Each thread has a set of attributes when it is created such as:

• Initialize attributes
• Destroy attributes
• Detach state
• Scheduling parameters
• Stack size
• Stack Address
• Thread scope

• When a thread is created by pthread_create() API, and the attribute parameter is NULL, it is created with the default values for the above attributes.
• The threads attributes are specified only during the creation time, they cannot be altered once it is in use.
• But any thread can be created with values different from that of default values.

Initialized Attributes:
The pthread_attr_init() API can be used to initialize the object attributes to their default values. The storage is allocated by the thread system during execution.

/* Protocol for initializing the attributes */
int pthread_attr_init (pthread_attr_t *attr);

#include <pthread.h>
pthread_attr_t attr;
int ret = pthread_attr_init (&attr);

Return values:
It returns 0 on success and non zero value in case of any failure.

Few of the important attributes with their default value are:

Destroy Attributes

The thread attributes can be destroyed by using pthread_attr_destroy() API

/* Protocol for destroying the attributes */
int pthread_attr_destroy (pthread_attr_t *attr);

#include <pthread.h>
pthread_attr_t attr;
int res = pthread_attr_destroy (&attr);

Return values:
It returns 0 on success and non-zero in case of failure.

Detached state

• By default, a thread is created in a joinable mode that its exit status and an identifier exist in a system even when a thread is terminated.
• Thus pthread_join() must be used to get the exit status and release all the resources of the tread so that it can be reuse.
• This default behavior can be changed to a detached state by using pthread_attr_setdetachstate() API
• Refer to pthread_detach() for detaching a thread.

/* Protocol for setting the detachstate */
int pthread_attr_setdetachstate (pthread_attr_t *attr, int             detachstate);

/*Protocol for getting the detachstate */
int pthreead_attr_getdetachstate(pthread_attr_t *attr, int detahcstate);

Sample Code:

#include <pthread.h>
pthread_attr_t attr;
pthread_t th_id;
int res, detach_state;

/* Initialize the attribute */
res = pthread_attr_init (&attr);

/*Set the detach state */
res = pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACH);

/*Create the thread with detach state */
res = pthread_create (&th_id, &attr, fun, NULL);

/*Get the detach state value */
res = pthread_attr_getdetachstate(&attr, &detach_state);

Return values:
It returns 0 on success and non-zero on failure.

Scheduling Attributes:

• The POSIX thread specifies three attributes for scheduling policy:
  • SCHED_FIFO
  • SCHED_RR
  • SCHED_OTHERS
• SCHED_OTHERS is the default scheduling policy attribute.

/*Protocol for setting the scheduling policy */
int pthread_attr_setschedpolicy (pthread_attr_t *attr, int policy);

/*Protocol for getting the scheduling policy */
int pthread_attr_getschedpolicy (pthread_attr_t *attr, int *policy);

Sample Code:

#include <pthread.h>

pthread_attr_t attr;
int policy, res;
pthread_t th_id;

/* Initialize the attribute *
res = pthread_attr_init (&attr);

/* Get the default scheduling policy */
res = pthread_attr_getschedpolicy (&attr, &policy);
printf("\n Default policy is %d\n", policy);

/* Setting a new scheduling policy */
res = pthread_attr_setschedpolicy (&attr, SCHED_FIFO);
res = pthread_create (&th_id, &attr, fun, NULL);

/* Fetching the new value */
res = pthread_attr_getschedpolicy (&attr, &policy);
printf("\n New policy is %d\n", policy);

Return values:
It returns 0 on success and non-zero in case of failure

Relevant Post:

• POSIX Thread APIs
• Thread creates attributes
• Thread attributes(TCB)
• Characteristics of a Thread
• Odd-even problem

Reference:
https://docs.oracle.com/cd/E19455-01/806-5257/6je9h032l/index.html#attrib-76485

Categories: Operating system (OS)