This section describes functions in the GNU C library for setting the absolute priority and scheduling policy of a process.
Portability Note: On systems that have the functions in this section, the macro _POSIX_PRIORITY_SCHEDULING is defined in `<unistd.h>'.
For the case that the scheduling policy is traditional scheduling, more functions to fine tune the scheduling are in section Traditional Scheduling.
Don't try to make too much out of the naming and structure of these functions. They don't match the concepts described in this manual because the functions are as defined by POSIX.1b, but the implementation on systems that use the GNU C library is the inverse of what the POSIX structure contemplates. The POSIX scheme assumes that the primary scheduling parameter is the scheduling policy and that the priority value, if any, is a parameter of the scheduling policy. In the implementation, though, the priority value is king and the scheduling policy, if anything, only fine tunes the effect of that priority.
The symbols in this section are declared by including file `sched.h'.
int sched_priority
This function sets both the absolute priority and the scheduling policy for a process.
It assigns the absolute priority value given by param and the
scheduling policy policy to the process with Process ID pid,
or the calling process if pid is zero. If policy is
negative, sched_setschedule
keeps the existing scheduling policy.
The following macros represent the valid values for policy:
SCHED_OTHER
SCHED_FIFO
SCHED_RR
On success, the return value is 0
. Otherwise, it is -1
and ERRNO
is set accordingly. The errno
values specific
to this function are:
EPERM
CAP_SYS_NICE
permission and
policy is not SCHED_OTHER
(or it's negative and the
existing policy is not SCHED_OTHER
.
CAP_SYS_NICE
permission and its
owner is not the target process' owner. I.e. the effective uid of the
calling process is neither the effective nor the real uid of process
pid.
ESRCH
EINVAL
sched_get_priority_max
and sched_get_priority_min
tell you what the valid range is.
This function returns the scheduling policy assigned to the process with Process ID (pid) pid, or the calling process if pid is zero.
The return value is the scheduling policy. See
sched_setscheduler
for the possible values.
If the function fails, the return value is instead -1
and
errno
is set accordingly.
The errno
values specific to this function are:
ESRCH
EINVAL
Note that this function is not an exact mate to sched_setscheduler
because while that function sets the scheduling policy and the absolute
priority, this function gets only the scheduling policy. To get the
absolute priority, use sched_getparam
.
This function sets a process' absolute priority.
It is functionally identical to sched_setscheduler
with
policy = -1
.
This function returns a process' absolute priority.
pid is the Process ID (pid) of the process whose absolute priority you want to know.
param is a pointer to a structure in which the function stores the absolute priority of the process.
On success, the return value is 0
. Otherwise, it is -1
and ERRNO
is set accordingly. The errno
values specific
to this function are:
ESRCH
EINVAL
This function returns the lowest absolute priority value that is allowable for a process with scheduling policy policy.
On Linux, it is 0 for SCHED_OTHER and 1 for everything else.
On success, the return value is 0
. Otherwise, it is -1
and ERRNO
is set accordingly. The errno
values specific
to this function are:
EINVAL
This function returns the highest absolute priority value that is allowable for a process that with scheduling policy policy.
On Linux, it is 0 for SCHED_OTHER and 99 for everything else.
On success, the return value is 0
. Otherwise, it is -1
and ERRNO
is set accordingly. The errno
values specific
to this function are:
EINVAL
This function returns the length of the quantum (time slice) used with the Round Robin scheduling policy, if it is used, for the process with Process ID pid.
It returns the length of time as interval.
With a Linux kernel, the round robin time slice is always 150 microseconds, and pid need not even be a real pid.
The return value is 0
on success and in the pathological case
that it fails, the return value is -1
and errno
is set
accordingly. There is nothing specific that can go wrong with this
function, so there are no specific errno
values.
This function voluntarily gives up the process' claim on the CPU.
Technically, sched_yield
causes the calling process to be made
immediately ready to run (as opposed to running, which is what it was
before). This means that if it has absolute priority higher than 0, it
gets pushed onto the tail of the queue of processes that share its
absolute priority and are ready to run, and it will run again when its
turn next arrives. If its absolute priority is 0, it is more
complicated, but still has the effect of yielding the CPU to other
processes.
If there are no other processes that share the calling process' absolute priority, this function doesn't have any effect.
To the extent that the containing program is oblivious to what other processes in the system are doing and how fast it executes, this function appears as a no-op.
The return value is 0
on success and in the pathological case
that it fails, the return value is -1
and errno
is set
accordingly. There is nothing specific that can go wrong with this
function, so there are no specific errno
values.
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