ipc_lock.h

/* Copyright 2012, Google Inc.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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#ifndef IPC_LOCK_H__
#define IPC_LOCK_H__

#include <sys/ipc.h>

struct ipc_lock {
	key_t key;         /* provided by the developer */
	int sem;           /* internal */
	int is_held;       /* internal */
};

/* don't use C99 initializers here, so this can be used in C++ code */
#define IPC_LOCK_INIT(key) \
	{ \
		key,       /* name */ \
		-1,        /* sem */ \
		0,         /* is_held */ \
	}

/*
 * acquire_lock: acquire a lock
 *
 * timeout <0 = no timeout (try forever)
 * timeout 0  = do not wait (return immediately)
 * timeout >0 = wait up to $timeout milliseconds (subject to kernel scheduling)
 *
 * return 0   = lock acquired
 * return >0  = lock was already held
 * return <0  = failed to acquire lock
 */
extern int acquire_lock(struct ipc_lock *lock, int timeout_msecs);

/*
 * release_lock: release a lock
 *
 * returns 0 if lock was released successfully
 * returns -1 if lock had not been held before the call
 */
extern int release_lock(struct ipc_lock *lock);

#endif /* IPC_LOCK_H__ */