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
#include
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.
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For inter-process synchronization, a mutex needs to be allo-
cated 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 ini-
tialization 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 attri-
bute 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 con-
sistent, since this state could have been left incon-
sistent 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
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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 ENOTRECOVER-
ABLE. 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
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Standard C Library Functions mutex_init(3C)
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 attri-
bute is the same as the POSIX PTHREAD_PRIO_PROTECT attri-
bute. 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 mutu-
ally 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 initial-
ized 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_ROBUSTabove. If default mutex attri-
butes are used, the macro DEFAULTMUTEX can be used to ini-
tialize mutexes that are statically allocated.
Default mutex initialization (intra-process):
mutex_t mp;
mutex_init(&mp, USYNC_THREAD, NULL);
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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.
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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:
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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 ini-
tialized and has not yet been destroyed.
EINVAL The mutex pointed to by mp was previously initial-
ized 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 max-
imum 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.
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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. Subse-
quent 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 pro-
tecting 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 EOWNER-
DEAD 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.
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/* cc thisfile.c -lthread */
#define _REENTRANT
#include
#include
#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
#include
#include
#include
#include
#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);
};
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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
#include
#include
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Standard C Library Functions mutex_init(3C)
#include
#include
#include
#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 */
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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 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.
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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
#include
#include
#include
#include
#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);
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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 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.
*/
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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(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Committed |
|_____________________________|_____________________________|
| MT-Level | MT-Safe |
|_____________________________|_____________________________|
SEE ALSO
mmap(2), shmop(2), pthread_mutexattr_getprioceiling(3C),
pthread_mutexattr_getprotocol(3C),
pthread_mutexattr_getrobust(3C),
pthread_mutexattr_gettype(3C),
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Standard C Library Functions mutex_init(3C)
pthread_mutex_getprioceiling(3C), pthread_mutex_init(3C),
attributes(5), mutex(5), standards(5)
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 restor-
ing 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) func-
tions. 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 cir-
cumstances.
The mutex_lock(), mutex_unlock(), and mutex_trylock() func-
tions 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 unspeci-
fied behavior.
Uninitialized mutexes that are allocated locally could con-
tain 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.
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