1 前言

在Linux C/C++编程时不可避免的会遇到以下的需求,全局变量线程共享;最为典型的功能则是errno,变量 在程序的任何地方都可以访问,但是不会影响到其他线程,这就是本文档说明的TLS(线程局部存储变量)

]]>https://mengdemao.com/2022/07/pthread_lock/Mon, 18 Jul 2022 21:27:24 +0800mengdemaohttps://mengdemao.com/2022/07/pthread_lock/

9 futex原理

高级锁的实现都是与futex实现相关

Futex是Fast Userspace muTexes的缩写

常用的锁都是通过futex实现的

• mutex (互斥锁)
• rwlock (读写锁)
• cond (条件变量)

graph LR
    atom[原子操作] --> spin[自旋锁]
	futex --> mutex[互斥锁]
	futex --> rwlock[读写锁]
	futex --> cond[条件变量]

c

#include <linux/futex.h>      /* Definition of FUTEX_* constants */
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>

long syscall(SYS_futex, uint32_t *uaddr, int futex_op, uint32_t val,
             const struct timespec *timeout,   /* or: uint32_t val2 */
             uint32_t *uaddr2, uint32_t val3);

• uaddr
• futex_op
• val
• timeout
• uaddr2
• val3

c

// 进入解析
long do_futex(u32 __user *uaddr, int op, u32 val,
              ktime_t *timeout,
			  u32 __user *uaddr2, u32 val2, u32 val3)
{
	int cmd = op & FUTEX_CMD_MASK;
	unsigned int flags = 0;

	if (!(op & FUTEX_PRIVATE_FLAG))
		flags |= FLAGS_SHARED;

	if (op & FUTEX_CLOCK_REALTIME) {
		flags |= FLAGS_CLOCKRT;
		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
			return -ENOSYS;
	}

	switch (cmd) {
	case FUTEX_LOCK_PI:
	case FUTEX_UNLOCK_PI:
	case FUTEX_TRYLOCK_PI:
	case FUTEX_WAIT_REQUEUE_PI:
	case FUTEX_CMP_REQUEUE_PI:
		if (!futex_cmpxchg_enabled)
			return -ENOSYS;
	}

	switch (cmd) {
	case FUTEX_WAIT:
		val3 = FUTEX_BITSET_MATCH_ANY;
	case FUTEX_WAIT_BITSET:
		return futex_wait(uaddr, flags, val, timeout, val3);
	case FUTEX_WAKE:
		val3 = FUTEX_BITSET_MATCH_ANY;
	case FUTEX_WAKE_BITSET:
		return futex_wake(uaddr, flags, val, val3);
	case FUTEX_REQUEUE:
		return futex_requeue(uaddr, flags, uaddr2, val, val2, NULL, 0);
	case FUTEX_CMP_REQUEUE:
		return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 0);
	case FUTEX_WAKE_OP:
		return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
	case FUTEX_LOCK_PI:
		return futex_lock_pi(uaddr, flags, timeout, 0);
	case FUTEX_UNLOCK_PI:
		return futex_unlock_pi(uaddr, flags);
	case FUTEX_TRYLOCK_PI:
		return futex_lock_pi(uaddr, flags, NULL, 1);
	case FUTEX_WAIT_REQUEUE_PI:
		val3 = FUTEX_BITSET_MATCH_ANY;
		return futex_wait_requeue_pi(uaddr, flags, val, timeout, val3,
					     uaddr2);
	case FUTEX_CMP_REQUEUE_PI:
		return futex_requeue(uaddr, flags, uaddr2, val, val2, &val3, 1);
	}
	return -ENOSYS;
}

但是我并没有搞明白futex实现的原理,造成难以继续分析, 因此只能分析别人的文档(注:不分析原理,只进行使用)

]]>https://mengdemao.com/2022/07/pthread_thread/Mon, 18 Jul 2022 21:27:23 +0800mengdemaohttps://mengdemao.com/2022/07/pthread_thread/

基于musl源码库与glibc库,其中musl提供分析的思路,glibc分析具体实现

7 调试环境搭建

7.1 下载源码

执行/usr/lib/libc.so.6,确定版本

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