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Full Discussion: help with shared memory
Top Forums Programming help with shared memory Post 302113435 by Hitori on Friday 6th of April 2007 02:36:15 AM
Old 04-06-2007
Quote:
Originally Posted by ddx08
what i want to do is have an int that can been written into by 2 processes but my code doesn't seem to work.

Code:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include<stdio.h>
#define KEY1 (1492)
int main()
{

	int shmid;
	volatile int * addr;
	pid_t  pid1;
	
	shmid=shmget(KEY1,4*1,SHM_R|SHM_W);
	addr=shmat(shmid,0,4);
	addr=0;
	pid1=fork();
	if(pid1==0)
	{
		addr[0]=1;
		sleep(1);
		addr=2;
		sleep(1);
		addr=3;
	}
	else
	{	
		printf("addr is %i \n",addr);
		sleep(1);
		printf("addr is %i \n",addr);
		sleep(1);
		printf("addr is %i \n",addr);
		sleep(1);

		
		if((shmctl(shmid,IPC_RMID, 0)) < 0) 
		{
			printf("shmctl shared mem delete NG\n");
			exit(1);
		} 				
		printf("shmctl shared mem delete OK\n"); 
	}

	return 0;
}


try

shmid=shmget(KEY1, 4*1, SHM_R | SHM_W| IPC_CREAT);

This will create the segment if it does not exists
 

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LEARN ABOUT DEBIAN

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 <machine/param.h>
     #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
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