pthread_attr_getstacksize 问题

        最近公司里遇到一个线程栈大小的问题，借此机会刚好学习一下这个线程栈大小相关的函数。如果公司里用的还是比较老的代码的话，都是用的 pthread 库支持线程的，而不是 c++11 里的线程类。主要有两个相关函数：pthread_attr_setstacksize() 和 pthread_attr_getstacksize()。

        下面看一下简单的例子：

#include <pthread.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <ctype.h>#define handle_error_en(en, msg) \do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)#define handle_error(msg) \do { perror(msg); exit(EXIT_FAILURE); } while (0)static void *thread_start(void *arg)
{	size_t stack_size = 0;pthread_attr_t attr;int ret = pthread_attr_init(&attr);if(ret != 0){handle_error_en(ret, "pthread_attr_init");}ret = pthread_attr_getstacksize(&attr, &stack_size);if (ret != 0){handle_error_en(ret, "pthread_attr_setstacksize");}printf("stack_size = %lu\n", stack_size);return 0;
}int main(int argc, char *argv[])
{int s, tnum, opt, num_threads;pthread_t thread_id;pthread_attr_t attr;int stack_size;void *res;/* The "-s" option specifies a stack size for our threads */stack_size = -1;while ((opt = getopt(argc, argv, "s:")) != -1) {switch (opt) {case 's':stack_size = strtoul(optarg, NULL, 0);break;default:fprintf(stderr, "Usage: %s [-s stack-size] arg...\n", argv[0]);exit(EXIT_FAILURE);}}/* Initialize thread creation attributes */s = pthread_attr_init(&attr);if (s != 0){handle_error_en(s, "pthread_attr_init");}if (stack_size > 0) {s = pthread_attr_setstacksize(&attr, stack_size);if (s != 0){handle_error_en(s, "pthread_attr_setstacksize");}}printf("set stack size %lu\n", stack_size);s = pthread_create(&thread_id, &attr, &thread_start, &thread_id);if (s != 0){handle_error_en(s, "pthread_create");}/* Destroy the thread attributes object, since it is nolonger needed */s = pthread_attr_destroy(&attr);if (s != 0){handle_error_en(s, "pthread_attr_destroy");}s = pthread_join(thread_id, &res);if (s != 0){handle_error_en(s, "pthread_join");}exit(EXIT_SUCCESS);
}

代码里设置了 512 Kb 大小的线程栈，而用 pthread_attr_getstacksize（）获取到的却是 8MB。为什么呢？其实这个 8388608 是系统默认的线程栈大小，可以用 ulimit -a 查看相关信息：

  

找到 pthread_attr_getstacksize 的源码看一下，大概就知道它为什么是这个返回值了

/* Copyright (C) 2002-2022 Free Software Foundation, Inc.This file is part of the GNU C Library.The GNU C Library is free software; you can redistribute it and/ormodify it under the terms of the GNU Lesser General PublicLicense as published by the Free Software Foundation; eitherversion 2.1 of the License, or (at your option) any later version.The GNU C Library is distributed in the hope that it will be useful,but WITHOUT ANY WARRANTY; without even the implied warranty ofMERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNULesser General Public License for more details.You should have received a copy of the GNU Lesser General PublicLicense along with the GNU C Library; if not, see<https://www.gnu.org/licenses/>.  */
#include "pthreadP.h"
#include <shlib-compat.h>
int
__pthread_attr_getstacksize (const pthread_attr_t *attr, size_t *stacksize)
{struct pthread_attr *iattr;iattr = (struct pthread_attr *) attr;size_t size = iattr->stacksize;/* If the user has not set a stack size we return what the systemwill use as the default.  */if (size == 0){lll_lock (__default_pthread_attr_lock, LLL_PRIVATE);size = __default_pthread_attr.internal.stacksize;lll_unlock (__default_pthread_attr_lock, LLL_PRIVATE);}*stacksize = size;return 0;
}
versioned_symbol (libc, __pthread_attr_getstacksize,pthread_attr_getstacksize, GLIBC_2_34);
#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
compat_symbol (libpthread, __pthread_attr_getstacksize,pthread_attr_getstacksize, GLIBC_2_1);
#endif

 可以看到如果是用户没有设置栈大小的话，就会返回系统默认值，即 8MB。但上面的代码中已经调用 pthread_attr_setstacksize() 设置栈大小了，为什么还会返回默认值呢？仔细看 pthread_attr_getstacksize 的实现，它返回的栈大小其实是从入参 attr 里取的 stacksize 值，所以回到我们的代码中，我们是用 pthread_attr_init(&attr) 来初始化 attr 的，所以返回的也就是这个初始化后 attr 里的 stacksize 值。可以看看 pthread_attr_init 的源码：

#include <errno.h>
#include <string.h>
#include <unistd.h>
#include "pthreadP.h"
#include <shlib-compat.h>
struct pthread_attr *__attr_list;
int __attr_list_lock = LLL_LOCK_INITIALIZER;
int
__pthread_attr_init (pthread_attr_t *attr)
{struct pthread_attr *iattr;ASSERT_TYPE_SIZE (pthread_attr_t, __SIZEOF_PTHREAD_ATTR_T);ASSERT_PTHREAD_INTERNAL_SIZE (pthread_attr_t, struct pthread_attr);/* Many elements are initialized to zero so let us do it all atonce.  This also takes care of clearing the bytes which are notinternally used.  */memset (attr, '\0', __SIZEOF_PTHREAD_ATTR_T);iattr = (struct pthread_attr *) attr;/* Default guard size specified by the standard.  */iattr->guardsize = __getpagesize ();return 0;
}
libc_hidden_def (__pthread_attr_init)
versioned_symbol (libc, __pthread_attr_init, pthread_attr_init, GLIBC_2_1);
#if SHLIB_COMPAT(libc, GLIBC_2_0, GLIBC_2_1)
int
__pthread_attr_init_2_0 (pthread_attr_t *attr)
{/* This code is specific to the old LinuxThread code which has a toosmall pthread_attr_t definition.  The struct looked likethis:  */struct old_attr{int detachstate;int schedpolicy;struct sched_param schedparam;int inheritsched;int scope;};struct pthread_attr *iattr;/* Many elements are initialized to zero so let us do it all atonce.  This also takes care of clearing the bytes which are notinternally used.  */memset (attr, '\0', sizeof (struct old_attr));iattr = (struct pthread_attr *) attr;iattr->flags |= ATTR_FLAG_OLDATTR;/* We cannot enqueue the attribute because that member is not in theold attribute structure.  */return 0;
}
compat_symbol (libc, __pthread_attr_init_2_0, pthread_attr_init, GLIBC_2_0);
#endif

就是把入参 attr 给 memset 了一下，所以在 pthead_attr_getstacksize() 里，size = iattr->stacksize是为 0 的，所以 pthead_attr_getstacksize 取到的就是默认的栈大小。那要真正取到当前线程的 stacksize 怎么操作呢？使用 pthread_getattr_np() 这个函数，取指定线程的 attr，如：

static void *thread_start(void *arg)
{	size_t stack_size = 0;pthread_attr_t attr;int ret = pthread_getattr_np(pthread_self(), &attr);if(ret != 0){handle_error_en(ret, "pthread_attr_init");}ret = pthread_attr_getstacksize(&attr, &stack_size);if (ret != 0){handle_error_en(ret, "pthread_attr_setstacksize");}printf("current thread stack_size = %lu\n", stack_size);return 0;
}