Instruction

Shared Memory

We learn how to create and use shared memory.

1. introduction
   
   • Use the shmget
     function to create a shared memory segment
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>
     int shmget (key_t key, int size, int shmflag);
     
     
     • returns the memory ID or -1 on error
     • key - key identifying shared memory
     • size - specifies the memory size in bytes
     • semflag - is a combination of symbolic constants specifying access rights
     
     
     

     Numerical value	Symbolic constant	Meaning
     0400	SHM_R	reading by the owner
     0200	SHM_A	change by owner
     0040	SHM_R>>3	reading by the group
     0020	SHM_A>>3	changing by group
     0004	SHM_R>>6	reading by others
     0002	SHM_A>>6	changing by others
     1000	IPC_CREAT
     2000	IPC_EXCL

   • shmget creates or opens a shared memory segment, but to use it you must attach it
     
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>
     char *shmat(int shmid, char *shmaddr, int shmflag);
     
     • the function returns the starting address of the shared memory segment, or -1 on error
     • the address is determined according to the following rules:
       • when shmaddr = 0 then the system chooses the address itself
       • when shmaddr != 0 then the forwarded address depends on whether the SHM_RND tag is set
         • if not set, the shared memory segment will be attached from the address specified by the shmaddr argument
         • if set, it will start with an address rounded down by the value of the constant SHMLBA (Lower Boundary Address)
     • for most uses it is enough to give 0
     • shmid - shared memory identifier returned by shmget
     • shmflag - may have a tag, e.g. SHM_RDONLY
   • disconnection of shared memory is done with
     
     int shmdt (char * shmaddr);
     This function does not delete the shared memory segment
   • to delete a shared memory segment use the following function:
     int shmctl (int shmid, int cmd, struct shmid_ds * buf);
     • with the cmd argument as IPC_RMID
       
       
2. Write a server program that:
   • creates a shared memory segment of a size that allows writing to a variable of typeint (0,4p)
     
   • creates two semaphores: 1 lowered, 2 raised (0,4p)
   • in the loop executes: lower semaphore 2, waits a random time from 0 to 1 second, enters the next number (starting from 1) into the shared variable, raises semaphore 1 (1.5p)
   • program should remove shared memory and semaphores after pressing ctrl ^ c (0,5p)
   • make sure that only one server program creates semaphores and memory (exclusive mode) (0,2p)
     
     
3. Write a client program that:
   • attaches to memory segments and semaphores(0,5)
   • looped (10 times), lower semaphore 1, reads the value from shared memory, raises semaphore 2, displays value, waits a random time from 0 to 1 second (1)
   • check the operation of several clients at once (0,5).
     Launch clients in one terminal, e.g. this way(./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & ./client & )
   • is there a danger that two clients will get the same number?
     
     
4. Report
   Don't forget the report