ptfork.c

/* Linuxthreads - a simple clone()-based implementation of Posix        */
/* threads for Linux.                                                   */
/* Copyright (C) 1996 Xavier Leroy (Xavier.Leroy@inria.fr)              */
/*                                                                      */
/* This program is free software; you can redistribute it and/or        */
/* modify it under the terms of the GNU Library General Public License  */
/* as published by the Free Software Foundation; either version 2       */
/* of the License, or (at your option) any later version.               */
/*                                                                      */
/* This program is distributed in the hope that it will be useful,      */
/* but WITHOUT ANY WARRANTY; without even the implied warranty of       */
/* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        */
/* GNU Library General Public License for more details.                 */

/* The "atfork" stuff */

#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include "pthread.h"
#include "internals.h"

struct handler_list {
  void (*handler)(void);
  struct handler_list * next;
};
// 用于互斥访问链表的互斥变量
static pthread_mutex_t pthread_atfork_lock = PTHREAD_MUTEX_INITIALIZER;
// 三个链表
static struct handler_list * pthread_atfork_prepare = NULL;
static struct handler_list * pthread_atfork_parent = NULL;
static struct handler_list * pthread_atfork_child = NULL;
// 生成一个新的handler_list节点插入到list中
static void pthread_insert_list(struct handler_list ** list,
                                void (*handler)(void),
                                struct handler_list * newlist,
                                int at_end)
{
  if (handler == NULL) return;
  // 插入到最后，则先把直接指向尾节点
  if (at_end) {
    while(*list != NULL) list = &((*list)->next);
  }
  // 保存数据到新节点
  newlist->handler = handler;
  // *list即第一个节点的地址
  newlist->next = *list;
  // *list的内容修改为新节点
  *list = newlist;
}

struct handler_list_block {
  struct handler_list prepare, parent, child;
};

int pthread_atfork(void (*prepare)(void),
                   void (*parent)(void),
                   void (*child)(void))
{
  struct handler_list_block * block =
    (struct handler_list_block *) malloc(sizeof(struct handler_list_block));
  if (block == NULL) return ENOMEM;
  pthread_mutex_lock(&pthread_atfork_lock);
  /* "prepare" handlers are called in LIFO */
  // 把三个函数保存到一个节点中，如果这个节点分别插入三个handle_list队列
  pthread_insert_list(&pthread_atfork_prepare, prepare, &block->prepare, 0);
  /* "parent" handlers are called in FIFO */
  pthread_insert_list(&pthread_atfork_parent, parent, &block->parent, 1);
  /* "child" handlers are called in FIFO */
  pthread_insert_list(&pthread_atfork_child, child, &block->child, 1);
  pthread_mutex_unlock(&pthread_atfork_lock);
  return 0;
}
// handle_list链表中每个节点的函数
static inline void pthread_call_handlers(struct handler_list * list)
{
  for (/*nothing*/; list != NULL; list = list->next) (list->handler)();
}

extern int __fork(void);
// http://man7.org/linux/man-pages/man3/pthread_atfork.3.html
/*
glibc中定义了fork和__fork的关系。
weak_alias (__fork, fork)
# define weak_alias(name, aliasname) _weak_alias (name, aliasname)
# define _weak_alias(name, aliasname) \
  extern __typeof (name) aliasname __attribute__ ((weak, alias (#name)))
fork是弱符号，并且是__fork的的别名。即如果定义了fork，则会覆盖glibc中的fork。
这里就是覆盖glibc的fork，然后在调用glibc的__fork之前执行一些额外的操作。这样用户在执行fork的时候，
就会执行下面这个fork函数，从而执行glibc的__fork
*/
int fork(void)
{
  int pid;
  struct handler_list * prepare, * child, * parent;

  pthread_mutex_lock(&pthread_atfork_lock);
  prepare = pthread_atfork_prepare;
  child = pthread_atfork_child;
  parent = pthread_atfork_parent;
  pthread_mutex_unlock(&pthread_atfork_lock);
  // 调fork之前调用函数列表
  pthread_call_handlers(prepare);
  pid = __fork();
  // 子进程
  if (pid == 0) {
    __pthread_reset_main_thread();
    __fresetlockfiles();
    pthread_call_handlers(child);
  } else {
    // 父进程
    pthread_call_handlers(parent);
  }
  return pid;
}