man-pages/man3/CPU_SET.3

.\" Copyright (C) 2006 Michael Kerrisk
.\" and Copyright (C) 2008 Linux Foundation, written by Michael Kerrisk
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.TH CPU_SET 2 2008-11-14 "Linux" "Linux Programmer's Manual"
.SH NAME
CPU_SET, CPU_CLR, CPU_ISSET, CPU_ZERO, CPU_COUNT,
CPU_AND, CPU_OR, CPU_XOR, CPU_EQUAL,
CPU_ALLOC, CPU_ALLOC_SIZE, CPU_FREE,
CPU_SET_S, CPU_CLR_S, CPU_ISSET_S, CPU_ZERO_S,
CPU_COUNT_S, CPU_AND_S, CPU_OR_S, CPU_XOR_S, CPU_EQUAL_S \-
macros for manipulating CPU sets
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <sched.h>
.sp
.BI "void CPU_ZERO(cpu_set_t *" set );
.sp
.BI "void CPU_SET(int " cpu ", cpu_set_t *" set );
.BI "void CPU_CLR(int " cpu ", cpu_set_t *" set );
.BI "int  CPU_ISSET(int " cpu ", cpu_set_t *" set );
.sp
.BI "void CPU_COUNT(cpu_set_t *" set );
.sp
.BI "void CPU_AND(cpu_set_t *" destset ,
.BI "             cpu_set_t *" srcset1 ", cpu_set_t *" srcset2 );
.BI "void CPU_OR(cpu_set_t *" destset ,
.BI "             cpu_set_t *" srcset1 ", cpu_set_t *" srcset2 );
.BI "void CPU_XOR(cpu_set_t *" destset ,
.BI "             cpu_set_t *" srcset1 ", cpu_set_t *" srcset2 );
.sp
.BI "int  CPU_EQUAL(cpu_set_t *" set1 ", cpu_set_t *" set2 );
.sp
.BI "cpu_set_t *CPU_ALLOC(int " num_cpus );
.BI "void CPU_FREE(cpu_set_t *" set );
.BI "size_t CPU_ALLOC_SIZE(int " num_cpus );
.sp
.BI "void CPU_ZERO_S(size_t " setsize ", cpu_set_t *" set );
.sp
.BI "void CPU_SET_S(int " cpu ", size_t " setsize ", cpu_set_t *" set );
.BI "void CPU_CLR_S(int " cpu ", size_t " setsize ", cpu_set_t *" set );
.BI "int  CPU_ISSET_S(int " cpu ", size_t " setsize ", cpu_set_t *" set );
.sp
.BI "void CPU_COUNT_S(size_t " setsize ", cpu_set_t *" set );
.sp
.BI "void CPU_AND_S(size_t " setsize ", cpu_set_t *" destset ,
.BI "             cpu_set_t *" srcset1 ", cpu_set_t *" srcset2 );
.BI "void CPU_OR_S(size_t " setsize ", cpu_set_t *" destset ,
.BI "             cpu_set_t *" srcset1 ", cpu_set_t *" srcset2 );
.BI "void CPU_XOR_S(size_t " setsize ", cpu_set_t *" destset ,
.BI "             cpu_set_t *" srcset1 ", cpu_set_t *" srcset2 );
.sp
.BI "int  CPU_EQUAL_S(size_t " setsize ", cpu_set_t *" set1 \
", cpu_set_t *" set2 );
.fi
.SH DESCRIPTION
The
.I cpu_set_t
data structure represents a set of CPUs.
CPU sets are used by
.BR sched_setaffinity (2)
and similar interfaces.

The
.I cpu_set_t
data type is implemented as a bitset.
However, the data structure treated as considered opaque:
all manipulation of CPU sets should be done via the macros
described in this page.

The following macros are provided to operate on the CPU set
.IR set :
.TP 17
.BR CPU_ZERO ()
Clears
.IR set ,
so that it contains no CPUs.
.TP
.BR CPU_SET ()
Add CPU
.I cpu
to
.IR set .
.TP
.BR CPU_CLR ()
Remove CPU
.I cpu
from
.IR set .
.TP
.BR CPU_ISSET ()
Test to see if CPU
.I cpu
is a member of
.IR set .
.TP
.BR CPU_COUNT ()
Return the number of CPUs in
.IR set .
.PP
Where a
.I cpu
argument is specified, it should not produce side effects,
since the above macros may evaluate the argument more than once.
.PP
The first available CPU on the system corresponds to a
.I cpu
value of 0, the next CPU corresponds to a
.I cpu
value of 1, and so on.
The constant
.B CPU_SETSIZE
(currently 1024) specifies a value one greater than the maximum CPU
number that can be stored in
.IR cpu_set_t .

The following macros perform logical operations on CPU sets:
.TP 17
.BR CPU_AND ()
Store the logical AND of the sets
.I srcset1
and
.I srcset2
in
.I destset
(which may be one of the source sets).
.TP
.BR CPU_OR ()
Store the logical OR of the sets
.I srcset1
and
.I srcset2
in
.I destset
(which may be one of the source sets).
.TP
.BR CPU_XOR ()
Store the logical XOR of the sets
.I srcset1
and
.I srcset2
in
.I destset
(which may be one of the source sets).
.TP
.BR CPU_EQUAL ()
Test whether two CPU set contain exactly the same CPUs.
.SS Dynamically sized CPU sets
Because some applications may require the ability to dynamically
size CPU sets (e.g., to allocate sets larger than that
defined by the standard
.I cpu_set_t
data type), glibc nowadays provides a set of macros to support this.

The following macros are used to allocate and deallocate CPU sets:
.TP 17
.BR CPU_ALLOC ()
Allocate a CPU set large enough to hold CPUs
in the range 0 to
.IR num_cpus-1 .
.TP
.BR CPU_ALLOC_SIZE ()
Return the size in bytes of the CPU set that would be needed to
hold CPUs in the range 0 to
.IR num_cpus-1 .
This macro provides the value that can be used for the
.I setsize
argument in the
.BR CPU_*_S ()
macros described below.
.TP
.BR CPU_FREE ()
Free a CPU set previously allocated by
.BR CPU_ALLOC ().
.PP
The macros whose names end with "_S" are the analogs of
the similarly named macros without the suffix.
These macros perform the same tasks as their analogs,
but operate on the dynamically allocated CPU set(s) whose size is
.I setsize
bytes.
.SH "RETURN VALUE"
.BR CPU_ISSET ()
and
.BR CPU_ISSET_S ()
return non-zero if
.I cpu
is in
.IR set ;
otherwise, it returns 0.

.BR CPU_COUNT ()
and
.BR CPU_COUNT_S ()
return the number of CPUs in
.IR set .

.BR CPU_EQUAL ()
and
.BR CPU_EQUAL_S ()
return non-zero if the two CPU sets are equal; otherwise it returns 0.

.BR CPU_ALLOC ()
returns a pointer on success, or NULL on failure.
(Errors are as for
.BR malloc (3).)

.BR CPU_ALLOC_SIZE ()
returns the number of bytes required to store a
CPU set of the specified cardinality.

The other functions do not return a value.
.SH VERSIONS
The
.BR CPU_ZERO (),
.BR CPU_SET (),
.BR CPU_CLR (),
and
.BR CPU_ISSET ()
macros were added in glibc 2.3.3.

.BR CPU_COUNT ()
first appeared in glibc 2.6.

.BR CPU_AND (),
.BR CPU_OR (),
.BR CPU_XOR (),
.BR CPU_EQUAL (),
.BR CPU_ALLOC (),
.BR CPU_ALLOC_SIZE (),
.BR CPU_FREE (),
.BR CPU_ZERO_S (),
.BR CPU_SET_S (),
.BR CPU_CLR_S (),
.BR CPU_ISSET_S (),
.BR CPU_AND_S (),
.BR CPU_OR_S (),
.BR CPU_XOR_S (),
and
.BR CPU_EQUAL_S ()
first appeared in glibc 2.7.
.SH "CONFORMING TO"
These interfaces are Linux-specific.
.SH NOTES
To duplicate a CPU set, use
.BR memcpy (3).

Since CPU sets are bitsets allocated in units of long words,
the actual number of CPUs in a dynamically
allocated CPU set will be rounded up to the next multiple of
.IR "sizeof(unsigned long)" .
An application should consider the contents of these extra bits
to be undefined.

Notwithstanding the similarity in the names,
note that the constant
.B CPU_SETSIZE
indicates the number of CPUs in the
.I cpu_set_t
data type (thus, it is effectively a count of bits in the bitset),
while the
.I setsize
argument of the
.BR CPU_*_S ()
macros is a size in bytes.

The data types for arguments and return values shown
in the SYNOPSIS are hints what about is expected in each case.
However, since these these interfaces are implemented as macros,
the compiler won't necessarily catch all type errors
if you violate the suggestions.
.SH EXAMPLE
The following program demonstrates the use of some of the macros
used for dynamically allocated CPU sets.

.nf
#define _GNU_SOURCE
#include <sched.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <assert.h>

int
main(int argc, char *argv[])
{
    cpu_set_t *cpusetp;
    size_t size;
    int num_cpus, cpu;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <num\-cpus>\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    num_cpus = atoi(argv[1]);

    cpusetp = CPU_ALLOC(num_cpus);
    if (cpusetp == NULL) {
        perror("CPU_ALLOC");
        exit(EXIT_FAILURE);
    }

    size = CPU_ALLOC_SIZE(num_cpus);

    CPU_ZERO_S(size, cpusetp);
    for (cpu = 0; cpu < num_cpus; cpu += 2)
        CPU_SET_S(cpu, size, cpusetp);

    printf("CPU_COUNT() of set:    %d\\n", CPU_COUNT_S(size, cpusetp));

    CPU_FREE(cpusetp);
    exit(EXIT_SUCCESS);
}
.fi
.SH BUGS
On 32-bit platforms with glibc 2.8 and earlier,
.BR CPU_ALLOC ()
allocates twice as much space as is required, and
.BR CPU_ALLOC_SIZE ()
returns a value twice as large as it should.
This bug should not affect the semantics of a program,
but does result in wasted memory
and less efficient operation of the macros that
operate on dynamically allocated CPU sets.
These bugs are fixed in glibc 2.9.
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=7029
.SH "SEE ALSO"
.BR sched_setaffinity (2),
.BR pthread_attr_setaffinity_np (3),
.BR pthread_setaffinity_np (3),
.BR cpuset (7)