TLPI - The Linux Programming Int

2019-01-30 本文已影响0人帆子_8c3a

Chapter 4. File I/O: The Universal I/O Model

Changing the File Offset: lseek()

off_t lseek(int fd, off_t offset, int whence);

SEEK_SET
SEEK_CUR
SEEK_END
如果文件系统产生hole,这段内容读出来是0

Chapter 5. File I/O: Further Details

Relationship Between File Descriptors and Open Files

file offset记录在open file table中

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Duplicating File Descriptors

Chapter 13. File I/O Buffering

Kernel Buffering of File I/O: The Buffer Cache

read()、write()系统调用并不直接写到磁盘，而是和cache打交道。
虽然read()、write()很快，但最好以BUF_SIZE传输效率最高，大小4K。系统调用负担也很大。

Controlling Kernel Buffering of File I/O

int fsync(int fd); //针对某个文件
void sync(void);//针对整个系统

打开文件用O_SYNC参数,每次write后自动flush

fd = open(pathname, O_WRONLY | O_SYNC);

Summary of I/O Buffering

fflush()保证调用了write()系统调用,fsync(),sync()保证写到了磁盘上。

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Bypassing the Buffer Cache: Direct I/O

用O_DIRECT参数打开文件，不走kernel cache，影响性能。

Chapter 21. Signals: Signal Handlers

Interruption and Restarting of System Calls

We establish a handler for some signal.
We make a blocking system call, for example, a read() from a terminal device, which blocks until input is supplied.
While the system call is blocked, the signal for which we established a handler is delivered, and its signal handler is invoked.
默认情况下，read()会返回EINTR。在设置flag为SA_RESTART时，system call会自动重启。

Chapter 23. Timers and Sleeping

Setting Timeouts on Blocking Operations

参见书中例子

Chapter 24. Process Creation

Overview

fork()
exit()
wait()
execve() //argv + envp，系统调用，其他函数(exec)调用这个函数

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File Sharing Between Parent and Child

When a fork() is performed, the child receives duplicates of all of the parent’s file descriptors. These duplicates are made in the manner of dup().
下面的例子证明了，父子进程共享文件，子进程修改描述符的offset,父进程也同样结果。

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Memory Semantics of fork()

为了避免拷贝内存：

代码段设置为只读，父子进程的代码段指向同一段物理内存。
对于 data, heap, and stack，Linux内核采用copy-on-write.

The vfork() System Call

和fork的区别:

父子进程共享内存
父进程不需要调用wait()，就可以等待子进程退出。

Chapter 27. Program Execution

Executing a New Program: execve()

exec家族函数是调用execve()实现的
/proc/PID/exe是对运行文件的符号链接。

Interpreter Scripts

#! interpreter-path [optional-arg]

File Descriptors and exec()

$ ls /tmp > dir.txt

以上的shell脚本做了几件事

A fork() is performed to create a child process that is also running a copy of the shell
The child shell opens dir.txt for output using file descriptor 1
1. The child shell closes descriptor 1 (STDOUT_FILENO) and then opens the file dir.txt.
2. The shell opens dir.txt, obtaining a new file descriptor.
The child shell execs the ls program.

The close-on-exec flag (FD_CLOEXEC)

在执行exec函数前,关掉FD。此功能可以用fcntl()查看，并且修改。

Chapter 55. File Locking

Overview

flock():整个文件上锁
fcntl():文件的某个范围上锁
为了使用lock，而不被stdio的缓存机制破坏，调用setbuf() 关闭stdio缓存。

File Locking with flock()

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Semantics of Lock Inheritance and Release

释放锁可以通过:

LOCK_UN操作
所有关联的FD都关闭
锁的信息存在Open file table中，而不是存在FD中。flock操作的是内核中的file object,而不是FD。下图通过dup句柄，然后对新句柄操作，就可以释放锁(因为这两个FD都指向同一个file object)。
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下图说明两个不同open()会产生两个不同file。第二次lock会被锁住。
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fork()后得到的子进程的FD，其实dup父进程得到的，所以子进程可以解锁lock.
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