# shm_open

## NAME



           shm_open, shm_unlink - create/open or unlink POSIX shared memory
           objects
    


## LIBRARY



           Real-time library (_librt_ , _-lrt_)
    


## SYNOPSIS



           **#include <sys/mman.h>**
           **#include <sys/stat.h>        **/* For mode constants */
           **#include <fcntl.h>           **/* For O_* constants */
    
           **int shm_open(const char ***_name_**, int** _oflag_**, mode_t** _mode_**);**
           **int shm_unlink(const char ***_name_**);**
    


## DESCRIPTION



           **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 mmap(2) the
           same region of shared memory.  The **shm_unlink**() function performs
           the converse operation, removing an object previously created by
           **shm_open**().
    
           The operation of **shm_open**() is analogous to that of open(2).  _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 _/somename_ ; that is, a null-terminated string of
           up to **NAME_MAX**(i.e., 255) characters consisting of an initial
           slash, followed by one or more characters, none of which are
           slashes.
    
           _oflag_ is a bit mask created by ORing together exactly one of
           **O_RDONLY** or **O_RDWR** and any of the other flags listed here:
    
           **O_RDONLY**
                  Open the object for read access.  A shared memory object
                  opened in this way can be mmap(2)ed only for read
                  (**PROT_READ**) access.
    
           **O_RDWR** Open the object for read-write access.
    
           **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, and the
                  object's permission bits are set according to the low-order
                  9 bits of _mode_ , except that those bits set in the process
                  file mode creation mask (see umask(2)) are cleared for the
                  new object.  A set of macro constants which can be used to
                  define _mode_ is listed in open(2).  (Symbolic definitions of
                  these constants can be obtained by including _< sys/stat.h>_.)
    
                  A new shared memory object initially has zero length—the
                  size of the object can be set using ftruncate(2).  The
                  newly allocated bytes of a shared memory object are
                  automatically initialized to 0.
    
           **O_EXCL** If **O_CREAT** was also specified, and a shared memory object
                  with the given _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.
    
           **O_TRUNC**
                  If the shared memory object already exists, truncate it to
                  zero bytes.
    
           Definitions of these flag values can be obtained by including
           _< fcntl.h>_.
    
           On successful completion **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 **FD_CLOEXEC** flag (see
           fcntl(2)) is set for the file descriptor.
    
           The file descriptor is normally used in subsequent calls to
           ftruncate(2) (for a newly created object) and mmap(2).  After a
           call to mmap(2) the file descriptor may be closed without
           affecting the memory mapping.
    
           The operation of **shm_unlink**() is analogous to 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 **shm_unlink**(),
           attempts to **shm_open**() an object with the same _name_ fail (unless
           **O_CREAT** was specified, in which case a new, distinct object is
           created).
    


## RETURN VALUE



           On success, **shm_open**() returns a file descriptor (a nonnegative
           integer).  On success, **shm_unlink**() returns 0.  On failure, both
           functions return -1 and set _errno_ to indicate the error.
    


## ERRORS



           **EACCES** Permission to **shm_unlink**() the shared memory object was
                  denied.
    
           **EACCES** Permission was denied to **shm_open**() _name_ in the specified
                  _mode_ , or **O_TRUNC** was specified and the caller does not have
                  write permission on the object.
    
           **EEXIST** Both **O_CREAT** and **O_EXCL** were specified to **shm_open**() and
                  the shared memory object specified by _name_ already exists.
    
           **EINVAL** The _name_ argument to **shm_open**() was invalid.
    
           **EMFILE** The per-process limit on the number of open file
                  descriptors has been reached.
    
           **ENAMETOOLONG**
                  The length of _name_ exceeds **PATH_MAX**.
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENOENT** An attempt was made to **shm_open**() a _name_ that did not
                  exist, and **O_CREAT** was not specified.
    
           **ENOENT** An attempt was to made to **shm_unlink**() a _name_ that does not
                  exist.
    


## ATTRIBUTES



           For an explanation of the terms used in this section, see
           attributes(7).
           ┌───────────────────────────────┬───────────────┬────────────────┐
           │ **Interface** │ **Attribute** │ **Value** │
           ├───────────────────────────────┼───────────────┼────────────────┤
           │ **shm_open**(), **shm_unlink**()      │ Thread safety │ MT-Safe locale │
           └───────────────────────────────┴───────────────┴────────────────┘
    


## VERSIONS



           POSIX leaves the behavior of the combination of **O_RDONLY** and
           **O_TRUNC** unspecified.  On Linux, this will successfully truncate an
           existing shared memory object—this may not be so on other UNIX
           systems.
    
           The POSIX shared memory object implementation on Linux makes use
           of a dedicated tmpfs(5) filesystem that is normally mounted under
           _/dev/shm_.
    


## STANDARDS



           POSIX.1-2008.
    


## 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:
    
               $ **./pshm_ucase_bounce /myshm &**
               [1] 270171
               $ **./pshm_ucase_send /myshm hello** ;
               HELLO
    
           Further detail about these programs is provided below.
    
       **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.
    
               #ifndef PSHM_UCASE_H
               #define PSHM_UCASE_H
    
               #include <semaphore.h>
               #include <stddef.h>
               #include <stdio.h>
               #include <stdlib.h>
    
               #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                                       } 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 'buf' */
                   char   buf[BUF_SIZE];   /* Data being transferred */
               };
    
               #endif  // include guard
    
       **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 _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.
    
           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.
    
               /* pshm_ucase_bounce.c
    
                  Licensed under GNU General Public License v2 or later.
               */
               #include <ctype.h>
               #include <fcntl.h>
               #include <stdio.h>
               #include <stdlib.h>
               #include <sys/mman.h>
               #include <unistd.h>
    
               #include "pshm_ucase.h"
    
               int
               main(int argc, char *argv[])
               {
                   int            fd;
                   char           *shmpath;
                   struct shmbuf  *shmp;
    
                   if (argc != 2) {
                       fprintf(stderr, "Usage: %s /shm-path\n", argv[0]);
                       exit(EXIT_FAILURE);
                   }
    
                   shmpath = argv[1];
    
                   /* Create shared memory object and set its size to the size
                      of our structure. */
    
                   fd = shm_open(shmpath, O_CREAT | O_EXCL | O_RDWR, 0600);
                   if (fd == -1)
                       errExit("shm_open");
    
                   if (ftruncate(fd, sizeof(struct shmbuf)) == -1)
                       errExit("ftruncate");
    
                   /* Map the object into the caller's address space. */
    
                   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 'sem1' 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 (size_t j = 0; j < shmp->cnt; j++)
                       shmp->buf[j] = toupper((unsigned char) shmp->buf[j]);
    
                   /* Post 'sem2' 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);
               }
    
       **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.
    
           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.
    
               /* pshm_ucase_send.c
    
                  Licensed under GNU General Public License v2 or later.
               */
               #include <fcntl.h>
               #include <stddef.h>
               #include <stdio.h>
               #include <stdlib.h>
               #include <string.h>
               #include <sys/mman.h>
               #include <unistd.h>
    
               #include "pshm_ucase.h"
    
               int
               main(int argc, char *argv[])
               {
                   int            fd;
                   char           *shmpath, *string;
                   size_t         len;
                   struct shmbuf  *shmp;
    
                   if (argc != 3) {
                       fprintf(stderr, "Usage: %s /shm-path string\n", argv[0]);
                       exit(EXIT_FAILURE);
                   }
    
                   shmpath = argv[1];
                   string = argv[2];
                   len = strlen(string);
    
                   if (len > BUF_SIZE) {
                       fprintf(stderr, "String is too long\n");
                       exit(EXIT_FAILURE);
                   }
    
                   /* Open the existing shared memory object and map it
                      into the caller's address space. */
    
                   fd = shm_open(shmpath, O_RDWR, 0);
                   if (fd == -1)
                       errExit("shm_open");
    
                   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. */
    
                   if (write(STDOUT_FILENO, &shmp->buf, len) == -1)
                       errExit("write");
                   if (write(STDOUT_FILENO, "\n", 1) == -1)
                       errExit("write");
    
                   exit(EXIT_SUCCESS);
               }
    


* * *