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shmop (2)
  • >> shmop (2) ( Solaris man: Системные вызовы )
  • shmop (2) ( Русские man: Системные вызовы )
  • shmop (2) ( Linux man: Системные вызовы )
  •  

    NAME

    shmop, shmat, shmdt - shared memory operations
     
    

    SYNOPSIS

    #include <sys/types.h>
    #include <sys/shm.h>
    
    void *shmat(int shmid, const void *shmaddr, int shmflg);
    

    int shmdt(char *shmaddr);
    

     

    Standard conforming

    int shmdt(const void *shmaddr);
    

     

    DESCRIPTION

    The shmat() function attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of the calling process.

    The permission required for a shared memory control operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

    00400    READ by user
    00200    WRITE by user
    00040    READ by group
    00020    WRITE by group
    00004    READ by others
    00002    WRITE by others
    

    See the Shared Memory Operation Permissions section of Intro(2) for more information.

    For shared memory segments created with the SHM_SHARE_MMU or SHM_PAGEABLE flags, the default protections cannot be changed so as to prevent a single process from affecting other processes sharing the same shared segment.

    When (shmflg&SHM_SHARE_MMU) is true, virtual memory resources in addition to shared memory itself are shared among processes that use the same shared memory.

    When (shmflg&SHM_PAGEABLE) is true, virtual memory resources are shared and the dynamic shared memory (DISM) framework is created. The dynamic shared memory can be resized dynamically within the specified size in shmget(2). The DISM shared memory is pageable unless it is locked.

    The shared memory segment is attached to the data segment of the calling process at the address specified based on one of the following criteria:

    o If shmaddr is equal to (void *) 0, the segment is attached to the first available address as selected by the system.
    o If shmaddr is equal to (void *) 0 and ( shmflg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE) is true, then the segment is attached to the first available suitably aligned address. When (shmflg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE) is set, however, the permission given by shmget() determines whether the segment is attached for reading or reading and writing.
    o If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is true, the segment is attached to the address given by (shmaddr- (shmaddr modulus SHMLBA)).
    o If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is false, the segment is attached to the address given by shmaddr.
    o The segment is attached for reading if (shmflg&SHM_RDONLY) is true {READ}, otherwise it is attached for reading and writing {READ/WRITE}.

    The shmdt() function detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr. If the application is standard-conforming (see standards(5)), the shmaddr argument is of type const void *. Otherwise it is of type char *.

    Shared memory segments must be explicitly removed after the last reference to them has been removed.  

    RETURN VALUES

    Upon successful completion, shmat() returns the data segment start address of the attached shared memory segment; shmdt() returns 0. Otherwise, -1 is returned, the shared memory segment is not attached, and errno is set to indicate the error.  

    ERRORS

    The shmat() function will fail if:

    EACCES

    Operation permission is denied to the calling process (see Intro(2)).

    EINVAL

    The shmid argument is not a valid shared memory identifier.

    The shmaddr argument is not equal to 0, and the value of (shmaddr- (shmaddr modulus SHMLBA)) is an illegal address.

    The shmaddr argument is not equal to 0, is an illegal address, and (shmflg&SHM_RND) is false.

    The shmaddr argument is not equal to 0, is not properly aligned, and (shmfg&SHM_SHARE_MMU) is true.

    SHM_SHARE_MMU is not supported in certain architectures.

    Both (shmflg&SHM_SHARE_MMU) and (shmflg&SHM_PAGEABLE) are true.

    (shmflg&SHM_SHARE_MMU) is true and the shared memory segment specified by shmid() had previously been attached by a call to shmat() in which (shmflg&SHM_PAGEABLE) was true.

    (shmflg&SHM_PAGEABLE) is true and the shared memory segment specified by shmid() had previously been attached by a call to shmat() in which (shmflg&SHM_SHARE_MMU) was true.

    EMFILE

    The number of shared memory segments attached to the calling process would exceed the system-imposed limit.

    ENOMEM

    The available data space is not large enough to accommodate the shared memory segment.

    The shmdt() function will fail if:

    EINVAL

    The shmaddr argument is not the data segment start address of a shared memory segment.

    ENOMEM

    (shmflg&SHM_SHARE_MMU) is true and attaching to the shared memory segment would exceed a limit or resource control on locked memory.

     

    WARNINGS

    Using a fixed value for the shmaddr argument can adversely affect performance on certain platforms due to D-cache aliasing.  

    ATTRIBUTES

    See attributes(5) for descriptions of the following attributes:

    ATTRIBUTE TYPEATTRIBUTE VALUE

    Interface StabilityCommitted

    MT-Level

    Standard

     

    SEE ALSO

    Intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2), attributes(5), standards(5)


     

    Index

    NAME
    SYNOPSIS
    Standard conforming
    DESCRIPTION
    RETURN VALUES
    ERRORS
    WARNINGS
    ATTRIBUTES
    SEE ALSO


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