sched_setscheduler in POSIX, about thread

liuake

NAME
sched_setscheduler - set scheduling policy and parameters (REALTIME)
SYNOPSIS
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#include
int sched_setscheduler(pid_t pid, int policy,
        const struct sched_param *param);

DESCRIPTION
The sched_setscheduler() function shall set the scheduling policy and scheduling parameters of the process specified by
pid to policy and the parameters specified in the sched_param structure pointed to by param,
respectively. The value of the sched_priority member in the sched_param structure shall be any integer within the
inclusive priority range for the scheduling policy specified by policy. If the value of pid is negative, the behavior
of the sched_setscheduler() function is unspecified.
The possible values for the policy parameter are defined in the

header.
If a process specified by pid exists, and if the calling process has permission, the scheduling policy and scheduling
parameters shall be set for the process whose process ID is equal to pid.
If pid is zero, the scheduling policy and scheduling parameters shall be set for the calling process.
The conditions under which one process has the appropriate privilege to change the scheduling parameters of another process are
implementation-defined.
Implementations may require that the requesting process have permission to set its own scheduling parameters or those of another
process. Additionally, implementation-defined restrictions may apply as to the appropriate privileges required to set a process'
own scheduling policy, or another process' scheduling policy, to a particular value.
The sched_setscheduler() function shall be considered successful if it succeeds in setting the scheduling policy and
scheduling parameters of the process specified by pid to the values specified by policy and the structure pointed to
by param, respectively.
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If
the scheduling policy specified by policy is SCHED_SPORADIC, the value specified by the sched_ss_low_priority member
of the param argument shall be any integer within the inclusive priority range for the sporadic server policy. The
sched_ss_repl_period and sched_ss_init_budget members of the param argument shall represent the time
parameters used by the sporadic server scheduling policy for the target process. The sched_ss_max_repl member of the
param argument shall represent the maximum number of replenishments that are allowed to be pending simultaneously for the
process scheduled under this scheduling policy.
The specified sched_ss_repl_period shall be greater than or equal to the specified sched_ss_init_budget for the
function to succeed; if it is not, then the function shall fail.
The value of sched_ss_max_repl shall be within the inclusive range [1, {SS_REPL_MAX}] for the function to succeed; if
not, the function shall fail.
If the scheduling policy specified by policy is either SCHED_FIFO or SCHED_RR, the sched_ss_low_priority,
sched_ss_repl_period, and sched_ss_init_budget members of the param argument shall have no effect on the
scheduling behavior.

The effect of this function on individual threads is dependent on the scheduling contention scope of the threads:

• 
  For threads with system scheduling contention scope, these functions shall have no effect on their scheduling.
  
• 
  For threads with process scheduling contention scope, the threads' scheduling policy and associated parameters shall not be
  affected. However, the scheduling of these threads with respect to threads in other processes may be dependent on the scheduling
  parameters of their process, which are governed using these functions.
  

If an implementation supports a two-level scheduling model in which library threads are multiplexed on top of several
kernel-scheduled entities, then the underlying kernel-scheduled entities for the system contention scope threads shall not be
affected by these functions.
The underlying kernel-scheduled entities for the process contention scope threads shall have their scheduling policy and
associated scheduling parameters changed to the values specified in policy and param, respectively. Kernel-scheduled
entities for use by process contention scope threads that are created after this call completes shall inherit their scheduling
policy and associated scheduling parameters from the process.
This function is not atomic with respect to other threads in the
process. Threads may continue to execute while this function
call is in the process of changing the scheduling policy and associated
scheduling parameters for the underlying kernel-scheduled
entities used by the process contention scope threads.
RETURN VALUE
Upon successful completion, the function shall return the former scheduling policy of the specified process. If the
sched_setscheduler() function fails to complete successfully, the policy and scheduling parameters shall remain unchanged,
and the function shall return a value of -1 and set errno to indicate the error.
ERRORS
The sched_setscheduler() function shall fail if:
[EINVAL]The value of the policy parameter is invalid, or one or more of the parameters contained in param is outside the
valid range for the specified scheduling policy.[EPERM]The requesting process does not have permission to set either or both of the scheduling parameters or the scheduling policy of
the specified process.[ESRCH]No process can be found corresponding to that specified by pid.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
sched_getparam()
,
sched_getscheduler()
,
sched_setparam()
, the Base Definitions volume of IEEE Std 1003.1-2001,

CHANGE HISTORY
First released in Issue 5. Included for alignment with the POSIX Realtime Extension.
Issue 6
The sched_setscheduler() function is marked as part of the Process Scheduling option.
The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Process
Scheduling option.
The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• 
  In the DESCRIPTION, the effect of this function on a thread's scheduling parameters is added.
  
• 
  Sections describing two-level scheduling and atomicity of the function are added.
  

The SCHED_SPORADIC scheduling policy is added for alignment with IEEE Std 1003.1d-1999.
               
               
               

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