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. No current type uses arg although a future type may specify additional behavior parameters by way of arg. The type argument can be one of the following:

USYNC_THREAD

The mutex can synchronize threads only in this process. The arg argument is ignored.

USYNC_PROCESS

The mutex can synchronize threads in this process and other processes. The arg argument is ignored. 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.

USYNC_PROCESS_ROBUST

The mutex can synchronize threads in this process and other processes robustly. At the time of process death, if the lock is held by the process, it is unlocked. The next owner of this 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 reinitialize the mutex by calling mutex_init() and then unlock the mutex. Only the new owner can make the mutex consistent. If another process then calls mutex_init(), the call will return EBUSY. If for any reason the new owner is not able to make the state consistent, it should not reinitialize the mutex 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. The mutex can be reused by uninitializing the mutex with mutex_destroy() and reinitializing it with mutex_init(). If the process that acquired the lock with EOWNERDEAD died, the next owner will acquire the lock with an error value of EOWNERDEAD. The arg argument is ignored. 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)) and memory must be zeroed before initialization. All the processes interested in the robust lock must call mutex_init() at least once to register robust mutex with the system and potentially initialize it. If the object is not allocated in such shared memory, it will not be shared between processes. If mutex_init() is called on a previously initialized mutex, mutex_init() will not reinitialize the mutex and will return the error valaue EBUSY.

If type is either USYNC_THREAD or USYNC_PROCESS, it can be augmented by the bitwise-inclusive-OR of one or both of the following flags:

LOCK_RECURSIVE

A thread attempting to relock this mutex without first unlocking it will succeed in locking the mutex.

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.

See pthread_mutexattr_settype(3THR) 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.

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:

• The same mutex must not be simultaneously initialized by multiple threads.
• A mutex lock must not be reinitialized while in use by other threads.

These rules do not apply to USYNC_PROCESS_ROBUST mutexes. See the description for USYNC_PROCESS_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, NULL, NULL);

or

mutex_init(&mp, USYNC_THREAD, NULL);

or

mutex_t  mp  =  DEFAULTMUTEX;

or

mutex_t mp;
mp = calloc(1, sizeof (mutex_t));

or

mutex_t mp;
mp = malloc(sizeof (mutex_t));
memset(mp, 0, sizeof (mutex_t));

Customized mutex initialization (inter-process):

mutex_init(&mp, USYNC_PROCESS, NULL);

Customized mutex initialization (inter-process):

mutex_init(&mp, USYNC_PROCESS_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)

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.

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.

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;
}

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;
}

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);
    }
}

A mutex can protect data that is shared among processes robustly. The mutex would need to be initialized as USYNC_PROCESS_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_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_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_ROBUST,0);
     }
     for(;;) {
         rc = mutex_lock(&buffer->Interprocess_mutex);
         switch (rc) {
             case EOWNERDEAD:
               /* lock acquired.
                * last owner died holding the lock, try to make
                * the state associated with the mutex consistent.
                * If so, make the robust lock consistent by
                * reinitializing it.
                */
               if (make_data_consistent())
                   mutex_init(&buffer->Interprocess_mutex,
                          USYNC_PROCESS_ROBUST,0);
               mutex_unlock(&buffer->Interprocess_mutex);
               case ENOTRECOVERABLE:
                 /* lock not acquired.
                  * last owner got the mutex with EOWNERDEAD
                  * mutex is not consistent (and data?),
                  * so return from here
                  */
                 exit(1);
                 break;
           case 0:
               /* 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);
 }

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.