oracular (2) shmat.2.gz

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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);
           }

SEE ALSO

       brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), sysvipc(7)