Programming Forums > Application Development > C

SAWG · Apr 26th, 2008, 7:02 PM

Hi. I'm having difficulties with shared memory.
I have a structure which looks like this:

 c Syntax (Toggle Plain Text) 
struct sharemem{
  int *actual
  int *end;
  int *sharearr;
}
struct sharemem{
  int *actual
  int *end;
  int *sharearr;
}

I use shmget (IPC_PRIVATE, sizeof(struct sharemem), IPC_CREAT | 0777) to get an ID.
Afterwards I use fork(), the child jumps into a function which attaches the shared memory ID of the structure, mallocs the *sharearr pointer(I pass on the size to the function via an argument),and fills it with integers ranging from 1 to (size+1).
The problem comes up when I try to access the array with the parent function(the parent is suspended until the child is done filling the array). The pointers seem to be actually shared, I just can't get access to any of the members of the array.
Could someone please tell me what I'm doing wrong?

The Dark · Apr 28th, 2008, 12:24 AM

I haven't used shmget, but I would imagine you need to allocate the sharearr array using smget as well. This means that your structure would need to contain the sharearr id rather than the pointer. Then your calling process would have to call shmat to get a pointer to the memory.

Apr 26th, 2008, 7:02 PM	#1
SAWG Newbie Join Date: Apr 2008 Posts: 1 Rep Power: 0	structure with an array in shared memory Hi. I'm having difficulties with shared memory. I have a structure which looks like this: c Syntax (Toggle Plain Text) struct sharemem{ int actual int end; int sharearr; } struct sharemem{ int actual int end; int sharearr; } I use shmget (IPC_PRIVATE, sizeof(struct sharemem), IPC_CREAT \| 0777) to get an ID. Afterwards I use fork(), the child jumps into a function which attaches the shared memory ID of the structure, mallocs the *sharearr pointer(I pass on the size to the function via an argument),and fills it with integers ranging from 1 to (size+1). The problem comes up when I try to access the array with the parent function(the parent is suspended until the child is done filling the array). The pointers seem to be actually shared, I just can't get access to any of the members of the array. Could someone please tell me what I'm doing wrong?

Apr 28th, 2008, 12:24 AM	#2
The Dark Expert Programmer Join Date: Jun 2005 Posts: 816 Rep Power: 4	Re: structure with an array in shared memory I haven't used shmget, but I would imagine you need to allocate the sharearr array using smget as well. This means that your structure would need to contain the sharearr id rather than the pointer. Then your calling process would have to call shmat to get a pointer to the memory.

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Programming Forums > Application Development > C

structure with an array in shared memory