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Dereferencing pointer to a shared memory struct

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# 1  
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
# 2  
Old 02-05-2011
Typo.

Code:
struct iostruct *test;

Code:
struct io_struct *test;

You can actually declare pointers to structure types you've never declared anywhere if you never dereference them, but when you do try and use their members, you get something like this... Smilie
# 3  
Old 02-05-2011
Wow - no wonder I was Smilie! Sometimes it helps to take a step back lol. Now it compiles and runs fine! Thank you!
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