There have been several shared-memory extensions written for Tcl,
including
   * [David Welton]'s tclshmem [http://www.dedasys.com/freesoftware/files/tclshmem.c] ("it's not complete")
   * David's libmm-based one, called TclMM [http://www.dedasys.com/freesoftware/files/TclMM.tgz]
   * [Till Immanuel Patzschke]'s interface to System V shared memory system calls [[email mailto:tip@prs.de for details]]
   * [[list others]]











[[Explain what shared memory is.]]

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See [Inventory of IPC methods] for other ways to let tcl processes communicate.




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"Shared memory" and "[memory mapping]" are roughly synonymous.
[[Explain distinction, usage.]]


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I've been using these two little packages "shm" and "bob" (binary object) for several years and its very simple. This only works on systems where sizeof int == sizeof pointer.

 package require critcl
 package provide shm     1.0

 namespace eval shm {
    critcl::ccode {
        #include <sys/ipc.h>
        #include <sys/shm.h>
    }
     critcl::cproc get  { int key } int {
        return shmget(key, 0, 0);
    }
    critcl::cproc new { int key int size int perm } int {
        return shmget(key, size, perm | IPC_CREAT);
    }
    critcl::cproc siz { int id  } int {
        struct shmid_ds shm;
        if ( shmctl(id, IPC_STAT, &shm) != -1 ) {
            return shm.shm_segsz;
        }
        return -1;
    }
    critcl::cproc att { int id  } int {
        return (int) shmat(id, 0, 0);
    }
 }

 package require critcl
 package provide bob     1.0

 namespace eval bob {
    namespace export {[a-z]*}
    critcl::cproc get { Tcl_Interp* interp int att int off int len } ok {
        Tcl_SetObjResult(interp, Tcl_NewByteArrayObj((char *) att + off, len));
        return TCL_OK;
    }
    critcl::cproc set { Tcl_Interp* interp int att int off int len Tcl_Obj* data } void {
        memcpy((char *) att + off, Tcl_GetByteArrayFromObj(data, NULL), len);
    }
    critcl::ccommand write { data interp objc objv } {
        char *att;
        int   off;
        int   len;
        Tcl_Channel ofp;

        int   mode;

        if ( objc != 5 ) {
            Tcl_SetResult(interp, "bob::write pointer offset length file", TCL_STATIC);
            return TCL_ERROR;
        }
        Tcl_GetIntFromObj(interp, objv[1], &att);
        Tcl_GetIntFromObj(interp, objv[2], &off);
        Tcl_GetIntFromObj(interp, objv[3], &len);

        if ( (ofp = Tcl_GetChannel(interp, Tcl_GetString(objv[4]), &mode)) == NULL ) {
            Tcl_SetResult(interp, "bob::read cannot get channel", TCL_STATIC);
            return TCL_ERROR;
        }
        Tcl_Write(ofp, att + off, len);
        return TCL_OK;
    }
    critcl::ccommand read { data interp objc objv } {
        char *att;
        int   off;
        int   len;
        Tcl_Channel ifp;

        int   mode;

        if ( objc != 5 ) {
            Tcl_SetResult(interp, "bob::read data offset length file", TCL_STATIC);
            return TCL_ERROR;
        }
        Tcl_GetIntFromObj(interp, objv[1], &att);
        Tcl_GetIntFromObj(interp, objv[2], &off);
        Tcl_GetIntFromObj(interp, objv[3], &len);

        if ( (ifp = Tcl_GetChannel(interp, Tcl_GetString(objv[4]), &mode)) == NULL ) {
            Tcl_SetResult(interp, "bob::read cannot get channel", TCL_STATIC);
            return TCL_ERROR;
        }
        Tcl_Read(ifp, att + off, len);
        return TCL_OK;
    }
 }


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