Книга: Programming with POSIX® Threads

5.2.2 Condition variable attributes

5.2.2 Condition variable attributes

pthread_condattr_t attr;

int pthread_condattr_init (pthread_condattr_t *attr);

int pthread_condattr_destroy (

pthread_condattr_t *attr);

#ifdef _POSIX_THREAD_PROCESS_SHARED

int pthread_condattr_getpshared (

pthread_condattr_t *attr, int *pshared);

int pthread_condattr_setpshared (

pthread_condattr_t *attr, int pshared);

#endif

Pthreads defines only one attribute for condition variable creation, pshared. No system is required to implement this attribute, so check the system documentation before using it. You initialize a condition variable attributes object using pthread_condattr_init, specifying a pointer to a variable of type pthread_ condattr_t, as in cond_attr.c, shown next. You use that attributes object by passing its address to pthread_cond_init instead of the NULL value we've been using so far.

If your system defines _POSIX_THREAD_PROCESS_SHARED then it supports the pshared attribute. You set the pshared attribute by calling the function pthread_ condattr_setpshared. If you set the pshared attribute to the value PTHREAD_ PROCESS_SHARED, the condition variable can be used by threads in separate processes that have access to the memory where the condition variable (pthread_ cond_t) is initialized. The default value for this attribute is PTHREAD_PROCESS_PRIVATE.

The cond_attr.c program shows how to set a condition variable attributes object to create a condition variable using the pshared attribute. This example uses the default value, PTHREAD_PROCESS_PRIVATE, to avoid the additional complexity of creating shared memory and forking a process.

? cond_attr.c

1 #include <pthread.h>
2 #include "errors.h"

3
4 pthread_cond_t cond;

5
6 int main (int argc, char *argv[])
7 {
8  pthread_condattr_t cond_attr;
9  int status;

10
11  status = pthread_condattr_init (&cond_attr);
12  if (status != 0)
13  err_abort (status, "Create attr");
14 #ifdef _POSIX_THREAD_PROCESS_SHARED
15  status = pthread_condattr_setpshared (
16  &cond_attr, PTHREAD_PROCESS_PRIVATE);
17  if (status != 0)
18  err_abort (status, "Set pshared");
19 #endif
20  status = pthread_cond_init (&cond, &cond_attr);
21  if (status != 0)
22  err_abort (status, "Init cond");
23 return 0;
24 }

To make use of a PTHREAD_PROCESS_SHARED condition variable, you must also use a PTHREAD_PROCESS_SHARED mutex. That's because two threads that synchronize using a condition variable must also use the same mutex. Waiting for a condition variable automatically unlocks, and then locks, the associated mutex. So if the mutex isn't also created with PTHREAD_PROCESS_SHARED, the synchronization won't work.

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