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Full Discussion: Dereferencing pointer to a shared memory struct
Top Forums Programming Dereferencing pointer to a shared memory struct Post 302494138 by arette on Saturday 5th of February 2011 02:37:58 PM
Old 02-05-2011
Dereferencing pointer to a shared memory struct
I have what should be a relatively simple program (fadec.c) that maps a struct from an included header file (fadec.h) to a shared memory region, but I’m struggling accessing members in the struct from the pointer returned by shmat. Ultimately, I want to access members in the shared memory structure with a globally declared version of the struct, ‘shm__’. Not only do I not know how to accomplish that, but I can’t even seem to access members of the shared struct directly from the pointer itself (compiler complains about ‘dereferencing pointer to incomplete type’). I’m at a loss and could use another set of eyes if you guys don’t mind taking a gander:


Compile Errors:
tony-pc:/cygdrive/p/test> cc -o fadec fadec.c
fadec.c: In function 'main':
fadec.c:30: error: dereferencing pointer to incomplete type
fadec.c:31: error: dereferencing pointer to incomplete type


fadec.h:
struct io_struct // sample, full structure is closer to 16384 bytes (see below)
{
int number;
};


fadec.c:
#include <sys/shm.h>
#include <sys/ipc.h>
#include <sys/types.h>
#include <string.h>
#include <stdio.h>
#include "fadec.h"

#define SHM_KEY 1000

struct iostruct *test; // pointer to structure
volatile struct io_struct shm__; // this is ultimately how I want to reference the
// struct in shared memory

int main(int argc, char *argv[])
{
int shmid = shmget(SHM_KEY, 16384, IPC_CREAT);
if( shmid == -1) return -1;

test = shmat(shmid, NULL, 0);
if((int)test == -1)
{
shmctl(shmid, IPC_RMID, NULL);
return -1;
}
printf("Memory attached at %X\n", (int)test);
fprintf(stderr,"test = %X\n",test);

// Need to somehow map 'test' to 'shm__' so that I can access with: shm__.number = 0;
// But, short of that, just try to set a value through 'test' directly:
(* test).number = 0; // or should be able to use ‘test->number=0;’ here
fprintf(stderr,"(* test).number = %d\n",(* test).number);

return(0);
}


Thanks for your help!
Tony
 

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shmctl

shmctl(2)							System Calls Manual							 shmctl(2)

NAME

       shmctl() - shared memory control operations

SYNOPSIS

DESCRIPTION

       The  system  call provides a variety of shared memory control operations as specified by the cmd argument.  cmd can have the following val-
       ues:

	      Place the current value of each member of the data structure associated with
			     shmid into the structure pointed to by buf.  The contents of this structure are defined in glossary(9).

	      Set the value of the following members of the data structure associated with
			     shmid to the corresponding value found in the structure pointed to by buf:

				       shm_perm.uid
				       shm_perm.gid
				       shm_perm.mode  /* only low 9 bits */

			     This cmd can only be executed by a process that has an effective user ID equal to either that of a user having appro-
			     priate privileges or to the value of either or in the data structure associated with shmid.

			     This cmd cannot be executed on a shared memory segment that has been created with relaxed isolation.

	      Remove the shared memory identifier specified by
			     shmid from the system and destroy the shared memory segment and data structure associated with it.  If the segment is
			     attached to one or more processes, then the segment key is changed to and the segment is marked removed.  The segment
			     disappears when the last attached process detaches it.  This cmd can only be executed by a process that has an effec-
			     tive user ID equal to either that of a user with appropriate privileges or to the value of  either  or  in  the  data
			     structure associated with shmid.

	      Lock the shared memory segment specified by
			     shmid  in memory.	This cmd can only be executed by a process that either has an effective user ID equal to that of a
			     user having appropriate privileges or has an effective user ID equal to the value of either or in the data  structure
			     associated with shmid and has the privilege (see getprivgrp(2)).

	      Unlock the shared memory segment specified by
			     shmid.  This cmd can only be executed by a process that either has an effective user ID equal to a user having appro-
			     priate privileges or has an effective user ID equal to the value of either or in the data structure  associated  with
			     shmid and has the privilege (see getprivgrp(2)).

RETURN VALUE

       returns the following values:

	      Successful completion.
	      Failure.
		     is set to indicate the error.

ERRORS

       If fails, is set to one of the following values.

	      cmd	     is equal to and Read operation permission is denied to the calling process (see in glossary(9)).

	      buf	     points to an illegal address.  The reliable detection of this error is implementation dependent.

	      cmd	     is equal to and the shared-memory segment specified by shmid is not locked in memory.

	      shmid	     is not a valid shared memory identifier.

	      cmd	     is not a valid command, or the command contains invalid parameters.

	      cmd	     is equal to and there is not sufficient lockable memory to fill the request.

	      cmd	     is  equal	to  The  attached process count of a shared memory object is too large for the field in the data structure
			     provided by the application.  Recompile the application with the latest file to resolve this issue.

	      cmd	     is equal to or and the effective user ID of the calling process is not equal to that of  a  user  having  appropriate
			     privileges and it is not equal to the value of either or in the data structure associated with shmid.

			     cmd is equal to and the shared memory segment has

	      cmd	     is  equal	to  or	and the effective user ID of the calling process is not equal to that of a user having appropriate
			     privileges and the calling process does not have the privilege (see getprivgrp(2)).

EXAMPLES

       The following call to locks in memory the shared memory segment represented by This example assumes the process has a  valid  shmid,  which
       can be obtained by calling shmget(2).

       The  following  call  to  removes the shared memory segment represented by This example assumes the process has a valid shmid, which can be
       obtained by calling (see shmget(2).

AUTHOR

       was developed by AT&T and HP.

SEE ALSO

       ipcrm(1), ipcs(1), shmget(2), shmop(2), stdipc(3C), privileges(5),

STANDARDS CONFORMANCE

																	 shmctl(2)
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