pthread_getattr_np - get attributes of created thread

#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <pthread.h>
int pthread_getattr_np(pthread_t thread, pthread_attr_t *attr);

The pthread_getattr_np() function initializes the thread attributes object referred to by attr so that it contains actual attribute values describing the running thread thread.

The returned attribute values may differ from the corresponding attribute values passed in the attr object that was used to create the thread using pthread_create(3). In particular, the following attributes may differ:

• the detach state, since a joinable thread may have detached itself after creation;

• the stack size, which the implementation may align to a suitable boundary.

• and the guard size, which the implementation may round upward to a multiple of the page size, or ignore (i.e., treat as 0), if the application is allocating its own stack.

Furthermore, if the stack address attribute was not set in the thread attributes object used to create the thread, then the returned thread attributes object will report the actual stack address that the implementation selected for the thread.

When the thread attributes object returned by pthread_getattr_np() is no longer required, it should be destroyed using pthread_attr_destroy(3).

On success, this function returns 0; on error, it returns a nonzero error number.

The program below demonstrates the use of pthread_getattr_np(). The program creates a thread that then uses pthread_getattr_np() to retrieve and display its guard size, stack address, and stack size attributes. Command-line arguments can be used to set these attributes to values other than the default when creating the thread. The shell sessions below demonstrate the use of the program.

In the first run, on an x86-32 system, a thread is created using default attributes:

$ ulimit -s # No stack limit ==> default stack size is 2 MB
unlimited
$ ./a.out
Attributes of created thread:
        Guard size          = 4096 bytes
        Stack address       = 0x40196000 (EOS = 0x40397000)
        Stack size          = 0x201000 (2101248) bytes

In the following run, we see that if a guard size is specified, it is rounded up to the next multiple of the system page size (4096 bytes on x86-32):

$ ./a.out -g 4097
Thread attributes object after initializations:
        Guard size          = 4097 bytes
        Stack address       = (nil)
        Stack size          = 0x0 (0) bytes
Attributes of created thread:
        Guard size          = 8192 bytes
        Stack address       = 0x40196000 (EOS = 0x40397000)
        Stack size          = 0x201000 (2101248) bytes

In the last run, the program manually allocates a stack for the thread. In this case, the guard size attribute is ignored.

$ ./a.out -g 4096 -s 0x8000 -a
Allocated thread stack at 0x804d000
Thread attributes object after initializations:
        Guard size          = 4096 bytes
        Stack address       = 0x804d000 (EOS = 0x8055000)
        Stack size          = 0x8000 (32768) bytes
Attributes of created thread:
        Guard size          = 0 bytes
        Stack address       = 0x804d000 (EOS = 0x8055000)
        Stack size          = 0x8000 (32768) bytes

Program source

#define _GNU_SOURCE     /* To get pthread_getattr_np() declaration */
#include <err.h>
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static void
display_stack_related_attributes(pthread_attr_t *attr, char *prefix)
{
    int s;
    size_t stack_size, guard_size;
    void *stack_addr;
    s = pthread_attr_getguardsize(attr, &guard_size);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_attr_getguardsize");
    printf("%sGuard size          = %zu bytes\n", prefix, guard_size);
    s = pthread_attr_getstack(attr, &stack_addr, &stack_size);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_attr_getstack");
    printf("%sStack address       = %p", prefix, stack_addr);
    if (stack_size > 0)
        printf(" (EOS = %p)", (char *) stack_addr + stack_size);
    printf("\n");
    printf("%sStack size          = %#zx (%zu) bytes\n",
           prefix, stack_size, stack_size);
}
static void
display_thread_attributes(pthread_t thread, char *prefix)
{
    int s;
    pthread_attr_t attr;
    s = pthread_getattr_np(thread, &attr);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_getattr_np");
    display_stack_related_attributes(&attr, prefix);
    s = pthread_attr_destroy(&attr);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_attr_destroy");
}
static void *           /* Start function for thread we create */
thread_start(void *arg)
{
    printf("Attributes of created thread:\n");
    display_thread_attributes(pthread_self(), "\t");
    exit(EXIT_SUCCESS);         /* Terminate all threads */
}
static void
usage(char *pname, char *msg)
{
    if (msg != NULL)
        fputs(msg, stderr);
    fprintf(stderr, "Usage: %s [-s stack-size [-a]]"
            " [-g guard-size]\n", pname);
    fprintf(stderr, "\t\t-a means program should allocate stack\n");
    exit(EXIT_FAILURE);
}
static pthread_attr_t *   /* Get thread attributes from command line */
get_thread_attributes_from_cl(int argc, char *argv[],
                              pthread_attr_t *attrp)
{
    int s, opt, allocate_stack;
    size_t stack_size, guard_size;
    void *stack_addr;
    pthread_attr_t *ret_attrp = NULL;   /* Set to attrp if we initialize
                                           a thread attributes object */
    allocate_stack = 0;
    stack_size = -1;
    guard_size = -1;
    while ((opt = getopt(argc, argv, "ag:s:")) != -1) {
        switch (opt) {
        case 'a':   allocate_stack = 1;                     break;
        case 'g':   guard_size = strtoul(optarg, NULL, 0);  break;
        case 's':   stack_size = strtoul(optarg, NULL, 0);  break;
        default:    usage(argv[0], NULL);
        }
    }
    if (allocate_stack && stack_size == -1)
        usage(argv[0], "Specifying -a without -s makes no sense\n");
    if (argc > optind)
        usage(argv[0], "Extraneous command-line arguments\n");
    if (stack_size != -1 || guard_size > 0) {
        ret_attrp = attrp;
        s = pthread_attr_init(attrp);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_init");
    }
    if (stack_size != -1) {
        if (!allocate_stack) {
            s = pthread_attr_setstacksize(attrp, stack_size);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_setstacksize");
        } else {
            s = posix_memalign(&stack_addr, sysconf(_SC_PAGESIZE),
                               stack_size);
            if (s != 0)
                errc(EXIT_FAILURE, s, "posix_memalign");
            printf("Allocated thread stack at %p\n\n", stack_addr);
            s = pthread_attr_setstack(attrp, stack_addr, stack_size);
            if (s != 0)
                errc(EXIT_FAILURE, s, "pthread_attr_setstacksize");
        }
    }
    if (guard_size != -1) {
        s = pthread_attr_setguardsize(attrp, guard_size);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_setstacksize");
    }
    return ret_attrp;
}
int
main(int argc, char *argv[])
{
    int s;
    pthread_t thr;
    pthread_attr_t attr;
    pthread_attr_t *attrp = NULL;    /* Set to &attr if we initialize
                                        a thread attributes object */
    attrp = get_thread_attributes_from_cl(argc, argv, &attr);
    if (attrp != NULL) {
        printf("Thread attributes object after initializations:\n");
        display_stack_related_attributes(attrp, "\t");
        printf("\n");
    }
    s = pthread_create(&thr, attrp, &thread_start, NULL);
    if (s != 0)
        errc(EXIT_FAILURE, s, "pthread_create");
    if (attrp != NULL) {
        s = pthread_attr_destroy(attrp);
        if (s != 0)
            errc(EXIT_FAILURE, s, "pthread_attr_destroy");
    }
    pause();    /* Terminates when other thread calls exit() */
}

pthread_attr_getaffinity_np(3), pthread_attr_getdetachstate(3), pthread_attr_getguardsize(3), pthread_attr_getinheritsched(3), pthread_attr_getschedparam(3), pthread_attr_getschedpolicy(3), pthread_attr_getscope(3), pthread_attr_getstack(3), pthread_attr_getstackaddr(3), pthread_attr_getstacksize(3), pthread_attr_init(3), pthread_create(3), pthreads(7)