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    Command:

    shm_open

    
    
    
    

    SYNOPSIS

         #include <sys/types.h>
         #include <sys/mman.h>
    
         int
         shm_open(const char *path, int flags, mode_t mode);
    
         int
         shm_unlink(const char *path);
    
    
    

    DESCRIPTION

         The shm_open() system call opens (or optionally creates) a POSIX shared
         memory object named path.  The flags argument contains a subset of the
         flags used by open(2).  An access mode of either O_RDONLY or O_RDWR must
         be included in flags.  The optional flags O_CREAT, O_EXCL, and O_TRUNC
         may also be specified.
    
         If O_CREAT is specified, then a new shared memory object named path will
         be created if it does not exist.  In this case, the shared memory object
         is created with mode mode subject to the process' umask value.  If both
         the O_CREAT and O_EXCL flags are specified and a shared memory object
         named path already exists, then shm_open() will fail with EEXIST.
    
         Newly created objects start off with a size of zero.  If an existing
         shared memory object is opened with O_RDWR and the O_TRUNC flag is speci-
         fied, then the shared memory object will be truncated to a size of zero.
         The size of the object can be adjusted via ftruncate(2) and queried via
         fstat(2).
    
         The new descriptor is set to close during execve(2) system calls; see
         close(2) and fcntl(2).
    
         As a FreeBSD extension, the constant SHM_ANON may be used for the path
         argument to shm_open().  In this case, an anonymous, unnamed shared mem-
         ory object is created.  Since the object has no name, it cannot be
         removed via a subsequent call to shm_unlink().  Instead, the shared mem-
         ory object will be garbage collected when the last reference to the
         shared memory object is removed.  The shared memory object may be shared
         with other processes by sharing the file descriptor via fork(2) or
         sendmsg(2).  Attempting to open an anonymous shared memory object with
         O_RDONLY will fail with EINVAL.  All other flags are ignored.
    
         The shm_unlink() system call removes a shared memory object named path.
    
    
    

    RETURN VALUES

         If successful, shm_open() returns a non-negative integer, and
         shm_unlink() returns zero.  Both functions return -1 on failure, and set
         errno to indicate the error.
    
    
    

    COMPATIBILITY

         The path argument does not necessarily represent a pathname (although it
         across reboots.
    
    
    

    ERRORS

         The following errors are defined for shm_open():
    
         [EINVAL]           A flag other than O_RDONLY, O_RDWR, O_CREAT, O_EXCL,
                            or O_TRUNC was included in flags.
    
         [EMFILE]           The process has already reached its limit for open
                            file descriptors.
    
         [ENFILE]           The system file table is full.
    
         [EINVAL]           O_RDONLY was specified while creating an anonymous
                            shared memory object via SHM_ANON.
    
         [EFAULT]           The path argument points outside the process' allo-
                            cated address space.
    
         [ENAMETOOLONG]     The entire pathname exceeded 1023 characters.
    
         [EINVAL]           The path does not begin with a slash ('/') character.
    
         [ENOENT]           O_CREAT is specified and the named shared memory
                            object does not exist.
    
         [EEXIST]           O_CREAT and O_EXCL are specified and the named shared
                            memory object dies exist.
    
         [EACCES]           The required permissions (for reading or reading and
                            writing) are denied.
    
         The following errors are defined for shm_unlink():
    
         [EFAULT]           The path argument points outside the process' allo-
                            cated address space.
    
         [ENAMETOOLONG]     The entire pathname exceeded 1023 characters.
    
         [ENOENT]           The named shared memory object does not exist.
    
         [EACCES]           The required permissions are denied.  shm_unlink()
                            requires write permission to the shared memory object.
    
    
    

    SEE ALSO

         close(2), ftruncate(2), fstat(2), mmap(2), munmap(2)
    
    
    

    STANDARDS

         The shm_open() and shm_unlink() functions are believed to conform to IEEE
         Std 1003.1b-1993 ("POSIX.1").
    
    
    

    HISTORY

    
    
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