit (2),
all attached shared memory segments are detached from the process.
.PP
Using
.BR shmat ()
with
.I shmaddr
equal to NULL
is the preferred, portable way of attaching a shared memory segment.
Be aware that the shared memory segment attached in this way
may be attached at different addresses in different processes.
Therefore, any pointers maintained within the shared memory must be
made relative (typically to the starting address of the segment),
rather than absolute.
.PP
On Linux, it is possible to attach a shared memory segment even if it
is already marked to be deleted.
However, POSIX.1 does not specify this behavior and
many other implementations do not support it.
.PP
The following system parameter affects
.BR shmat ():
.TP
.B SHMLBA
Segment low boundary address multiple.
When explicitly specifying an attach address in a call to
.BR shmat (),
the caller should ensure that the address is a multiple of this value.
This is necessary on some architectures,
in order either to ensure good CPU cache performance or to ensure that
different attaches of the same segment have consistent views
within the CPU cache.
.B SHMLBA
is normally some multiple of the system page size.
(On many Linux architectures,
.B SHMLBA
is the same as the system page size.)
.PP
The implementation places no intrinsic per-process limit on the
number of shared memory segments
.RB ( SHMSEG ).
.SH EXAMPLES
The two programs shown below exchange a string using a shared memory segment.
Further details about the programs are given below.
First, we show a shell session demonstrating their use.
.PP
In one terminal window, we run the "reader" program,
which creates a System V shared memory segment and a System V semaphore set.
The program prints out the IDs of the created objects,
and then waits for the semaphore to change value.
.PP
.in +4n
.EX
$ \fB./svshm_string_read\fP
shmid = 1114194; semid = 15
.EE
.in
.PP
In another terminal window, we run the "writer" program.
The "writer" program takes three command-line arguments:
the IDs of the shared memory segment and semaphore set created
by the "reader", and a string.
It attaches the existing shared memory segment,
copies the string to the shared memory, and modifies the semaphore value.
.PP
.in +4n
.EX
$ \fB./svshm_string_write 1114194 15 \(aqHello, world\(aq\fP
.EE
.in
.PP
Returning to the terminal where the "reader" is running,
we see that the program has ceased waiting on the semaphore
and has printed the string that was copied into the
shared memory segment by the writer:
.PP
.in +4n
.EX
Hello, world
.EE
.in
.\"
.SS Program source: svshm_string.h
The following header file is included by the "reader" and "writer" programs.
.PP
.in +4n
.EX
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

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

union semun {                   /* Used in calls to semctl() */
    int                 val;
    struct semid_ds *   buf;
    unsigned short *    array;
#if defined(__linux__)
    struct seminfo *    __buf;
#endif
};

#define MEM_SIZE 4096
.EE
.in
.\"
.SS Program source: svshm_string_read.c
The "reader" program creates a shared memory segment and a semaphore set
containing one semaphore.
It then attaches the shared memory object into its address space
and initializes the semaphore value to 1.
Finally, the program waits for the semaphore value to become 0,
and afterwards prints the string that has been copied into the
shared memory segment by the "writer".
.PP
.in +4n
.EX
/* svshm_string_read.c

   Licensed under GNU General Public License v2 or later.
*/
#include "svshm_string.h"

int
main(int argc, char *argv[])
{
    int semid, shmid;
    union semun arg, dummy;
    struct sembuf sop;
    char *addr;

    /* Create shared memory and semaphore set containing one
       semaphore. */

    shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
    if (shmid == \-1)
        errExit("shmget");

    semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
    if (shmid == \-1)
        errExit("shmget");

    /* Attach shared memory into our address space. */

    addr = shmat(shmid, NULL, SHM_RDONLY);
    if (addr == (void *) \-1)
        errExit("shmat");

    /* Initialize semaphore 0 in set with value 1. */

    arg.val = 1;
    if (semctl(semid, 0, SETVAL, arg) == \-1)
        errExit("semctl");

    printf("shmid = %d; semid = %d\en", shmid, semid);

    /* Wait for semaphore value to become 0. */

    sop.sem_num = 0;
    sop.sem_op = 0;
    sop.sem_flg = 0;

    if (semop(semid, &sop, 1) == \-1)
        errExit("semop");

    /* Print the string from shared memory. */

    printf("%s\en", addr);

    /* Remove shared memory and semaphore set. */

    if (shmctl(shmid, IPC_RMID, NULL) == \-1)
        errExit("shmctl");
    if (semctl(semid, 0, IPC_RMID, dummy) == \-1)
        errExit("semctl");

    exit(EXIT_SUCCESS);
}
.EE
.in
.\"
.SS Program source: svshm_string_write.c
The writer program takes three command-line arguments:
the IDs of the shared memory segment and semaphore set
that have already been created by the "reader", and a string.
It attaches the shared memory segment into its address space,
and then decrements the semaphore value to 0 in order to inform the
"reader" that it can now examine the contents of the shared memory.
.PP
.in +4n
.EX
/* svshm_string_write.c

   Licensed under GNU General Public License v2 or later.
*/
#include "svshm_string.h"

int
main(int argc, char *argv[])
{
    int semid, shmid;
    struct sembuf sop;
    char *addr;
    size_t len;

    if (argc != 4) {
        fprintf(stderr, "Usage: %s shmid semid string\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    len = strlen(argv[3]) + 1;  /* +1 to include trailing \(aq\e0\(aq */
    if (len > MEM_SIZE) {
        fprintf(stderr, "String is too big!\en");
        exit(EXIT_FAILURE);
    }

    /* Get object IDs from command\-line. */

    shmid = atoi(argv[1]);
    semid = atoi(argv[2]);

    /* Attach shared memory into our address space and copy string
       (including trailing null byte) into memory. */

    addr = shmat(shmid, NULL, 0);
    if (addr == (void *) \-1)
        errExit("shmat");

    memcpy(addr, argv[3], len);

    /* Decrement semaphore to 0. */

    sop.sem_num = 0;
    sop.sem_op = \-1;
    sop.sem_flg = 0;

    if (semop(semid, &sop, 1) == \-1)
        errExit("semop");

    exit(EXIT_SUCCESS);
}
.EE
.in
.SH SEE ALSO
.BR brk (2),
.BR mmap (2),
.BR shmctl (2),
.BR shmget (2),
.BR capabilities (7),
.BR shm_overview (7),
.BR sysvipc (7)