# shmop

## 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.
    


## 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         size;
                   struct sembuf  sop;
    
                   if (argc != 4) {
                       fprintf(stderr, "Usage: %s shmid semid string\n", argv[0]);
                       exit(EXIT_FAILURE);
                   }
    
                   size = strlen(argv[3]) + 1;  /* +1 to include trailing '\0' */
                   if (size > 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], size);
    
                   /* 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);
               }
    


* * *