Linux interprocess communication (IPC) programming Practice (11) System v Semaphore---Implement a first in, out shared memory Shmfifo

Message Queuing is used to implement the FIFO of the message, but it is faster to use shared memory for the FIFO of the message;

We first complete the C language version of Shmfifo (based on the procedure call), and then implement the C + + version of Shmfifo on this basis, will be 1 blocks of shared memory with 3 semaphores (1 mutext semaphores, 1 full semaphores, 1 empty semaphores) Encapsulated into a class Shmfifo, and then write their own test code;

Shmfifo Description:

The requested shared memory is used as a buffer, and the header (P_shm to P_payload) of the memory is formatted as shown in the form;

Read/write process constantly in accordance with the principle of cash first out of the read/write data from it, the read/write process can be regarded as producer/consumer, therefore, using the semaphore Sem_mutex (initial value of 1) to mutually exclusive access to shared memory, using Sem_full (initial value is the number of shared buffer blocks), sem_ Empty (initial value is 0) to synchronize two processes.

We use C + + to achieve:

Shmfifo class Design class  Shmfifo  {public  :      shmfifo (int _key, int _blksize, int _blocks);      ~shmfifo ();      void put (const void *buf);      void get (void *buf);      void Destroy ();    Private:      typedef struct SHMHEAD      {          unsigned int blksize;   Block size          unsigned int blocks;    Total block number          unsigned int rd_index;  Read index block          unsigned int wr_index;  Write index block      } shmhead_t;    Private:      shmhead_t *p_shm;   Shared memory head pointer      char *p_payload;    Payload in fact address        int shmid;      Shared memory ID      int Sem_mutex;  Mutex semaphore      int sem_full;   Full semaphore      int sem_empty;  Empty signal volume  };  

/** constructor **/shmfifo::shmfifo (int _key, int _blksize, int _blocks) {//Open a piece of shared memory Shmid = Shmget (_key, 0, 0); If the open fails, the shared memory is not created, then the IF (Shmid = =-1) {/** Sets the shared memory **/int size = _blksize*_block          s + sizeof (shmhead_t);          Create shared memory Shmid = Shmget (_key, size, ipc_creat|0666);            if (Shmid = =-1) err_exit ("Shmget error");          To create the shared memory successfully, you need to connect it to the process's address space P_SHM = (shmhead_t *) Shmat (shmid, NULL, 0);            if (P_shm = = (void *)-1) err_exit ("Shmat error");          Initializes the header of the shared memory to a struct shmhead p_shm->blksize = _blksize;          P_shm->blocks = _blocks;          P_shm->rd_index = 0;            P_shm->wr_index = 0;            P_payload = (char *) (P_SHM+1);          /** set three semaphores **/Sem_mutex = sem_create (_key);            Sem_setval (Sem_mutex, 1);          Sem_full = Sem_create (_key+1);            Sem_setval (Sem_full, _blocks); Sem_empty = SEM_create (_key+2);      Sem_setval (sem_empty, 0);          } else {//shared memory already exists and opened successfully, you only need to connect it to the process's address space P_SHM = (shmhead_t *) Shmat (shmid, NULL, 0);            if (P_shm = = (void *)-1) err_exit ("Shmat error");            P_payload = (char *) (P_SHM+1);          /** Open three semaphores **/Sem_mutex = Sem_open (_key);          Sem_full = Sem_open (_key+1);      Sem_empty = Sem_open (_key+2);   }}/** destructor **/Shmfifo::~shmfifo () {SHMDT (P_SHM);      Unmount shared Memory P_shm = NULL;  P_payload = NULL;   }

/** destroy function **/  void Shmfifo::d Estroy ()  {      sem_delete (Sem_mutex);      Sem_delete (sem_full);      Sem_delete (sem_empty);      if (Shmctl (Shmid, Ipc_rmid, NULL) = =-1)          err_exit ("Remove share memory Error");  }  

/** put function **/  void Shmfifo::p ut (const void *buf)  {      sem_p (sem_full);      Sem_p (Sem_mutex);        /** enters the critical section **/      //Gets the write position from the structure      char *index = p_payload +                    (P_shm->wr_index * p_shm->blksize);      Write      memcpy (index, buf, p_shm->blksize);      Index shift      P_shm->wr_index = (p_shm->wr_index+1)%p_shm->blocks;      /** exit Critical Zone **/        Sem_v (Sem_mutex);      Sem_v (Sem_empty);  }  

/** get function **/  void shmfifo::get (void *buf)  {      sem_p (sem_empty);      Sem_p (Sem_mutex);        /** enters the critical section **/      //obtains the readout position from the structure      char *index = p_payload +                    (P_shm->rd_index * p_shm->blksize);      Read      memcpy (buf, index, p_shm->blksize);      P_shm->rd_index = (p_shm->rd_index+1)%p_shm->blocks;      /** exit Critical Zone **/        Sem_v (Sem_mutex);      Sem_v (Sem_full);  }  

. Phony:clean all  CC = g++  cppflags =-wall-g  bin = Write Read free  SOURCES = $ (bin.=.cpp) All  : $ (BIN)    %.O:%.cpp      $ (cc) $ (cppflags)-C $^-o [email protected]    WRITE:WRITE.O shmfifo.o IPC.O      $ (cc) $ (cppflags) $^-lrt-o [email protected]  READ:READ.O shmfifo.o ipc.o      $ (CC) $ (cppflags) $^-lrt-o [email protected]    FR EE:FREE.O shmfifo.o ipc.o      $ (CC) $ (cppflags) $^-lrt-o [email protected] Clean    :      -rm-rf $ (BIN) *.O Bin/ob J/core  

Need real-time link library, only need to modify the makefile file, plus-LRT can be

Linux interprocess communication (IPC) programming Practice (11) System v Semaphore---Implement a first in, out shared memory Shmfifo