qsem.c File Reference

Semaphore APIs. More...

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Functions
int	qsem_init (const char *keyfile, int keyid, int nsems, bool recreate)
	Initialize semaphore.
int	qsem_getid (const char *keyfile, int keyid)
	Get semaphore identifier by keyfile and keyid for the existing semaphore.
bool	qsem_enter (int semid, int semno)
	Turn on the flag of semaphore then entering critical section.
bool	qsem_enter_nowait (int semid, int semno)
	Try to turn on the flag of semaphore.
bool	qsem_enter_force (int semid, int semno, int maxwaitms, bool *forceflag)
	Force to turn on the flag of semaphore.
bool	qsem_leave (int semid, int semno)
	Turn off the flag of semaphore then leaving critical section.
bool	qsem_check (int semid, int semno)
	Get the status of semaphore.
bool	qsem_free (int semid)
	Release semaphore to system.

Detailed Description

Semaphore APIs.

Note

[daemon main]
#define MAX_SEMAPHORES (2)
 
// create semaphores
int semid = qsem_init("/some/file/for/generating/unique/key", 'q', MAX_SEMAPHORES, true);
if(semid < 0) {
  printf("ERROR: Can't initialize semaphores.\n");
  return -1;
}
 
// fork childs
(... child forking codes ...)
 
// at the end of daemon, free semaphores
if(semid >= 0) qsem_free(semid);
 
[forked child]
// critical section for resource 0
qsem_enter(semid, 0);
(... guaranteed as atomic procedure ...)
qsem_leave(semid, 0);
 
(... some codes ...)
 
// critical section for resource 1
qsem_enter(semid, 1);
(... guaranteed as atomic procedure ...)
qsem_leave(semid, 1);
 
[other program which uses resource 1]
int semid = qsem_getid("/some/file/for/generating/unique/key", 'q');
if(semid < 0) {
  printf("ERROR: Can't get semaphore id.\n");
  return -1;
}
 
// critical section for resource 1
qsem_enter(semid, 1);
(... guaranteed as atomic procedure ...)
qsem_leave(semid, 1);

Definition in file qsem.c.

Function Documentation

qsem_init()

int qsem_init	(	const char *	keyfile,
		int	keyid,
		int	nsems,
		bool	recreate
	)

Initialize semaphore.

Parameters

keyfile	seed for generating unique IPC key
keyid	seed for generating unique IPC key
nsems	number of semaphores to initialize
recreate	set to true to re-create semaphore if exists

Returns

non-negative shared memory identifier if successful, otherwise returns -1

int semid = qsem_init("/tmp/mydaemon.pid", 'q', 10, true);

Definition at line 102 of file qsem.c.

qsem_getid()

int qsem_getid	(	const char *	keyfile,
		int	keyid
	)

Get semaphore identifier by keyfile and keyid for the existing semaphore.

Parameters

keyfile	seed for generating unique IPC key
keyid	seed for generating unique IPC key

Returns

non-negative shared memory identifier if successful, otherwise returns -1

Definition at line 155 of file qsem.c.

qsem_enter()

bool qsem_enter	(	int	semid,
		int	semno
	)

Turn on the flag of semaphore then entering critical section.

Parameters

semid	semaphore identifier
semno	semaphore number

Returns

true if successful, otherwise returns false

Note

If the semaphore is already turned on, this will wait until released

Definition at line 180 of file qsem.c.

qsem_enter_nowait()

bool qsem_enter_nowait	(	int	semid,
		int	semno
	)

Try to turn on the flag of semaphore.

If it is already turned on, do not wait.

Parameters

semid	semaphore identifier
semno	semaphore number

Returns

true if successful, otherwise(already turned on by other) returns false

Definition at line 202 of file qsem.c.

qsem_enter_force()

bool qsem_enter_force	(	int	semid,
		int	semno,
		int	maxwaitms,
		bool *	forceflag
	)

Force to turn on the flag of semaphore.

Parameters

semid	semaphore identifier
semno	semaphore number
maxwaitms	maximum waiting micro-seconds to release
forceflag	status will be stored, it can be NULL if you don't need this information

Returns

true if successful, otherwise returns false

Note

This will wait the semaphore to be released with in maxwaitms. If it it released by locker normally with in maxwaitms, forceflag will be set to false. But if maximum maxwaitms is exceed and the semaphore is released forcely, forceflag will be set to true.

Definition at line 232 of file qsem.c.

qsem_leave()

bool qsem_leave	(	int	semid,
		int	semno
	)

Turn off the flag of semaphore then leaving critical section.

Parameters

semid	semaphore identifier
semno	semaphore number

Returns

true if successful, otherwise returns false

Definition at line 263 of file qsem.c.

qsem_check()

bool qsem_check	(	int	semid,
		int	semno
	)

Get the status of semaphore.

Parameters

semid	semaphore identifier
semno	semaphore number

Returns

true for the flag on, false for the flag off

Definition at line 285 of file qsem.c.

qsem_free()

bool qsem_free

(

int

semid

)

Release semaphore to system.

Parameters

semid

semaphore identifier

Returns

true if successful, otherwise returns false

Definition at line 298 of file qsem.c.