NAME
pthread_setschedparam, pthread_getschedparam - set/get scheduling
policy and parameters of a thread
SYNOPSIS
#include <pthread.h>
pthread_setschedparam(pthread_t thread, int policy,
const struct sched_param *param);
pthread_getschedparam(pthread_t thread, int *policy,
struct sched_param *param);
Compile and link with -pthread.
DESCRIPTION
The pthread_setschedparam() function sets the scheduling policy and
parameters of the thread thread.
policy specifies the new scheduling policy for thread. The supported
values for policy, and their semantics, are described in
sched_setscheduler(2).
The structure pointed to by param specifies the new scheduling
parameters for thread. Scheduling parameters are maintained in the
following structure:
struct sched_param {
int sched_priority; /* Scheduling priority */
};
As can be seen, only one scheduling parameter is supported. For
details of the permitted ranges for scheduling priorities in each
scheduling policy, see sched_setscheduler(2).
The pthread_getschedparam() function returns the scheduling policy and
parameters of the thread thread, in the buffers pointed to by policy
and param, respectively. The returned priority value is that set by
the most recent pthread_setschedparam(), pthread_setschedprio(3), or
pthread_create(3) call that affected thread. The returned priority
does not reflect any temporary priority adjustments as a result of
calls to any priority inheritance or priority ceiling functions (see,
for example, pthread_mutexattr_setprioceiling(3) and
pthread_mutexattr_setprotocol(3)).
RETURN VALUE
On success, these functions return 0; on error, they return a nonzero
error number. If pthread_setschedparam() fails, the scheduling policy
and parameters of thread are not changed.
ERRORS
Both of these functions can fail with the following error:
ESRCH No thread with the ID thread could be found.
pthread_setschedparam() may additionally fail with the following
errors:
EINVAL policy is not a recognized policy, or param does not make sense
for the policy.
EPERM The caller does not have appropriate privileges to set the
specified scheduling policy and parameters.
POSIX.1-2001 also documents an ENOTSUP ("attempt was made to set the
policy or scheduling parameters to an unsupported value") error for
pthread_setschedparam().
CONFORMING TO
POSIX.1-2001.
NOTES
For a description of the permissions required to, and the effect of,
changing a thread's scheduling policy and priority, and details of the
permitted ranges for priorities in each scheduling policy, see
sched_setscheduler(2).
EXAMPLE
The program below demonstrates the use of pthread_setschedparam() and
pthread_getschedparam(), as well as the use of a number of other
scheduling-related pthreads functions.
In the following run, the main thread sets its scheduling policy to
SCHED_FIFO with a priority of 10, and initializes a thread attributes
object with a scheduling policy attribute of SCHED_RR and a scheduling
priority attribute of 20. The program then sets (using
pthread_attr_setinheritsched(3)) the inherit scheduler attribute of the
thread attributes object to PTHREAD_EXPLICIT_SCHED, meaning that
threads created using this attributes object should take their
scheduling attributes from the thread attributes object. The program
then creates a thread using the thread attributes object, and that
thread displays its scheduling policy and priority.
$ su # Need privilege to set real-time scheduling policies
Password:
# ./a.out -mf10 -ar20 -i e
Scheduler settings of main thread
policy=SCHED_FIFO, priority=10
Scheduler settings in 'attr'
policy=SCHED_RR, priority=20
inheritsched is EXPLICIT
Scheduler attributes of new thread
policy=SCHED_RR, priority=20
In the above output, one can see that the scheduling policy and
priority were taken from the values specified in the thread attributes
object.
The next run is the same as the previous, except that the inherit
scheduler attribute is set to PTHREAD_INHERIT_SCHED, meaning that
threads created using the thread attributes object should ignore the
scheduling attributes specified in the attributes object and instead
take their scheduling attributes from the creating thread.
# ./a.out -mf10 -ar20 -i i
Scheduler settings of main thread
policy=SCHED_FIFO, priority=10
Scheduler settings in 'attr'
policy=SCHED_RR, priority=20
inheritsched is INHERIT
Scheduler attributes of new thread
policy=SCHED_FIFO, priority=10
In the above output, one can see that the scheduling policy and
priority were taken from the creating thread, rather than the thread
attributes object.
Note that if we had omitted the -i i option, the output would have been
the same, since PTHREAD_INHERIT_SCHED is the default for the inherit
scheduler attribute.
Program source
/* pthreads_sched_test.c */
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#define handle_error_en(en, msg) \
do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
static void
usage(char *prog_name, char *msg)
{
if (msg != NULL)
fputs(msg, stderr);
fprintf(stderr, "Usage: %s [options]\n", prog_name);
fprintf(stderr, "Options are:\n");
#define fpe(msg) fprintf(stderr, "\t%s", msg); /* Shorter */
fpe("-a<policy><prio> Set scheduling policy and priority in\n");
fpe(" thread attributes object\n");
fpe(" <policy> can be\n");
fpe(" f SCHED_FIFO\n");
fpe(" r SCHED_RR\n");
fpe(" o SCHED_OTHER\n");
fpe("-A Use default thread attributes object\n");
fpe("-i {e|s} Set inherit scheduler attribute to\n");
fpe(" 'explicit' or 'inherit'\n");
fpe("-m<policy><prio> Set scheduling policy and priority on\n");
fpe(" main thread before pthread_create() call\n");
exit(EXIT_FAILURE);
}
static int
get_policy(char p, int *policy)
{
switch (p) {
case 'f': *policy = SCHED_FIFO; return 1;
case 'r': *policy = SCHED_RR; return 1;
case 'o': *policy = SCHED_OTHER; return 1;
default: return 0;
}
}
static void
display_sched_attr(int policy, struct sched_param *param)
{
printf(" policy=%s, priority=%d\n",
(policy == SCHED_FIFO) ? "SCHED_FIFO" :
(policy == SCHED_RR) ? "SCHED_RR" :
(policy == SCHED_OTHER) ? "SCHED_OTHER" :
"???",
param->sched_priority);
}
static void
display_thread_sched_attr(char *msg)
{
int policy, s;
struct sched_param param;
s = pthread_getschedparam(pthread_self(), &policy, ¶m);
if (s != 0)
handle_error_en(s, "pthread_getschedparam");
printf("%s\n", msg);
display_sched_attr(policy, ¶m);
}
static void *
thread_start(void *arg)
{
display_thread_sched_attr("Scheduler attributes of new thread");
return NULL;
}
int
main(int argc, char *argv[])
{
int s, opt, inheritsched, use_null_attrib, policy;
pthread_t thread;
pthread_attr_t attr;
pthread_attr_t *attrp;
char *attr_sched_str, *main_sched_str, *inheritsched_str;
struct sched_param param;
/* Process command-line options */
use_null_attrib = 0;
attr_sched_str = NULL;
main_sched_str = NULL;
inheritsched_str = NULL;
while ((opt = getopt(argc, argv, "a:Ai:m:")) != -1) {
switch (opt) {
case 'a': attr_sched_str = optarg; break;
case 'A': use_null_attrib = 1; break;
case 'i': inheritsched_str = optarg; break;
case 'm': main_sched_str = optarg; break;
default: usage(argv[0], "Unrecognized option\n");
}
}
if (use_null_attrib &&
(inheritsched_str != NULL || attr_sched_str != NULL))
usage(argv[0], "Can't specify -A with -i or -a\n");
/* Optionally set scheduling attributes of main thread,
and display the attributes */
if (main_sched_str != NULL) {
if (!get_policy(main_sched_str[0], &policy))
usage(argv[0], "Bad policy for main thread (-s)\n");
param.sched_priority = strtol(&main_sched_str[1], NULL, 0);
s = pthread_setschedparam(pthread_self(), policy, ¶m);
if (s != 0)
handle_error_en(s, "pthread_setschedparam");
}
display_thread_sched_attr("Scheduler settings of main thread");
printf("\n");
/* Initialize thread attributes object according to options */
attrp = NULL;
if (!use_null_attrib) {
s = pthread_attr_init(&attr);
if (s != 0)
handle_error_en(s, "pthread_attr_init");
attrp = &attr;
}
if (inheritsched_str != NULL) {
if (inheritsched_str[0] == 'e')
inheritsched = PTHREAD_EXPLICIT_SCHED;
else if (inheritsched_str[0] == 'i')
inheritsched = PTHREAD_INHERIT_SCHED;
else
usage(argv[0], "Value for -i must be 'e' or 'i'\n");
s = pthread_attr_setinheritsched(&attr, inheritsched);
if (s != 0)
handle_error_en(s, "pthread_attr_setinheritsched");
}
if (attr_sched_str != NULL) {
if (!get_policy(attr_sched_str[0], &policy))
usage(argv[0],
"Bad policy for 'attr' (-a)\n");
param.sched_priority = strtol(&attr_sched_str[1], NULL, 0);
s = pthread_attr_setschedpolicy(&attr, policy);
if (s != 0)
handle_error_en(s, "pthread_attr_setschedpolicy");
s = pthread_attr_setschedparam(&attr, ¶m);
if (s != 0)
handle_error_en(s, "pthread_attr_setschedparam");
}
/* If we initialized a thread attributes object, display
the scheduling attributes that were set in the object */
if (attrp != NULL) {
s = pthread_attr_getschedparam(&attr, ¶m);
if (s != 0)
handle_error_en(s, "pthread_attr_getschedparam");
s = pthread_attr_getschedpolicy(&attr, &policy);
if (s != 0)
handle_error_en(s, "pthread_attr_getschedpolicy");
printf("Scheduler settings in 'attr'\n");
display_sched_attr(policy, ¶m);
s = pthread_attr_getinheritsched(&attr, &inheritsched);
printf(" inheritsched is %s\n",
(inheritsched == PTHREAD_INHERIT_SCHED) ? "INHERIT" :
(inheritsched == PTHREAD_EXPLICIT_SCHED) ? "EXPLICIT" :
"???");
printf("\n");
}
/* Create a thread that will display its scheduling attributes */
s = pthread_create(&thread, attrp, &thread_start, NULL);
if (s != 0)
handle_error_en(s, "pthread_create");
/* Destroy unneeded thread attributes object */
s = pthread_attr_destroy(&attr);
if (s != 0)
handle_error_en(s, "pthread_attr_destroy");
s = pthread_join(thread, NULL);
if (s != 0)
handle_error_en(s, "pthread_join");
exit(EXIT_SUCCESS);
}
SEE ALSO
getrlimit(2), sched_get_priority_min(2), sched_setscheduler(2),
pthread_attr_init(3), pthread_attr_setinheritsched(3),
pthread_attr_setschedparam(3), pthread_attr_setschedpolicy(3),
pthread_create(3), pthread_self(3), pthread_setschedprio(3),
pthreads(7)
COLOPHON
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