Manpages - CPU_SET.3

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

  #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);

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.

*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.

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_ALLOC_SIZE*(), *CPU_FREE*(), *CPU_ZERO_S*(), *CPU_SET_S*(), *CPU_CLR_S*(), *CPU_ISSET_S*(), *CPU_AND_S*(), *CPU_OR_S*(), *CPU_XOR_S*(), and *CPU_EQUAL_S*() first appeared in glibc 2.7.

These interfaces are Linux-specific.

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 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 interfaces are implemented as macros, the compiler won’t necessarily catch all type errors if you violate the suggestions.

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.

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>

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

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

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