Shared Memory  Creating a Shared Memory Segment Allocated in byte amounts  Shared Memory Operations Create Attach Detach  Shared Memory Control Remove

shmget System Call Function  The shmget system call is used to create or access a shared memory segment. Include: Command: int shmget (key_t key, int size, int shmflg); Returns: Success: unique shared memory identifier. Failure: -1 ; Sets errno: Yes. Arguments: key_t key: key for creating or accessing shared memory int size: size in bytes of shared memory segment to create. Use 0 for accessing an existing segment. int shmflg: segment creation condition and access permission.

shmget(): Argument Values key same as before; we use getuid() to make it unique size number of bytes to allocate shmflag Creating: IPC_CREAT and permissions 0 for accessing only

Shared Memory Limits Shared Memory Segment Defaults Value Maximum segment size 1,048,576 bytes Minimum segment size 1 byte System wide maximum number of segments 100 Maximum number of segments per process 6? that can be attached

shmat System Call Function:  used to attach (map) the referenced shared memory segment into the calling process’s data segment. The pointer returned is to the first byte of the segment Include: Command: void *shmat ( int shmid, void *shmaddr, int shmflg); Returns: Success:Reference to the data segment address of shared memory; Failure:-1; Sets errno:Yes. Arguments: int shmid: a valid shared memory identifier. void *shmaddr: allows the calling process some flexibility in assigning the location of the shared memory. int shmflg: access permissions and attachment conditions.

Notes on shmat() arguments  shmid is as before  2 nd and 3 rd arguments can be used to attach to a specific address within shmem, but it is an absolute address, requiring knowledge of the address of the shared memory segment  Almost always, 2 nd and 3 rd arguments are 0

shmdt System Call Function shmdt is used to detach the calling process’s data segment from the shared memory segment. Include: Command: int shmdt (void * shmaddr); Returns: Success: 0; Failure: -1; Sets errno: Yes. Argument: void *shmaddr: a reference to an attached memory segment (the shared memory pointer).

shmctl call - Shared Memory Control Function: shmctl permits the user to perform a number of generalized control operations on an existing shared memory segment and on the system shared memory data structure. Include: Command: int shmctl(int shmid, int cmd, struct shmid_ds * buf); Return: Success: 0; Failure: -1; Sets errno: Yes. Arguments int shmid: a valid shared memory segment identifier. int cmd: the operation shmctl is t perform. struct shmid_ds * buf: a reference to the shmid_ds structure

Operations of shmctl()  IPC_STAT: Return the current value of the shmid_ds structure for the shared memory segment indicated by the shmid value.  IPC_SET: Modify a limited number of members in the permission structure found within the shmid_ds structure.  IPC_RMID: Remove the system data structure for the referenced shared memory identifier (shmid). Once all references to the shared memory segment are eliminated, the system will remove the actual shared memory segment.  SHM_LOCK: Lock, in memory, the shared memory segment referenced by the shmid argument. Superuser access required  SHM_UNLOCK: Unlock the shared memory segment referenced by the shmid argument. Superuser access required

Shared Memory Data Structure /* One shmid data structure for each shared memory segment in the system. */ struct shmid_ds { struct ipc_perm shm_perm; /* operation perms */ int shm_segsz; /* size of segment (bytes) */ time_t shm_atime; /* last attach time */ time_t shm_dtime; /* last detach time */ time_t shm_ctime; /* last change time */ unsigned short shm_cpid; /* pid of creator */ unsigned short shm_lpid; /* pid of last operator */ short shm_nattch; /* no. of current attaches */ /* the following are private */ unsigned short shm_npages; /* size of segment (pages) */ unsigned long *shm_pages; /* array of ptrs to frames -> SHMMAX */ struct vm_area_struct *attaches; /* descriptors for attaches */ };

Virtual Memory Area Structure vm_area_struct  defines a memory VMM memory area. There is one of these per VM-area/task. A VM area is any part of the process virtual memory space that has a special rule for the page-fault handlers (ie a shared library, the executable area etc). struct vm_area_struct { struct mm_struct * vm_mm; unsigned long vm_start; unsigned long vm_end; struct vm_area_struct *vm_next; pgprot_t vm_page_prot; unsigned long vm_flags; rb_node_t vm_rb; struct vm_area_struct *vm_next_share; struct vm_area_struct **vm_pprev_share; struct vm_operations_struct * vm_ops; unsigned long vm_pgoff; struct file * vm_file; unsigned long vm_raend; void * vm_private_data; };

ipcs & ipcrm Command * % ipcs -b (-b to display the maximum number of bytes ) T ID KEY MODE OWNER GROUP QBYTES Message Queues: q 50 0X67028a01 -Rrw-rw---- gray other 4096 Shared Memory facility not in system T ID KEY MODE OWNER GROUP NSEMS Semaphores: s 0 0X000187cf --ra-ra-ra- root root 2 s 1 0X000187ce --ra-ra-ra- root root 1