mutexing with sysv calls Post: 302286096

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Top Forums Programming mutexing with sysv calls Post 302286096 by otheus on Tuesday 10th of February 2009 10:09:47 AM

02-10-2009

Registered User

mutexing with sysv calls

My question is: what's the best way to implement mutual exclusion for a shared memory segment. I want to ensure that while updating a shared memory segment that another process will not get read part of the segment before and the other part after the update. (I also want to ensure that the update itself is not clobbered by another update.)

My update looks something like this:

Code:

      semkey = ftok(CONFIG_FILE,1);
      shmid = shmget(semkey, SHARED_POOL_SIZE, 0444);
      if (-1 == shmid) {
	  /* new shared memory segment -- initialize */
          isnew = 1;
          shmid = shmget(semkey, SHARED_POOL_SIZE,
                  IPC_CREAT | IPC_CREAT | SHM_NORESERVE | shm_unlocked );
          shmctl(shmid, IPC_STAT, &semstat);
      }
      else {
	  /* already exists -- grab pointer to it */
	  shmctl(shmid, IPC_STAT, &semstat);
	  stat(CONFIG_FILE, &confstat);
      }
      /* shmid is ID of our shared memory segment */
      if (shmid < 0 || isnew || confstat.st_mtime > semstat.shm_ctime) {
	  /* update it */
	  /* parse the file */
	  config_parse(&config);

	  pconfig = shmat(shmid, NULL, 0);
	  /* update the shared segment */
	  if ((void*)-1 != pconfig) {

	    /* MUTAL WRITE EXCLUSION NEEDED HERE */
	    memcpy(pconfig, &config, sizeof(struct config_data));
	    /* update the timestamp */
	    shmctl(shmid, IPC_SET, &semstat);
	    /* END MUTEX */
	    shmdt(pconfig);
	  }
      }

My reader code looks like this:

Code:

      semkey = ftok(CONFIG_FILE,1);
      shmid = shmget(semkey,128, 0444);
      pconfig = shmat(shmid, NULL, SHM_RDONLY);
      if ((void*)-1 == pconfig) {
	/* error recovery */
      }
      else {

        /* NEED MUTEX HERE */
        memcpy(&config, pconfig, sizeof(struct config_data));
        /* END MUTEX  */
      }

otheus

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shmat

shmop(2)							System Calls Manual							  shmop(2)

Name
       shmop, shmat, shmdt - shared memory operations

Syntax
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/shm.h>

       char *shmat (shmid, shmaddr, shmflg)
       int shmid;
       char *shmaddr;
       int shmflg;

       int shmdt (shmaddr)
       char *shmaddr;

Description
       The  system  call attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of
       the calling process.  The segment is attached at the address specified by one of the following criteria:

       If shmaddr is equal to zero, the segment is attached at the first available address as selected by the system.

       If shmaddr is not equal to zero and (shmflg & SHM_RND ) is true, the segment is attached at the address given by (shmaddr- (shmaddr modulus
       SHMLBA )).

       If shmaddr is not equal to zero and (shmflg & SHM_RND ) is false, the segment is attached at the address given by shmaddr.

       The segment is attached for reading if (shmflg & SHM_RDONLY ) is true. Otherwise, it is attached for reading and writing.

       The system call detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr.

Return Values
       Upon successful completion, the return values are as follows:

       o   The system call returns the data segment start address of the attached shared memory segment.

       o   The system call returns a value of zero (0).

       Otherwise, a value of -1 is returned, and errno is set to indicate the error.

Diagnostics
       The system call fails and not attach the shared memory segment, if any of the following is true:

       [EINVAL]       The shmid is not a valid shared memory identifier.

       [EACCES]       Operation permission is denied to the calling process.  For further information, see

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

       [EINVAL]       The shmaddr is not equal to zero, and the value of (shmaddr- (shmaddr modulus SHMLBA )) is an illegal address.

       [EINVAL]       The shmaddr is not equal to zero, (shmflg & SHM_RND ) is false, and the value of shmaddr is an illegal address.

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

       The fails and does not detach the shared memory segment if:

       [EINVAL]       The shmaddr is not the data segment start address of a shared memory segment.

See Also
       execve(2), exit(2), fork(2), shmctl(2), shmget(2)

																	  shmop(2)