pthread_create线程创建的过程剖析_pthread 创建线程被系统调用-优快云博客

概述

在Linux环境下，pthread库提供的pthread_create()API函数，用于创建一个线程。线程创建失败时，它可能会返回ENOMEM或EAGAIN。这篇文章主要讨论线程创建过程中碰到的一些问题和解决方法。

创建线程

首先，本文用的实例代码example.c:

/* example.c*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
void thread(void)
{
    int i;
    for(i=0;i<3;i++)
        printf("This is a pthread.\n");

    sleep(30);
}

int main(int argc,char **argv)
{
    pthread_t id;
    int i,ret;
    ret=pthread_create(&id,NULL,(void *) thread,NULL);
    if(ret!=0){
        printf ("Create pthread error!\n");
        exit (1);
    }
    for(i=0;i<3;i++)
        printf("This is the main process.\n");
    pthread_join(id,NULL);
    return 0;
}

编译，执行下面命令：

# example.c -lpthread -o example -g

用strace工具跟踪线程创建的过程：

# strace ./example

Strace工具输出：

getrlimit(RLIMIT_STACK, {rlim_cur=10240*1024, rlim_max=RLIM_INFINITY}) = 0
uname({sys="Linux", node="yjye", ...})  = 0
mmap2(NULL, 10489856, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0xb6d1c000
brk(0)                                  = 0x90e0000
brk(0x9101000)                          = 0x9101000
mprotect(0xb6d1c000, 4096, PROT_NONE)  = 0
clone(child_stack=0xb771c494, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE
_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0xb771cbd8, {entry_number:6, base_addr:0xb771cb70, limit:1048575, seg_32bi
t:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}, child_tidptr=0xb771cbd8) = 17209
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0

由上表中的输出可以看出创建线程过程中的调用步骤：

通过系统调用getrlimit() 获取线程栈的大小(参数中的RLIMIT_STACK)，在我的环境里(CentOS6)，缺省值是10M。
调用mmap2()分配内存，大小为10489856字节，合10244K，比栈空间大了4K。返回0xb6d1c000。
调用mprotect()，设置个内存页的保护区(大小为4K)，页面起始地址为0xb6d1c000。这个页面用于监测栈溢出，如果对这片内存有读写操作，那么将会触发一个SIGSEGV信号。下面布局图中的红色区域既是。
调用clone()创建线程。调用的第一个参数是一个地址：栈底的地址(这里具体为0xb771c494)。栈空间的内存使用，是从高位内存开始的。

从/proc/<pid>/smaps文件里，我们可以清楚地看到栈内存的映射情况：

090e0000-09101000 rw-p 00000000 00:00 0         [heap]
Size:                132 kB
Rss:                   4 kB
Pss:                   4 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         4 kB
Referenced:            4 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
b6d1c000-b6d1d000 ---p 00000000 00:00 0 #线程栈溢出监测区域
Size:                  4 kB
Rss:                   0 kB
Pss:                   0 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:            0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
b6d1d000-b771e000 rw-p 00000000 00:00 0 #线程栈
Size:              10244 kB
Rss:                   8 kB
Pss:                   8 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         8 kB
Referenced:            8 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB

从上面的映射文件的深蓝色部分中，我们看到，栈的空间总共为10244Kb，内存段是从b6d1d000到b771e000。从strace的输出中，我们看到栈底的地址为0xb771c494，那么，从0xb771c494到b771e000这段内存是做什么用的呢？它就是线程的TCB(thread's control block)和TLS区域(thread's local storage)。具体的线程内存空间布局如下：

GLIBC2.5与2.8

研究GLIBC2.5和2.8里的pthread_create()相关代码，会发现在mmap()调用失败并返回ENOMEM时，作了点变动，新版里替换了错误码。

V2.5相关代码.../nptl/allocatestack.c：

mem = mmap (NULL, size, prot,
              MAP_PRIVATE | MAP_ANONYMOUS | ARCH_MAP_FLAGS, -1, 0);

    if (__builtin_expect (mem == MAP_FAILED, 0))
        {
#ifdef ARCH_RETRY_MMAP
          mem = ARCH_RETRY_MMAP (size);
          if (__builtin_expect (mem == MAP_FAILED, 0))
#endif
        return errno;
        }

V2.8里的.../nptl/allocatestack.c：

mem = mmap (NULL, size, prot,
              MAP_PRIVATE | MAP_ANONYMOUS | ARCH_MAP_FLAGS, -1, 0);

    if (__builtin_expect (mem == MAP_FAILED, 0))
        {
#ifdef ARCH_RETRY_MMAP
          mem = ARCH_RETRY_MMAP (size, prot);
          if (__builtin_expect (mem == MAP_FAILED, 0))
#endif
        {
        if (errno == ENOMEM)
            errno = EAGAIN;

        return errno;
        }
        }

如上面的代码片段所示，在V2.5，简单地将mmap()调用结果返回给用户，而在V2.8里，如果mmap()返回ENOMEM，那么GLIBC会将错误码改成EAGAIN再返回。

为什么pthread_create()会调用失败?

随着运行中的线程数量的增大，pthread_create()失败的可能性也会增大。因为这会使分配给线程的内存空间(比如说线程栈)累积太多，导致mmap()系统调用失败。

比如说，/proc/<pid>/smaps里有这样一个内存映射片段：

[...]
7eb3d000-7f33c000 rw-p 7eb3d000 00:00 0
Size:               8188 kB
Rss:                  12 kB
Pss:                  12 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:        12 kB
Referenced:           12 kB
Swap:                  0 kB
7f8f5000-7f90a000 rw-p 7ffeb000 00:00 0          [stack]
Size:                 84 kB
Rss:                  16 kB
Pss:                  16 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:        16 kB
Referenced:           16 kB
Swap:                  0 kB

可用的内存空间是最后一个内存段和[stack]标签之间的空间：0x7F8F5000 - 0x7F33C000 = 0x5B9000 = 6000640字节(也就是6MB)。按缺省配置，小于一个线程栈的空间(10MB)。这时再创建线程就要失败。

解决方法

通常情况下，缺省10M的线程栈空间显然是太大了，所以建议通过调用pthread_attr_setstacksize()API来改变线程栈的大小。比如说以下代码片段：

//-------------------------------------------------------
// Name   : create_thd
// Usage : Create a thread
// Return : 0, if OK
//          -1, if error (errno is set)
//-------------------------------------------------------
static int create_thd(
                    void       *thd_par, // Thread parameters
                    size_t      stack_sz,
                    void       *(*entry)(void *),
                    pthread_t *pThreadId // Thread identifier
                     )
{
pthread_attr_t      attr;
int                 rc = 0;
int                 err_sav;

// Check the parameters
if (!pThreadId)
{
    fprintf(stderr, "NULL thread id\n");
    errno = EINVAL;
    return -1;
}

memset(&attr, 0, sizeof(attr));

errno = pthread_attr_init(&attr);
if (0 != errno)
{
    err_sav = errno;
    fprintf(stderr, "pthread_attr_init() failed (errno = %d)\n", errno);
    errno = err_sav;
    return -1;
}

errno = pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
if (0 != errno)
{
    err_sav = errno;
    fprintf(stderr, "pthread_attr_setscope() failed (errno = %d)\n", errno);
    errno = err_sav;
    rc = -1;
    goto err;
}

errno = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
if (0 != errno)
{
    err_sav = errno;
    fprintf(stderr, "pthread_attr_setdetachstate() failed (errno = %d)\n", errno);
    errno = err_sav;
    rc = -1;
    goto err;
}

// Set the stack size
errno = pthread_attr_setstacksize(&attr, stack_sz);
if (0 != errno)
{
    err_sav = errno;
    fprintf(stderr, "Error %d on pthread_attr_setstacksize()\n", errno);
    errno = err_sav;
    rc = -1;
    goto err;
}

// Thread creation
errno = pthread_create(pThreadId,
                         &attr,
                         entry,
                         thd_par);
if (0 != errno)
{
    err_sav = errno;
    fprintf(stderr, "pthread_create() failed (errno = %m - %d)\n", errno);
    errno = err_sav;
    rc = -1;
    goto err;
}

goto ok;

err:

ok:

// The following calls will alter errno
err_sav = errno;

errno = pthread_attr_destroy(&attr);
if (0 != errno)
{
    fprintf(stderr, "pthread_attr_destroy() failed (errno = %d)\n", errno);
    rc = -1;
}

errno = err_sav;

return rc;
} // create_thd

符号版本的链接问题

回到我们前面的示例代码中来，在里面，我们在主进程里直接调用pthread_create()函数。我们来看一下它的链接情况：

[root@yjye yeyj]# nm example | grep pthread
U pthread_create@@GLIBC_2.1
U pthread_join@@GLIBC_2.0

而上次在调试Freeswitch时，发现配置的栈大小居然不生效，所有子线程全部继承父线程的大小。这是怎么回事呢？Freeswitch调用的是apr封装后的接口，那我们看下apr的链接符号：

[root@yjye .libs]# nm libapr-1.a | grep pthread
         U pthread_rwlock_destroy
         U pthread_rwlock_init
         U pthread_rwlock_rdlock
         U pthread_rwlock_tryrdlock
         U pthread_rwlock_trywrlock
         U pthread_rwlock_unlock
         U pthread_rwlock_wrlock
         U pthread_mutex_destroy
         U pthread_mutex_init
         U pthread_mutex_lock
         U pthread_mutex_trylock
         U pthread_mutex_unlock
         U pthread_mutexattr_destroy
         U pthread_mutexattr_init
         U pthread_mutexattr_settype
         U pthread_cond_broadcast
         U pthread_cond_destroy
         U pthread_cond_init
         U pthread_cond_signal
         U pthread_cond_timedwait
         U pthread_cond_wait
00000080 d mutex_proc_pthread_methods
00000a10 t proc_mutex_proc_pthread_acquire
00000990 t proc_mutex_proc_pthread_cleanup
00000a50 t proc_mutex_proc_pthread_create
00000960 t proc_mutex_proc_pthread_release
         U pthread_mutex_destroy
         U pthread_mutex_init
         U pthread_mutex_lock
         U pthread_mutex_unlock
         U pthread_mutexattr_destroy
         U pthread_mutexattr_init
         U pthread_mutexattr_setprotocol
         U pthread_mutexattr_setpshared
         U pthread_mutexattr_setrobust_np
         U pthread_attr_destroy
         U pthread_attr_getdetachstate
         U pthread_attr_init
         U pthread_attr_setdetachstate
         U pthread_attr_setguardsize
         U pthread_attr_setstacksize
         U pthread_create
         U pthread_detach
         U pthread_exit
         U pthread_join
         U pthread_once
         U pthread_self
         U pthread_sigmask
         U pthread_getspecific
         U pthread_key_create
         U pthread_key_delete
         U pthread_setspecific

和前面相比，好像符号后面少了e@@GLIBC_2.1或者e@@GLIBC_2.0。通过GDB跟踪，发现最终调用的是pthread_join@@GLIBC_2.0。弄出两个版本来了。通过第三库调用pthread库，经常会出现这种情况。

我们看2.0的代码，打开文件…//nptl/pthread_create.c：

int
__pthread_create_2_0 (newthread, attr, start_routine, arg)
     pthread_t *newthread;
     const pthread_attr_t *attr;
     void *(*start_routine) (void *);
     void *arg;
{
/* The ATTR attribute is not really of type `pthread_attr_t *'. It has
     the old size and access to the new members might crash the program.
     We convert the struct now. */
struct pthread_attr new_attr;

  if (attr != NULL)
    {
      struct pthread_attr *iattr = (struct pthread_attr *) attr;
      size_t ps = __getpagesize ();

      /* Copy values from the user-provided attributes. */
      new_attr.schedparam = iattr->schedparam;
      new_attr.schedpolicy = iattr->schedpolicy;
      new_attr.flags = iattr->flags;

      /* Fill in default values for the fields not present in the old
    implementation. */
      new_attr.guardsize = ps;
      new_attr.stackaddr = NULL;
      new_attr.stacksize = 0;
      new_attr.cpuset = NULL;

      /* We will pass this value on to the real implementation. */
      attr = (pthread_attr_t *) &new_attr;
    }

  return __pthread_create_2_1 (newthread, attr, start_routine, arg);
}

很明显，如果链接到老版本，那么设置栈大小的属性完全被忽略掉了。

怎么解决这个问题呢？强制指定链接的符号，让它调用GLIBC_2.1。感谢Linux提供的系统调用，dlvsym()正好可以解决这个问题：

#include <dlfcn.h>
………

typedef int (*lxb_pcreate_t)(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void*), void *arg);

static lxb_pcreate_t lxb_pthread_create;
[...]
   void *pSym;

// Get the version GLIBC_2.1 of pthread_create() symbol
  pSym = dlvsym(RTLD_DEFAULT, "pthread_create", "GLIBC_2.1");
if (NULL == pSym)
{
    lxb_pthread_create = pthread_create;
}
else
{
    lxb_pthread_create = (lxb_pcreate_t)pSym;
    if (pSym != (void *)pthread_create)
    {
      LXB_PRINTF("Unexpected version of pthread_create() symbol ==> Forced to GLIBC_2.1\n");
    }
}