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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LEARN ABOUT FREEBSD

shmat

SHMAT(2)						      BSD System Calls Manual							  SHMAT(2)

NAME

     shmat, shmdt -- attach or detach shared memory

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

     void *
     shmat(int shmid, const void *addr, int flag);

     int
     shmdt(const void *addr);

DESCRIPTION

     The shmat() system call attaches the shared memory segment identified by shmid to the calling process's address space.  The address where the
     segment is attached is determined as follows:

     o	 If addr is 0, the segment is attached at an address selected by the kernel.

     o	 If addr is nonzero and SHM_RND is not specified in flag, the segment is attached the specified address.

     o	 If addr is specified and SHM_RND is specified, addr is rounded down to the nearest multiple of SHMLBA.

     The shmdt() system call detaches the shared memory segment at the address specified by addr from the calling process's address space.

RETURN VALUES

     Upon success, shmat() returns the address where the segment is attached; otherwise, -1 is returned and errno is set to indicate the error.

     The shmdt() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The shmat() system call will fail if:

     [EINVAL]		No shared memory segment was found corresponding to shmid.

     [EINVAL]		The addr argument was not an acceptable address.

     The shmdt() system call will fail if:

     [EINVAL]		The addr argument does not point to a shared memory segment.

SEE ALSO

     shmctl(2), shmget(2)

BSD
								  August 2, 1995							       BSD