Thread: problem with sending array of struct over shared memory

First of all, Hello to the comunity!
I hope someone will be able to help or at least give me some hint/guide on what I'm doing wrong here.
Here is the code:

Code:

#include <stdlib.h>
#include <stdio.h>
 
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
 
#define KEY (5679)
 
typedef struct _command
{
        int x;
        int y;
        int xo;
        int yo;
        int lock;
} command[20];
 
main ()
{
        int shmid, shmkeyid, i, size;
        command *tuio_cmd;
        size = sizeof( *tuio_cmd);
 
        shmid = shmget (KEY, size, 0666 | IPC_CREAT);
        if (shmid != -1)
        {
                tuio_cmd = shmat (shmid, NULL, 0);
                if (tuio_cmd != (void *) -1)
                {
                        printf ("Memory attached at:%X size:%d\n", (int) tuio_cmd, size);
                        tuio_cmd[0]->lock = 0;
                        while (1)
                        {
                                if (tuio_cmd[0]->lock)
                                {
                                if (tuio_cmd[0]->xo != 0)
                                        for( i=0; i<20; i++ )
                                                printf ("X:%d,Y:%d,Xo:%d,Yo:%d\n", tuio_cmd[i]->x,tuio_cmd[i]->y, tuio_cmd[i]->xo, tuio_cmd[i]->yo);
                                else
                                        for( i=0; i<20; i++ )
                                                printf ("X:%d,Y:%d\n", tuio_cmd[i]->x,tuio_cmd[i]->y);
 
                                tuio_cmd[0]->lock = 0;
                                }
                        }
                        if (shmdt (tuio_cmd) == -1)
                        {
                                fprintf (stderr, "shmdt failed\n");
                                exit (EXIT_FAILURE);
                        }
                        if (shmctl (shmid, IPC_RMID, 0) == -1)
                        {
                                fprintf (stderr, "shmctl(IPC_RMID) failed\n");
                                exit (EXIT_FAILURE);
                        }
                }
        else fprintf(stdout,"error attaching memory\n");
        }
        else fprintf(stdout,"error in allocating shm\n");
        return 0;
}

I'm trying to send array of struct over the shared memory, and after all day I can't find why this won't work. (don't look at locking It's remaining of old one dimensional struct, I still need to implement semaphores)

What are you expecting exactly? your shared segment seems to be created correctly.
After that, there is an infinite loop due to the 'tuio_cmd[0]->lock' condition never satisfied
(or is it unlocked by another process? in this case, the code is missing)

hmm here is the other half, and to repeat myself .. the lock is there just for the moment, as I wanted to try if it's working.

Code:

#include <stdlib.h>
#include <stdio.h>

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

#define KEY (5679)

typedef struct _command
{
	int x;
	int y;
	int xo;
	int yo;
	int lock;
} command[20], *command_ptr;

main ()
{
	int shmid, *shm_ptr, i, size;
	command *tuio_cmd;
	size = sizeof( *tuio_cmd);

	shmid = shmget (KEY, size, 0666 | IPC_CREAT);
	if (shmid != -1)
	{
		shm_ptr = shmat (shmid, NULL, 0);
		tuio_cmd = *shm_ptr;
		if (tuio_cmd != (void *) -1)
		{
			for( i=0; i<20; i++ )
				{
				tuio_cmd[i]->lock = 1;
				tuio_cmd[i]->x = i;
				tuio_cmd[i]->y = i;
				tuio_cmd[i]->x = i;
				tuio_cmd[i]->y = i;
				}
		}
	else fprintf(stdout,"error attaching memory\n");
	}
	else fprintf(stdout,"error in allocating shm\n");
return 0;
}

Actually this part I wrote just to test the fist program, as I'll send the data with python script. And to be more sure I'm not doing some python mistake I tried this way also. But with no luck. Here is also a simple python implementation of how I intend to send the data :

Code:

import shm
import sys
import ctypes
class comm(ctypes.Structure):
    _fields_ = [
	    ('x',ctypes.c_int),
	    ('y',ctypes.c_int),
	    ('xo',ctypes.c_int),
	    ('yo',ctypes.c_int),
	    ('lock',ctypes.c_int)
    ]
x = int(sys.argv[1])
y = int(sys.argv[2])
if len(sys.argv) >3:
    xo = int(sys.argv[3])
    yo = int(sys.argv[4])
key = 5679
mem = shm.memory(shm.getshmid(key))
mem.attach()
comm_array_class = comm * 20
an_array = comm_array_class()
if len(sys.argv) >3:
    #kom=comm(x=x,y=y,xo=xo,yo=yo,lock=1)
    for i in range(20):
	    an_array[i].x = x
            an_array[i].y = y
            an_array[i].xo = xo
	    an_array[i].yo = yo
	    an_array[i].lock = 1
    mem.write(an_array)
    print "Sent:",x,y,xo,yo;
else:
    for i in range(20):
	    an_array[i].x = x
            an_array[i].y = y
	    an_array[i].lock = 1
    mem.write(an_array)

I'm stuck here: tuio_cmd = shmat (shmid, NULL, 0);
I'm not sure if I did that right ..

What error do you get exactly? I pass this point when I try your code and enter the loop.

I think your problem comes from the initialization of tuio_cmd:

Code:

	shmid = shmget (KEY, size, 0666 | IPC_CREAT);
	if (shmid != -1)
	{
		shm_ptr = shmat (shmid, NULL, 0);
		tuio_cmd = *shm_ptr;          // <- no need to dereference here
		if (tuio_cmd != (void *) -1)
		{
			for( i=0; i<20; i++ )

As this var is a command*, the result of shmat() is directly this address
'tuio_cmd=(command*)shm_ptr;'.

Thats not the python problem, as now after I fixed the wrong dereferencing in second C program,
I got the same output (which I think I got in the first place as maybe I didn't recompile it after I was experimenting on changing various things). Anyway now If I run it :
gdb ./arr_send_struct
break 37
run
c <-- repeat 12 x
:::::
Breakpoint 1, main () at arr_send_struct.c:37
37 tuio_cmd[i]->lock = 1;
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x080484cc in main () at arr_send_struct.c:37
37 tuio_cmd[i]->lock = 1;
:::::::::::
and the output of ./arr_struct
X:0,Y:0
X:1,Y:1
X:2,Y:2
X:3,Y:3
X:4,Y:4
X:5,Y:5
X:6,Y:6
X:7,Y:7
X:8,Y:8
X:9,Y:9
X:10,Y:10
X:5,Y:7
X:-160678444,Y:-2008677909
X:32,Y:655394
X:1601399924,Y:1601463655
X:157536133,Y:20
X:-1996488699,Y:-1983685147
X:1358916351,Y:360
X:42304783,Y:-2088042496
X:-1947896016,Y:231555

ok I added / uncommented printf ("Memory attached at:&#37;X size:%d\n", (int) tuio_cmd, size);
and in second prog. I added
printf("tuio_cmd[i]:%p\n",tuio_cmd[i]);
after tuio_cmd[i]->yo = i; (I edited this also as it was wrong)

Its saying :
./arr_send_struct
Memory attached at:B7F1B000 size:400
tuio_cmd[i]:0xb7f1b000
tuio_cmd[i]:0xb7f1b190
tuio_cmd[i]:0xb7f1b320
tuio_cmd[i]:0xb7f1b4b0
tuio_cmd[i]:0xb7f1b640
tuio_cmd[i]:0xb7f1b7d0
tuio_cmd[i]:0xb7f1b960
tuio_cmd[i]:0xb7f1baf0
tuio_cmd[i]:0xb7f1bc80
tuio_cmd[i]:0xb7f1be10
tuio_cmd[i]:0xb7f1bfa0
Segmentation fault

./arr_struct
Memory attached at:B7EF7000 size:400
X:0,Y:0,tuio_cmd[i]:0xb7ef7000
X:1,Y:1,tuio_cmd[i]:0xb7ef7190
X:0,Y:0,tuio_cmd[i]:0xb7ef7320
X:3,Y:3,tuio_cmd[i]:0xb7ef74b0
X:4,Y:4,tuio_cmd[i]:0xb7ef7640
X:5,Y:5,tuio_cmd[i]:0xb7ef77d0
X:6,Y:6,tuio_cmd[i]:0xb7ef7960
X:7,Y:7,tuio_cmd[i]:0xb7ef7af0
X:8,Y:8,tuio_cmd[i]:0xb7ef7c80
X:9,Y:9,tuio_cmd[i]:0xb7ef7e10
X:10,Y:10,tuio_cmd[i]:0xb7ef7fa0
X:5,Y:7,tuio_cmd[i]:0xb7ef8130
X:-160678444,Y:-2008677909,tuio_cmd[i]:0xb7ef82c0
X:32,Y:655394,tuio_cmd[i]:0xb7ef8450
X:1601399924,Y:1601463655,tuio_cmd[i]:0xb7ef85e0
X:157536133,Y:20,tuio_cmd[i]:0xb7ef8770
X:-1996488699,Y:-1983685147,tuio_cmd[i]:0xb7ef8900
X:1358916351,Y:360,tuio_cmd[i]:0xb7ef8a90
X:42304783,Y:-2088042496,tuio_cmd[i]:0xb7ef8c20
X:-1947896016,Y:231555,tuio_cmd[i]:0xb7ef8db0