pthreads & shared memory
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From: Vandana on 26 Feb 2010 16:06 Hello All, I am using fork () command to create 2 processes and using shared memory to communicate between the processes. I have couple of questions. 1. When either of the process access the shared memory, do I need to have a mutex around the whole shared memory? If yes, then how do I create a mutex that can be seen by both process, do I have to declare it in the shared memory? for ex: struct shared { int data; char *something; pthread_spinlock_t shared_lock; } can i use this spinlock, so that each process acquires the lock then writes . reads to the shared memory? Is this correct? 2. How do I make the child process wait on a condition that is set by the parent. ie, I want the child process to do nothing until the condition set by the parent is reset again by the parent. Thanks, Vadana
From: David Schwartz on 26 Feb 2010 16:49 On Feb 26, 1:06 pm, Vandana <nair...(a)gmail.com> wrote: > I am using fork () command to create 2 processes and using shared > memory to communicate between the processes. I have couple of > questions. > 1. When either of the process access the shared memory, do I need to > have a mutex around the whole shared memory? You probably need a mutex if one thread might modify data while another thread is reading or modifying it. > If yes, then how do I > create a mutex that can be seen by both process, do I have to declare > it in the shared memory? Yes. See 'pthread_mutexattr_setpshared' too. > for ex: > struct shared { > > int data; > char *something; > pthread_spinlock_t shared_lock; } > > can i use this spinlock, so that each process acquires the lock then > writes . reads to the shared memory? Is this correct? I don't think you can create a process shared spinlock. I believe you have to use a mutex. > 2. How do I make the child process wait on a condition that is set by > the parent. ie, I want the child process to do nothing until the > condition set by the parent is reset again by the parent. The same way you would do it with two threads. Acquire a mutex, check a shared variable. If it's not set to allow forward progress, call pthread_cond_wait. Then check the variable again. Once it's set to allow forward progress, release the mutex, if appropriate. DS
From: ~Glynne on 26 Feb 2010 21:01 On Feb 26, 2:06 pm, Vandana <nair...(a)gmail.com> wrote: > Hello All, > > I am using fork () command to create 2 processes and using shared > memory to communicate between the processes. I have couple of > questions. > > 1. When either of the process access the shared memory, do I need to > have a mutex around the whole shared memory? If yes, then how do I > create a mutex that can be seen by both process, do I have to declare > it in the shared memory? > > for ex: > struct shared { > > int data; > char *something; > pthread_spinlock_t shared_lock; } > > can i use this spinlock, so that each process acquires the lock then > writes . reads to the shared memory? Is this correct? > > 2. How do I make the child process wait on a condition that is set by > the parent. ie, I want the child process to do nothing until the > condition set by the parent is reset again by the parent. > > Thanks, > Vadana Since you're talking about separate *processes* (not threads), you could use something much simpler: // create lock file int h= fileno( tmpfile() ); lseek( h, 0, SEEK_SET ); // usage lockf( h, F_LOCK, 0 ); // // do shared memory updates here // lockf( h, F_ULOCK, 0 );
From: ~Glynne on 26 Feb 2010 21:08 On Feb 26, 7:01 pm, "~Glynne" <glynnec2...(a)yahoo.com> wrote: > On Feb 26, 2:06 pm, Vandana <nair...(a)gmail.com> wrote: > > > > > Hello All, > > > I am using fork () command to create 2 processes and using shared > > memory to communicate between the processes. I have couple of > > questions. > > > 1. When either of the process access the shared memory, do I need to > > have a mutex around the whole shared memory? If yes, then how do I > > create a mutex that can be seen by both process, do I have to declare > > it in the shared memory? > > > for ex: > > struct shared { > > > int data; > > char *something; > > pthread_spinlock_t shared_lock; } > > > can i use this spinlock, so that each process acquires the lock then > > writes . reads to the shared memory? Is this correct? > > > 2. How do I make the child process wait on a condition that is set by > > the parent. ie, I want the child process to do nothing until the > > condition set by the parent is reset again by the parent. > > > Thanks, > > Vadana > > Since you're talking about separate *processes* (not threads), you > could use something much simpler: > > // create lock file > int h= fileno( tmpfile() ); > lseek( h, 0, SEEK_SET ); > > // usage > lockf( h, F_LOCK, 0 ); > // > // do shared memory updates here > // > lockf( h, F_ULOCK, 0 ); Obviously you should create the lock file *before* forking ;-)
From: David Schwartz on 26 Feb 2010 22:53
On Feb 26, 6:01 pm, "~Glynne" <glynnec2...(a)yahoo.com> wrote: > Since you're talking about separate *processes* (not threads), you > could use something much simpler: > > // create lock file > int h= fileno( tmpfile() ); > lseek( h, 0, SEEK_SET ); > > // usage > lockf( h, F_LOCK, 0 ); > // > // do shared memory updates here > // > lockf( h, F_ULOCK, 0 ); What keeps the memory accesses from migrating around the 'lockf' calls? Suppose his CPU has a write posting buffer and that posting buffer holds a write just before the unlock operation and doesn't commit it to memory/cache until after the unlock completes? DS |