P_ONLINE(2) System Calls P_ONLINE(2)


p_online - return or change processor operational status


#include <sys/types.h>
#include <sys/processor.h>

int p_online(processorid_t processorid, int flag);


The p_online() function changes or returns the operational status of
processors. The state of the processor specified by the processorid
argument is changed to the state represented by the flag argument.

Legal values for flag are P_STATUS, P_ONLINE, P_OFFLINE, P_NOINTR,
P_FAULTED, P_SPARE, and P_FORCED. P_DISABLED is only supported in the

When flag is P_STATUS, no processor status change occurs, but the current
processor status is returned.

refer to valid processor states. The P_ONLINE, P_OFFLINE, P_SPARE, and
P_FAULTED processor states can be combined with the P_FORCED flag.

A processor in the P_ONLINE state is allowed to process LWPs (lightweight
processes) and perform system activities. The processor is also
interruptible by I/O devices attached to the system.

A processor in the P_OFFLINE state is not allowed to process LWPs. The
processor is as inactive as possible. If the hardware supports such a
feature, the processor is not interruptible by attached I/O devices.

A processor in the P_NOINTR state is allowed to process LWPs, but it is
not interruptible by attached I/O devices. Typically, interrupts, when
they occur are routed to other processors in the system. Not all systems
support putting a processor into the P_NOINTR state. It is not permitted
to put all the processors of a system into the P_NOINTR state. At least
one processor must always be available to service system clock

A processor in the P_SPARE state is not allowed to process LWPs. In many
respects, the P_SPARE state is similar to the P_OFFLINE state, but
describes a processor that is available for reactivation by management
tools without administrator intervention.

A processor in the P_FAULTED state is not allowed to process LWPs. In
many respects, the P_FAULTED state is similar to the P_OFFLINE state, but
describes a processor that has been diagnosed as faulty. The privileged
caller can change the state of the processor from P_FAULTED to any of the
other states, but since the processor might generate additional errors,
electing to reactivate such a processor should be carefully considered.

A processor in the P_DISABLED state is not allowed to process LWPs. In
many respects, the P_DISABLED state is similar to the P_OFFLINE state,
but describes a processor explicitly disabled for general use.

Forced processor state transition can be requested if a new processor
state is specified with the bitwise-inclusive OR of the special P_FORCED
flag. Forcing transition of a processor to the P_OFFLINE, P_SPARE, or
P_FAULTED state revokes processor bindings for all threads that were
previously bound to that processor with processor_bind(2). There is no
guarantee that a forced processor state transition always succeeds.

Processor numbers are integers, greater than or equal to 0, and are
defined by the hardware platform. Processor numbers are not necessarily
contiguous, but "not too sparse." Processor numbers should always be
printed in decimal.

The maximum possible processorid value can be determined by calling
sysconf(_SC_CPUID_MAX). The list of valid processor numbers can be
determined by calling p_online() with processorid values from 0 to the
maximum returned by sysconf(_SC_CPUID_MAX). The EINVAL error is returned
for invalid processor numbers. See EXAMPLES below.

The special value P_ALL_SIBLINGS can be specified instead of a
processorid. In this case, the operation applies to all but one of the
SMT siblings on each CPU core. The only state transition supported is
P_DISABLED. The operation is only considered successful if all candidate
siblings could be disabled. Individual CPUs disabled in this manner can
transition to P_ONLINE only in combination with the P_FORCED flag, and
only if they were disabled via this system call.


On successful completion, the value returned is the previous state of the
P_POWEROFF. Otherwise, -1 is returned, the CPU state remains unchanged,
and errno is set to indicate the error.


The p_online() function will fail if:

The flag was P_OFFLINE or P_SPARE and the specified processor
is the only on-line processor, there are currently LWPs bound
to the processor, or the processor performs some essential
function that cannot be performed by another processor.

The flag was P_NOINTR and the specified processor is the only
interruptible processor in the system, or it handles
interrupts that cannot be handled by another processor.

The specified processor is powered off and cannot be powered
on because some platform- specific resource is not available.

A non-existent processor ID was specified or flag was

The caller is in a non-global zone, the pools facility is
active, and the processor is not a member of the zone's pool's
processor set.

The specified processor is powered off, and the platform does
not support power on of individual processors.

The flag was not P_STATUS and the {PRIV_SYS_RES_CONFIG}
privilege is not asserted in the effective set of the calling


Example 1: List the legal processor numbers.

The following code sample will list the legal processor numbers:

#include <sys/unistd.h>
#include <sys/processor.h>
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>

processorid_t i, cpuid_max;
cpuid_max = sysconf(_SC_CPUID_MAX);
for (i = 0; i <= cpuid_max; i++) {
if (p_online(i, P_STATUS) != -1)
printf("processor %d present\n", i);
return (0);


See attributes(7) for descriptions of the following attributes:

|MT-Level | MT-Safe |


processor_bind(2), processor_info(2), pset_create(2), sysconf(3C),
attributes(7), privileges(7), pooladm(8), psradm(8), psrinfo(8),

April 25, 2019 P_ONLINE(2)