CPU_SET(3) Library Functions Manual CPU_SET(3)
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
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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 once.
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 sets).
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 CPUs.
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 stan-
dard 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
CPU_ISSET() and CPU_ISSET_S() return nonzero if cpu is in set; other-
wise, 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.
STANDARDS
Linux.
HISTORY
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_AND(), CPU_OR(), CPU_XOR(), CPU_EQUAL(), CPU_ALLOC(), CPU_AL-
LOC_SIZE(), CPU_FREE(), CPU_ZERO_S(), CPU_SET_S(), CPU_CLR_S(), CPU_IS-
SET_S(), CPU_AND_S(), CPU_OR_S(), CPU_XOR_S(), and CPU_EQUAL_S() first
appeared in glibc 2.7.
NOTES
To duplicate a CPU set, use memcpy(3).
Since CPU sets are bit masks allocated in units of long words, the ac-
tual 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 undefined.
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 inter-
faces are implemented as macros, the compiler won't necessarily catch
all type errors if you violate the suggestions.
BUGS
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 opera-
tion of the macros that operate on dynamically allocated CPU sets.
These bugs are fixed in glibc 2.9.
EXAMPLES
The following program demonstrates the use of some of the macros used
for dynamically allocated CPU sets.
#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <assert.h>
int
main(int argc, char *argv[])
{
cpu_set_t *cpusetp;
size_t size, num_cpus;
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 (size_t 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);
}
SEE ALSO
sched_setaffinity(2), pthread_attr_setaffinity_np(3), pthread_setaffin-
ity_np(3), cpuset(7)
Linux man-pages 6.9.1 2024-06-15 CPU_SET(3)
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