Ubuntu Manpage: shmat, shmdt - System V shared memory operations

NAME

       shmat, shmdt - System V shared memory operations

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/shm.h>

       void *shmat(int shmid, const void *_Nullable shmaddr, int shmflg);
       int shmdt(const void *shmaddr);

DESCRIPTION

shmat()
shmat() attaches the System V shared memory segment identified by shmid to the address
space of the calling process. The attaching address is specified by shmaddr with one of
the following criteria:

• If shmaddr is NULL, the system chooses a suitable (unused) page-aligned address to
attach the segment.

• If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach occurs at the
address equal to shmaddr rounded down to the nearest multiple of SHMLBA.

• Otherwise, shmaddr must be a page-aligned address at which the attach occurs.

In addition to SHM_RND, the following flags may be specified in the shmflg bit-mask
argument:

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.

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.

SHM_REMAP (Linux-specific)
This flag specifies that the mapping of the segment should replace any existing
mapping in the range starting at shmaddr and continuing for the size of the
segment. (Normally, an EINVAL error would result if a mapping already exists in
this address range.) In this case, shmaddr must not be NULL.

The 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.

A successful shmat() call updates the members of the shmid_ds structure (see shmctl(2))
associated with the shared memory segment as follows:

• shm_atime is set to the current time.

• shm_lpid is set to the process-ID of the calling process.

• shm_nattch is incremented by one.

shmdt()
shmdt() detaches the shared memory segment located at the address specified by shmaddr
from the address space of the calling process. The to-be-detached segment must be
currently attached with shmaddr equal to the value returned by the attaching shmat() call.

On a successful shmdt() call, the system updates the members of the shmid_ds structure
associated with the shared memory segment as follows:

• shm_dtime is set to the current time.

• shm_lpid is set to the process-ID of the calling process.

• shm_nattch is decremented by one. If it becomes 0 and the segment is marked for
deletion, the segment is deleted.

RETURN VALUE

       On success, shmat() returns the address of the attached shared memory segment;  on  error,
       (void *) -1 is returned, and errno is set to indicate the error.

       On  success,  shmdt() returns 0; on error -1 is returned, and errno is set to indicate the
       error.

ERRORS

       shmat() can fail with one of the following errors:

       EACCES The calling process does not have the required permissions for the requested attach
              type,  and  does  not  have the CAP_IPC_OWNER capability in the user namespace that
              governs its IPC namespace.

       EIDRM  shmid points to a removed identifier.

       EINVAL Invalid shmid  value,  unaligned  (i.e.,  not  page-aligned  and  SHM_RND  was  not
              specified)  or  invalid  shmaddr  value,  or  can't  attach  segment at shmaddr, or
              SHM_REMAP was specified and shmaddr was NULL.

       ENOMEM Could not allocate memory for the descriptor or for the page tables.

       shmdt() can fail with one of the following errors:

       EINVAL There is no shared memory segment attached at shmaddr; or, shmaddr is  not  aligned
              on a page boundary.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, SVr4.

       In  SVID  3 (or perhaps earlier), the type of the shmaddr argument was changed from char *
       into const void *, and the returned type of shmat() from char * into void *.

NOTES

After a fork(2), the child inherits the attached shared memory segments.

After an execve(2), all attached shared memory segments are detached from the process.

Upon _exit(2), all attached shared memory segments are detached from the process.

Using shmat() with 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.

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.

The following system parameter affects shmat():

SHMLBA Segment low boundary address multiple. When explicitly specifying an attach
address in a call to 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. SHMLBA is normally some
multiple of the system page size. (On many Linux architectures, SHMLBA is the same
as the system page size.)

The implementation places no intrinsic per-process limit on the number of shared memory
segments (SHMSEG).

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.

       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.

           $ ./svshm_string_read
           shmid = 1114194; semid = 15

       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.

           $ ./svshm_string_write 1114194 15 'Hello, world'

       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:

           Hello, world

   Program source: svshm_string.h
       The following header file is included by the "reader" and "writer" programs:

           /* svshm_string.h

              Licensed under GNU General Public License v2 or later.
           */
           #ifndef SVSHM_STRING_H
           #define SVSHM_STRING_H

           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/sem.h>

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

           #endif  // include guard

   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".

           /* svshm_string_read.c

              Licensed under GNU General Public License v2 or later.
           */
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/ipc.h>
           #include <sys/sem.h>
           #include <sys/shm.h>

           #include "svshm_string.h"

           int
           main(void)
           {
               int            semid, shmid;
               char           *addr;
               union semun    arg, dummy;
               struct sembuf  sop;

               /* 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 (semid == -1)
                   errExit("semget");

               /* 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\n", 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\n", 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);
           }

   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.

           /* svshm_string_write.c

              Licensed under GNU General Public License v2 or later.
           */
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/sem.h>
           #include <sys/shm.h>

           #include "svshm_string.h"

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

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

               len = strlen(argv[3]) + 1;  /* +1 to include trailing '\0' */
               if (len > MEM_SIZE) {
                   fprintf(stderr, "String is too big!\n");
                   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);
           }