CPU_SET(3)                Linux Programmer's Manual               CPU_SET(3)

NAME         top

       CPU_XOR_S, CPU_EQUAL_S - macros for manipulating CPU sets

SYNOPSIS         top

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sched.h>

       void CPU_ZERO(cpu_set_t *set);

       void CPU_SET(int cpu, cpu_set_t *set);
       void CPU_CLR(int cpu, cpu_set_t *set);
       int  CPU_ISSET(int cpu, cpu_set_t *set);

       int  CPU_COUNT(cpu_set_t *set);

       void CPU_AND(cpu_set_t *destset,
                    cpu_set_t *srcset1, cpu_set_t *srcset2);
       void CPU_OR(cpu_set_t *destset,
                    cpu_set_t *srcset1, cpu_set_t *srcset2);
       void CPU_XOR(cpu_set_t *destset,
                    cpu_set_t *srcset1, cpu_set_t *srcset2);

       int  CPU_EQUAL(cpu_set_t *set1, cpu_set_t *set2);

       cpu_set_t *CPU_ALLOC(int num_cpus);
       void CPU_FREE(cpu_set_t *set);
       size_t CPU_ALLOC_SIZE(int num_cpus);

       void CPU_ZERO_S(size_t setsize, cpu_set_t *set);

       void CPU_SET_S(int cpu, size_t setsize, cpu_set_t *set);
       void CPU_CLR_S(int cpu, size_t setsize, cpu_set_t *set);
       int  CPU_ISSET_S(int cpu, size_t setsize, cpu_set_t *set);

       int  CPU_COUNT_S(size_t setsize, cpu_set_t *set);

       void CPU_AND_S(size_t setsize, cpu_set_t *destset,
                    cpu_set_t *srcset1, cpu_set_t *srcset2);
       void CPU_OR_S(size_t setsize, cpu_set_t *destset,
                    cpu_set_t *srcset1, cpu_set_t *srcset2);
       void CPU_XOR_S(size_t setsize, cpu_set_t *destset,
                    cpu_set_t *srcset1, cpu_set_t *srcset2);

       int  CPU_EQUAL_S(size_t setsize, cpu_set_t *set1, cpu_set_t *set2);

DESCRIPTION         top

       The cpu_set_t data structure represents a set of CPUs.  CPU sets are
       used by sched_setaffinity(2) and similar interfaces.

       The cpu_set_t data type is implemented as a bit mask.  However, the
       data structure should be treated as 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 set:

       CPU_ZERO()       Clears set, so that it contains no CPUs.

       CPU_SET()        Add CPU cpu to set.

       CPU_CLR()        Remove CPU cpu from set.

       CPU_ISSET()      Test to see if CPU cpu is a member of set.

       CPU_COUNT()      Return the number of CPUs in set.

       Where a cpu argument is specified, it should not produce side
       effects, since the above macros may evaluate the argument more than

       The first CPU on the system corresponds to a cpu value of 0, the next
       CPU corresponds to a cpu value of 1, and so on.  No assumptions
       should be made about particular CPUs being available, or the set of
       CPUs being contiguous, since CPUs can be taken offline dynamically or
       be otherwise absent.  The constant CPU_SETSIZE (currently 1024)
       specifies a value one greater than the maximum CPU number that can be
       stored in cpu_set_t.

       The following macros perform logical operations on CPU sets:

       CPU_AND()        Store the intersection of the sets srcset1 and
                        srcset2 in destset (which may be one of the source

       CPU_OR()         Store the union of the sets srcset1 and srcset2 in
                        destset (which may be one of the source sets).

       CPU_XOR()        Store the XOR of the sets srcset1 and srcset2 in
                        destset (which may be one of the source sets).  The
                        XOR means the set of CPUs that are in either srcset1
                        or srcset2, but not both.

       CPU_EQUAL()      Test whether two CPU set contain exactly the same

   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 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:

       CPU_ALLOC()      Allocate a CPU set large enough to hold CPUs in the
                        range 0 to num_cpus-1.

       CPU_ALLOC_SIZE() Return the size in bytes of the CPU set that would
                        be needed to hold CPUs in the range 0 to num_cpus-1.
                        This macro provides the value that can be used for
                        the setsize argument in the CPU_*_S() macros
                        described below.

       CPU_FREE()       Free a CPU set previously allocated by CPU_ALLOC().

       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 setsize bytes.

RETURN VALUE         top

       CPU_ISSET() and CPU_ISSET_S() return nonzero if cpu is in set;
       otherwise, it returns 0.

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

       CPU_EQUAL() and CPU_EQUAL_S() return nonzero if the two CPU sets are
       equal; otherwise they return 0.

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

       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.

VERSIONS         top

       The CPU_ZERO(), CPU_SET(), CPU_CLR(), and CPU_ISSET() macros were
       added in glibc 2.3.3.

       CPU_COUNT() first appeared in glibc 2.6.

       CPU_ISSET_S(), CPU_AND_S(), CPU_OR_S(), CPU_XOR_S(), and
       CPU_EQUAL_S() first appeared in glibc 2.7.

CONFORMING TO         top

       These interfaces are Linux-specific.

NOTES         top

       To duplicate a CPU set, use memcpy(3).

       Since CPU sets are bit masks 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 sizeof(unsigned long).  An
       application should consider the contents of these extra bits to be

       Notwithstanding the similarity in the names, note that the constant
       CPU_SETSIZE indicates the number of CPUs in the cpu_set_t data type
       (thus, it is effectively a count of the bits in the bit mask), while
       the setsize argument of the 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
       interfaces are implemented as macros, the compiler won't necessarily
       catch all type errors if you violate the suggestions.

BUGS         top

       On 32-bit platforms with glibc 2.8 and earlier, CPU_ALLOC() allocates
       twice as much space as is required, and 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.

EXAMPLE         top

       The following program demonstrates the use of some of the macros used
       for dynamically allocated CPU sets.

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

       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]);

           num_cpus = atoi(argv[1]);

           cpusetp = CPU_ALLOC(num_cpus);
           if (cpusetp == NULL) {

           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));


SEE ALSO         top

       sched_setaffinity(2), pthread_attr_setaffinity_np(3),
       pthread_setaffinity_np(3), cpuset(7)

COLOPHON         top

       This page is part of release 5.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2019-03-06                       CPU_SET(3)

Pages that refer to this page: sched_setaffinity(2)pthread_attr_setaffinity_np(3)pthread_setaffinity_np(3)cpuset(7)