MUTEX_INIT(3C) Standard C Library Functions MUTEX_INIT(3C)

NAME

mutex_init, mutex_lock, mutex_trylock, mutex_unlock,
mutex_consistent, mutex_destroy - mutual exclusion locks

SYNOPSIS

cc -mt [ flag... ] file... [ library... ]
#include <thread.h>
#include <synch.h>

int mutex_init(mutex_t *mp, int type, void *arg);


int mutex_lock(mutex_t *mp);


int mutex_trylock(mutex_t *mp);


int mutex_unlock(mutex_t *mp);


int mutex_consistent(mutex_t *mp);


int mutex_destroy(mutex_t *mp);

DESCRIPTION

Mutual exclusion locks (mutexes) prevent multiple threads from
simultaneously executing critical sections of code that access shared
data (that is, mutexes are used to serialize the execution of
threads). All mutexes must be global. A successful call for a mutex
lock by way of mutex_lock() will cause another thread that is also
trying to lock the same mutex to block until the owner thread unlocks
it by way of mutex_unlock(). Threads within the same process or
within other processes can share mutexes.


Mutexes can synchronize threads within the same process or in other
processes. Mutexes can be used to synchronize threads between
processes if the mutexes are allocated in writable memory and shared
among the cooperating processes (see mmap(2)), and have been
initialized for this task.

Initialize

Mutexes are either intra-process or inter-process, depending upon
the argument passed implicitly or explicitly to the initialization
of that mutex. A statically allocated mutex does not need to be
explicitly initialized; by default, a statically allocated mutex is
initialized with all zeros and its scope is set to be within the
calling process.


For inter-process synchronization, a mutex needs to be allocated in
memory shared between these processes. Since the memory for such a
mutex must be allocated dynamically, the mutex needs to be
explicitly initialized using mutex_init().


The mutex_init() function initializes the mutex referenced by mp
with the type specified by type. Upon successful initialization the
state of the mutex becomes initialized and unlocked. Only the
attribute type LOCK_PRIO_PROTECT uses arg. The type argument must be
one of the following:

USYNC_THREAD

The mutex can synchronize threads only in this process.


USYNC_PROCESS

The mutex can synchronize threads in this process and other
processes. The object initialized with this attribute must be
allocated in memory shared between processes, either in System V
shared memory (see shmop(2)) or in memory mapped to a file (see
mmap(2)). If the object is not allocated in such shared memory,
it will not be shared between processes.


The type argument can be augmented by the bitwise-inclusive-OR of
zero or more of the following flags:

LOCK_ROBUST

The mutex can synchronize threads robustly. At the time of thread
or process death, either by calling thr_exit() or exit() or due
to process abnormal termination, the lock is unlocked if is held
by the thread or process. The next owner of the mutex will
acquire it with an error return of EOWNERDEAD. The application
must always check the return value from mutex_lock() for a mutex
of this type. The new owner of this mutex should then attempt to
make the state protected by the mutex consistent, since this
state could have been left inconsistent when the last owner died.
If the new owner is able to make the state consistent, it should
call mutex_consistent() to restore the state of the mutex and
then unlock the mutex. All subsequent calls to mutex_lock() will
then behave normally. Only the new owner can make the mutex
consistent. If for any reason the new owner is not able to make
the state consistent, it should not call mutex_consistent() but
should simply unlock the mutex. All waiting processes will be
awakened and all subsequent calls to mutex_lock() will fail in
acquiring the mutex with an error value of ENOTRECOVERABLE. If
the thread or process that acquired the lock with EOWNERDEAD
terminates without unlocking the mutex, the next owner will
acquire the lock with an error value of EOWNERDEAD.

The memory for the object to be initialized with this attribute
must be zeroed before initialization. Any thread or process
interested in the robust lock can call mutex_init() to
potentially initialize it, provided that all such callers of
mutex_init() specify the same set of attribute flags. In this
situation, if mutex_init() is called on a previously initialized
robust mutex, mutex_init() will not reinitialize the mutex and
will return the error value EBUSY.


LOCK_RECURSIVE

A thread attempting to relock this mutex without first unlocking
it will succeed in locking the mutex. The mutex must be unlocked
as many times as it is locked.


LOCK_ERRORCHECK

Unless LOCK_RECURSIVE is also set, a thread attempting to relock
this mutex without first unlocking it will return with an error
rather than deadlocking itself. A thread attempting to unlock
this mutex without first owning it will return with an error.


LOCK_PRIO_INHERIT

When a thread is blocking higher priority threads because of
owning one or more mutexes with the LOCK_PRIO_INHERIT attribute,
it executes at the higher of its priority or the priority of the
highest priority thread waiting on any of the mutexes owned by
this thread and initialized with this attribute.


LOCK_PRIO_PROTECT

When a thread owns one or more mutexes initialized with the
LOCK_PRIO_PROTECT attribute, it executes at the higher of its
priority or the highest of the priority ceilings of all the
mutexes owned by this thread and initialized with this attribute,
regardless of whether other threads are blocked on any of these
mutexes. When this attribute is specified, arg must point to an
int containing the priority ceiling.


See pthread_mutexattr_getrobust(3C) for more information about robust
mutexes. The LOCK_ROBUST attribute is the same as the POSIX
PTHREAD_MUTEX_ROBUST attribute.


See pthread_mutexattr_settype(3C) for more information on recursive
and error checking mutex types. The combination (LOCK_RECURSIVE |
LOCK_ERRORCHECK) is the same as the POSIX PTHREAD_MUTEX_RECURSIVE
type. By itself, LOCK_ERRORCHECK is the same as the POSIX
PTHREAD_MUTEX_ERRORCHECK type.


The LOCK_PRIO_INHERIT attribute is the same as the POSIX
PTHREAD_PRIO_INHERIT attribute. The LOCK_PRIO_PROTECT attribute is
the same as the POSIX PTHREAD_PRIO_PROTECT attribute. See
pthread_mutexattr_getprotocol(3C),
pthread_mutexattr_getprioceiling(3C), and
pthread_mutex_getprioceiling(3C) for a full discussion. The
LOCK_PRIO_INHERIT and LOCK_PRIO_PROTECT attributes are mutually
exclusive. Specifying both of these attributes causes mutex_init() to
fail with EINVAL.


Initializing mutexes can also be accomplished by allocating in
zeroed memory (default), in which case a type of USYNC_THREAD is
assumed. In general, the following rules apply to mutex
initialization:

o The same mutex must not be simultaneously initialized by
multiple threads.

o A mutex lock must not be reinitialized while in use by
other threads.


These rules do not apply to LOCK_ROBUST mutexes. See the description
for LOCK_ROBUST above. If default mutex attributes are used, the
macro DEFAULTMUTEX can be used to initialize mutexes that are
statically allocated.


Default mutex initialization (intra-process):

mutex_t mp;
mutex_init(&mp, USYNC_THREAD, NULL);


or

mutex_t mp = DEFAULTMUTEX;


Customized mutex initialization (inter-process):

mutex_init(&mp, USYNC_PROCESS, NULL);


Customized mutex initialization (inter-process robust):

mutex_init(&mp, USYNC_PROCESS | LOCK_ROBUST, NULL);


Statically allocated mutexes can also be initialized with macros
specifying LOCK_RECURSIVE and/or LOCK_ERRORCHECK:

mutex_t mp = RECURSIVEMUTEX;

Same as (USYNC_THREAD | LOCK_RECURSIVE)


mutex_t mp = ERRORCHECKMUTEX;

Same as (USYNC_THREAD | LOCK_ERRORCHECK)


mutex_t mp = RECURSIVE_ERRORCHECKMUTEX;

Same as (USYNC_THREAD | LOCK_RECURSIVE | LOCK_ERRORCHECK)

Lock and Unlock

A critical section of code is enclosed by a the call to lock the
mutex and the call to unlock the mutex to protect it from
simultaneous access by multiple threads. Only one thread at a time
may possess mutually exclusive access to the critical section of code
that is enclosed by the mutex-locking call and the mutex-unlocking
call, whether the mutex's scope is intra-process or inter-process. A
thread calling to lock the mutex either gets exclusive access to the
code starting from the successful locking until its call to unlock
the mutex, or it waits until the mutex is unlocked by the thread that
locked it.


Mutexes have ownership, unlike semaphores. Although any thread,
within the scope of a mutex, can get an unlocked mutex and lock
access to the same critical section of code, only the thread that
locked a mutex should unlock it.


If a thread waiting for a mutex receives a signal, upon return from
the signal handler, the thread resumes waiting for the mutex as if
there was no interrupt. A mutex protects code, not data; therefore,
strongly bind a mutex with the data by putting both within the same
structure, or at least within the same procedure.


A call to mutex_lock() locks the mutex object referenced by mp. If
the mutex is already locked, the calling thread blocks until the
mutex is freed; this will return with the mutex object referenced by
mp in the locked state with the calling thread as its owner. If the
current owner of a mutex tries to relock the mutex, it will result in
deadlock.


The mutex_trylock() function is the same as mutex_lock(),
respectively, except that if the mutex object referenced by mp is
locked (by any thread, including the current thread), the call
returns immediately with an error.


The mutex_unlock() function are called by the owner of the mutex
object referenced by mp to release it. The mutex must be locked and
the calling thread must be the one that last locked the mutex (the
owner). If there are threads blocked on the mutex object referenced
by mp when mutex_unlock() is called, the mp is freed, and the
scheduling policy will determine which thread gets the mutex. If the
calling thread is not the owner of the lock, no error status is
returned, and the behavior of the program is undefined.

Destroy

The mutex_destroy() function destroys the mutex object referenced by
mp. The mutex object becomes uninitialized. The space used by the
destroyed mutex variable is not freed. It needs to be explicitly
reclaimed.

RETURN VALUES

If successful, these functions return 0. Otherwise, an error number
is returned.

ERRORS

The mutex_init() function will fail if:

EINVAL
The value specified by type is invalid, or the
LOCK_PRIO_INHERIT and LOCK_PRIO_PROTECT attributes are both
specified.


The mutex_init() function will fail for LOCK_ROBUST type mutex if:

EBUSY
The mutex pointed to by mp was previously initialized and
has not yet been destroyed.


EINVAL
The mutex pointed to by mp was previously initialized with
a different set of attribute flags.


The mutex_trylock() function will fail if:

EBUSY
The mutex pointed to by mp is already locked.


The mutex_lock() and mutex_trylock() functions will fail for a
LOCK_RECURSIVE mutex if:

EAGAIN
The mutex could not be acquired because the maximum number
of recursive locks for the mutex has been reached.


The mutex_lock() function will fail for a LOCK_ERRORCHECK and
non-LOCK_RECURSIVE mutex if:

EDEADLK
The caller already owns the mutex.


The mutex_lock() function may fail for a non-LOCK_ERRORCHECK and
non-LOCK_RECURSIVE mutex if:

EDEADLK
The caller already owns the mutex.


The mutex_unlock() function will fail for a LOCK_ERRORCHECK mutex if:

EPERM
The caller does not own the mutex.


The mutex_lock() or mutex_trylock() functions will fail for
LOCK_ROBUST type mutex if:

EOWNERDEAD
The last owner of this mutex died while holding
the mutex. This mutex is now owned by the caller.
The caller must now attempt to make the state
protected by the mutex consistent. If it is able
to clean up the state, then it should restore the
state of the mutex by calling mutex_consistent()
and unlock the mutex. Subsequent calls to
mutex_lock() will behave normally, as before. If
the caller is not able to clean up the state,
mutex_consistent() should not be called but the
mutex should be unlocked. Subsequent calls to
mutex_lock() will fail to acquire the mutex,
returning with the error value ENOTRECOVERABLE. If
the owner who acquired the lock with EOWNERDEAD
dies, the next owner will acquire the lock with
EOWNERDEAD.


ENOTRECOVERABLE
The mutex trying to be acquired was protecting the
state that has been left unrecoverable when the
mutex's last owner could not make the state
protected by the mutex consistent. The mutex has
not been acquired. This condition occurs when the
lock was previously acquired with EOWNERDEAD and
the owner was not able to clean up the state and
unlocked the mutex without calling
mutex_consistent().


The mutex_consistent() function will fail if:

EINVAL
The caller does not own the mutex or the mutex is not a
LOCK_ROBUST mutex having an inconsistent state
(EOWNERDEAD).

EXAMPLES

Single Gate

The following example uses one global mutex as a gate-keeper to
permit each thread exclusive sequential access to the code within the
user-defined function "change_global_data." This type of
synchronization will protect the state of shared data, but it also
prohibits parallelism.

/* cc thisfile.c -lthread */
#define _REENTRANT
#include <stdio.h>
#include <thread.h>
#define NUM_THREADS 12
void *change_global_data(void *); /* for thr_create() */
main(int argc,char * argv[]) {
int i=0;
for (i=0; i< NUM_THREADS; i++) {
thr_create(NULL, 0, change_global_data, NULL, 0, NULL);
}
while ((thr_join(NULL, NULL, NULL) == 0));
}

void * change_global_data(void *null){
static mutex_t Global_mutex;
static int Global_data = 0;
mutex_lock(&Global_mutex);
Global_data++;
sleep(1);
printf("%d is global data\n",Global_data);
mutex_unlock(&Global_mutex);
return NULL;
}

Multiple Instruction Single Data

The previous example, the mutex, the code it owns, and the data it
protects was enclosed in one function. The next example uses C++
features to accommodate many functions that use just one mutex to
protect one data:

/* CC thisfile.c -lthread use C++ to compile*/

#define _REENTRANT
#include <stdlib.h>
#include <stdio.h>
#include <thread.h>
#include <errno.h>
#include <iostream.h>
#define NUM_THREADS 16
void *change_global_data(void *); /* for thr_create() */

class Mutected {
private:
static mutex_t Global_mutex;
static int Global_data;
public:
static int add_to_global_data(void);
static int subtract_from_global_data(void);
};

int Mutected::Global_data = 0;
mutex_t Mutected::Global_mutex;

int Mutected::add_to_global_data() {
mutex_lock(&Global_mutex);
Global_data++;
mutex_unlock(&Global_mutex);
return Global_data;
}

int Mutected::subtract_from_global_data() {
mutex_lock(&Global_mutex);
Global_data--;
mutex_unlock(&Global_mutex);
return Global_data;
}

void
main(int argc,char * argv[]) {
int i=0;
for (i=0;i< NUM_THREADS;i++) {
thr_create(NULL,0,change_global_data,NULL,0,NULL);
}
while ((thr_join(NULL,NULL,NULL) == 0));
}

void * change_global_data(void *) {
static int switcher = 0;
if ((switcher++ % 3) == 0) /* one-in-three threads subtracts */
cout << Mutected::subtract_from_global_data() << endl;
else
cout << Mutected::add_to_global_data() << endl;
return NULL;
}

Interprocess Locking

A mutex can protect data that is shared among processes. The mutex
would need to be initialized as USYNC_PROCESS. One process
initializes the process-shared mutex and writes it to a file to be
mapped into memory by all cooperating processes (see mmap(2)).
Afterwards, other independent processes can run the same program
(whether concurrently or not) and share mutex-protected data.

/* cc thisfile.c -lthread */
/* To execute, run the command line "a.out 0 &; a.out 1" */

#define _REENTRANT
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <thread.h>
#define INTERPROCESS_FILE "ipc-sharedfile"
#define NUM_ADDTHREADS 12
#define NUM_SUBTRACTTHREADS 10
#define INCREMENT '0'
#define DECREMENT '1'
typedef struct {
mutex_t Interprocess_mutex;
int Interprocess_data;
} buffer_t;
buffer_t *buffer;

void *add_interprocess_data(), *subtract_interprocess_data();
void create_shared_memory(), test_argv();
int zeroed[sizeof(buffer_t)];
int ipc_fd, i=0;

void
main(int argc,char * argv[]){
test_argv(argv[1]);

switch (*argv[1]) {
case INCREMENT:
/* Initializes the process-shared mutex */
/* Should be run prior to running a DECREMENT process */
create_shared_memory();
ipc_fd = open(INTERPROCESS_FILE, O_RDWR);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
buffer->Interprocess_data = 0;
mutex_init(&buffer->Interprocess_mutex, USYNC_PROCESS,0);
for (i=0; i< NUM_ADDTHREADS; i++)
thr_create(NULL, 0, add_interprocess_data, argv[1],
0, NULL);
break;

case DECREMENT:
/* Should be run after the INCREMENT process has run. */
while(ipc_fd = open(INTERPROCESS_FILE, O_RDWR)) == -1)
sleep(1);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
for (i=0; i< NUM_SUBTRACTTHREADS; i++)
thr_create(NULL, 0, subtract_interprocess_data, argv[1],
0, NULL);
break;
} /* end switch */

while ((thr_join(NULL,NULL,NULL) == 0));
} /* end main */

void *add_interprocess_data(char argv_1[]){
mutex_lock(&buffer->Interprocess_mutex);
buffer->Interprocess_data++;
sleep(2);
printf("%d is add-interprocess data, and %c is argv1\n",
buffer->Interprocess_data, argv_1[0]);
mutex_unlock(&buffer->Interprocess_mutex);
return NULL;
}

void *subtract_interprocess_data(char argv_1[]) {
mutex_lock(&buffer->Interprocess_mutex);
buffer->Interprocess_data--;
sleep(2);
printf("%d is subtract-interprocess data, and %c is argv1\n",
buffer->Interprocess_data, argv_1[0]);
mutex_unlock(&buffer->Interprocess_mutex);
return NULL;
}

void create_shared_memory(){
int i;
ipc_fd = creat(INTERPROCESS_FILE, O_CREAT | O_RDWR );
for (i=0; i<sizeof(buffer_t); i++){
zeroed[i] = 0;
write(ipc_fd, &zeroed[i],2);
}
close(ipc_fd);
chmod(INTERPROCESS_FILE, S_IRWXU | S_IRWXG | S_IRWXO);
}

void test_argv(char argv1[]) {
if (argv1 == NULL) {
printf("use 0 as arg1 for initial process\n \
or use 1 as arg1 for the second process\n");
exit(NULL);
}
}

Solaris Interprocess Robust Locking

A mutex can protect data that is shared among processes robustly. The
mutex would need to be initialized as USYNC_PROCESS | LOCK_ROBUST.
One process initializes the robust process-shared mutex and writes it
to a file to be mapped into memory by all cooperating processes (see
mmap(2)). Afterwards, other independent processes can run the same
program (whether concurrently or not) and share mutex-protected data.


The following example shows how to use a USYNC_PROCESS | LOCK_ROBUST
type mutex.

/* cc thisfile.c -lthread */
/* To execute, run the command line "a.out & a.out 1" */
#include <sys/types.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <thread.h>
#define INTERPROCESS_FILE "ipc-sharedfile"
typedef struct {
mutex_t Interprocess_mutex;
int Interprocess_data;
} buffer_t;
buffer_t *buffer;
int make_date_consistent();
void create_shared_memory();
int zeroed[sizeof(buffer_t)];
int ipc_fd, i=0;
main(int argc,char * argv[]) {
int rc;
if (argc > 1) {
while((ipc_fd = open(INTERPROCESS_FILE, O_RDWR)) == -1)
sleep(1);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
mutex_init(&buffer->Interprocess_mutex,
USYNC_PROCESS | LOCK_ROBUST, 0);
} else {
create_shared_memory();
ipc_fd = open(INTERPROCESS_FILE, O_RDWR);
buffer = (buffer_t *)mmap(NULL, sizeof(buffer_t),
PROT_READ | PROT_WRITE, MAP_SHARED, ipc_fd, 0);
buffer->Interprocess_data = 0;
mutex_init(&buffer->Interprocess_mutex,
USYNC_PROCESS | LOCK_ROBUST, 0);
}
for(;;) {
rc = mutex_lock(&buffer->Interprocess_mutex);
switch (rc) {
case EOWNERDEAD:
/*
* The lock is acquired.
* The last owner died holding the lock.
* Try to make the state associated with
* the mutex consistent.
* If successful, make the robust lock consistent.
*/
if (make_data_consistent())
mutex_consistent(&buffer->Interprocess_mutex);
mutex_unlock(&buffer->Interprocess_mutex);
break;
case ENOTRECOVERABLE:
/*
* The lock is not acquired.
* The last owner got the mutex with EOWNERDEAD
* and failed to make the data consistent.
* There is no way to recover, so just exit.
*/
exit(1);
case 0:
/*
* There is no error - data is consistent.
* Do something with data.
*/
mutex_unlock(&buffer->Interprocess_mutex);
break;
}
}
} /* end main */
void create_shared_memory() {
int i;
ipc_fd = creat(INTERPROCESS_FILE, O_CREAT | O_RDWR );
for (i=0; i<sizeof(buffer_t); i++) {
zeroed[i] = 0;
write(ipc_fd, &zeroed[i],2);
}
close(ipc_fd);
chmod(INTERPROCESS_FILE, S_IRWXU | S_IRWXG | S_IRWXO);
}

/* return 1 if able to make data consistent, otherwise 0. */
int make_data_consistent () {
buffer->Interprocess_data = 0;
return (1);
}

Dynamically Allocated Mutexes

The following example allocates and frees memory in which a mutex is
embedded.

struct record {
int field1;
int field2;
mutex_t m;
} *r;
r = malloc(sizeof(struct record));
mutex_init(&r->m, USYNC_THREAD, NULL);
/*
* The fields in this record are accessed concurrently
* by acquiring the embedded lock.
*/


The thread execution in this example is as follows:

Thread 1 executes: Thread 2 executes:

... ...
mutex_lock(&r->m); mutex_lock(&r->m);
r->field1++; localvar = r->field1;
mutex_unlock(&r->m); mutex_unlock(&r->m);
... ...


Later, when a thread decides to free the memory pointed to by r, the
thread should call mutex_destroy() on the mutexes in this memory.


In the following example, the main thread can do a thr_join() on
both of the above threads. If there are no other threads using the
memory in r, the main thread can now safely free r:

for (i = 0; i < 2; i++)
thr_join(0, 0, 0);
mutex_destroy(&r->m); /* first destroy mutex */
free(r); /* then free memory */


If the mutex is not destroyed, the program could have memory leaks.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Stable |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

mmap(2), shmop(2), pthread_mutex_getprioceiling(3C),
pthread_mutex_init(3C), pthread_mutexattr_getprioceiling(3C),
pthread_mutexattr_getprotocol(3C), pthread_mutexattr_getrobust(3C),
pthread_mutexattr_gettype(3C), attributes(7), mutex(7), standards(7)

NOTES

Previous releases of Solaris provided the USYNC_PROCESS_ROBUST mutex
type. This type is now deprecated but is still supported for source
and binary compatibility. When passed to mutex_init(), it is
transformed into (USYNC_PROCESS | LOCK_ROBUST). The former method for
restoring a USYNC_PROCESS_ROBUST mutex to a consistent state was to
reinitialize it by calling mutex_init(). This method is still
supported for source and binary compatibility, but the proper method
is to call mutex_consistent().


The USYNC_PROCESS_ROBUST type permitted an alternate error value,
ELOCKUNMAPPED, to be returned by mutex_lock() if the process
containing a locked robust mutex unmapped the memory containing the
mutex or performed one of the exec(2) functions. The ELOCKUNMAPPED
error value implies all of the consequences of the EOWNERDEAD error
value and as such is just a synonym for EOWNERDEAD. For full source
and binary compatibility, the ELOCKUNMAPPED error value is still
returned from mutex_lock() in these circumstances, but only if the
mutex was initialized with the USYNC_PROCESS_ROBUST type. Otherwise,
EOWNERDEAD is returned in these circumstances.


The mutex_lock(), mutex_unlock(), and mutex_trylock() functions do
not validate the mutex type. An uninitialized mutex or a mutex with
an invalid type does not return EINVAL. Interfaces for mutexes with
an invalid type have unspecified behavior.


Uninitialized mutexes that are allocated locally could contain junk
data. Such mutexes need to be initialized using mutex_init().


By default, if multiple threads are waiting for a mutex, the order of
acquisition is undefined.

September 7, 2015 MUTEX_INIT(3C)