pthread的底层实现

pthread

// 在linux内核源码中，文件include/linux/sched.h说：

/**
 * cloning flags:
 */
#define CSIGNAL    	0x000000ff	/** signal mask to be sent at exit */
#define CLONE_VM	0x00000100	/** set if VM shared between processes */
#define CLONE_FS	0x00000200	/** set if fs info shared between processes */
#define CLONE_FILES	0x00000400	/** set if open files shared between processes */


// 在glibc中，文件nptl/sysdeps/pthread/createthread.c说:

static int
create_thread (struct pthread *pd, const struct pthread_attr *attr,
           STACK_VARIABLES_PARMS)
{
#ifdef TLS_TCB_AT_TP
  assert (pd->header.tcb != NULL);
#endif

  /** We rely heavily on various flags the CLONE function understands:

     CLONE_VM, CLONE_FS, CLONE_FILES
	These flags select semantics with shared address space and
	file descriptors according to what POSIX requires.

     CLONE_SIGNAL
	This flag selects the POSIX signal semantics.

     CLONE_SETTLS
	The sixth parameter to CLONE determines the TLS area for the
	new thread.

     CLONE_PARENT_SETTID
	The kernels writes the thread ID of the newly created thread
	into the location pointed to by the fifth parameters to CLONE.

	Note that it would be semantically equivalent to use
	CLONE_CHILD_SETTID but it is be more expensive in the kernel.

     CLONE_CHILD_CLEARTID
	The kernels clears the thread ID of a thread that has called
	sys_exit() in the location pointed to by the seventh parameter
	to CLONE.

     CLONE_DETACHED
	No signal is generated if the thread exists and it is
	automatically reaped.

     The termination signal is chosen to be zero which means no signal
     is sent.  */
  int clone_flags = (CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_SIGNAL
		     | CLONE_SETTLS | CLONE_PARENT_SETTID
		     | CLONE_CHILD_CLEARTID | CLONE_SYSVSEM
#if __ASSUME_NO_CLONE_DETACHED == 0
		     | CLONE_DETACHED
#endif
		     | 0);

// 于是：线程与其父进程共享VM、FS、FILES等。这也是线程与进程的区别点。通常Fork出来进程（复制资源），Clone出来线程（共享资源）。Fork调用无参数，Clone调用传参控制。