2 Blocking and Non-blocking IO

Course Content#

Understand the true meaning and differences between the two, and be able to answer the above questions.

Literal Meanings of Both#

Going to Work

Blocking: There is traffic on the way to work, wait or use other methods, in short, you have to go to work.
Non-blocking: There is traffic on the way to work, just don’t go.

Mom Asked to Buy Soy Sauce

Blocking: There is no soy sauce, ask mom if I should get something else, or buy something else.
Non-blocking: There is no soy sauce, just don’t buy it.

Let Xiao Ming Write a Report

Blocking: Wait for Xiao Ming to finish writing.
Non-blocking: No need to wait for Xiao Ming to write, inform the result later [ask Xiao Ming myself - synchronous / Xiao Ming tells me proactively - asynchronous].

Boiling Water

Blocking: During the process of boiling water, you cannot do other things, you can only stand there and wait for the water to boil.
Non-blocking: During the process of boiling water, you can do other things, like go to the living room to watch TV.
Reference Simple Explanation of Synchronous and Asynchronous, Blocking and Non-blocking——CSDN

Introduction#

Review O_NONBLOCK in the open function.
- Everything is a file, so everything can be blocking/non-blocking.
Analyze the process of writing a file.
- Open → write → close.
- Among them, write is a system call, the specific process is as follows:
  - Data → kernel → disk: The kernel copies the data and places it in the buffer [block buffer]. When flushing the buffer, it schedules the IO device, finds the inode and block, and writes the data to the disk.
    - The first step data → kernel is user mode, the second step kernel → disk is kernel mode, there is a conversion.
    - Reference How to Understand Linux User Mode and Kernel Mode?——Zhihu
  - NON_BLOCK sets the user mode [data → kernel or kernel → data] process.
    - Most of the blocking occurs when taking data out from the kernel.
  - The process from kernel → disk is actually blocking.
    - To allow upper-level applications to quickly return the write status, the kernel writes the data to the disk, which ordinary users do not need to perceive.
    - After the kernel copies the data, the program returns, and ordinary users think the write was successful, but the data is likely still in the buffer [Buffered IO].

fcntl#

Operate on file descriptors [can make files non-blocking].

man fcntl
Prototype
- fd: file descriptor. The most common file descriptors: 0, 1, 2.
- cmd: operation mode.
- ... /* arg */
  - Variable parameters.
  - arg indicates that this parameter is the parameter of the previous parameter [cmd], its meaning depends on cmd.
Description
- cmd in parentheses indicates whether there are variable parameters, at most one.
- The variable parameter type is generally int, using macro definitions, essentially a bitmask, changing the state through bitwise operations.
- Among them, there is a category of cmd: related to [file status flags].
  - You can obtain or set the file status [such as O_NONBLOCK].
Return Value
- Observe the return value, consider the judgments in the program.
- Errors return -1, successful setting operations return 0.

select#

Synchronous I/O multiplexing [interface].

man select
Prototype
- nfds: number of file descriptors.
- fd_set: set of file descriptors.
  - Readable, writable, exceptions.
  - Implemented using arrays at the bottom.
- timeout: time interval.
- Four macros are used to operate on the set, see below.
Description
- Allows the program to monitor multiple file descriptors, waiting for one or more file descriptors' I/O operations to be "ready".
  - ready: ready, can perform corresponding IO operations on the file, such as non-blocking reads or sufficiently small writes.
- The number of file descriptors that can be monitored is less than FD_SETSIZE [generally 1024].
- When exiting, each file descriptor set will be modified, leaving only the file descriptors whose status has changed, serving as an indication.
  - Therefore, if select is used in a loop, each set needs to be re-initialized before each call to select.
- The set can be NULL, indicating that no files are being monitored in this type of event.
- Macros for operating on the set:
  - FD_ZERO: clear a set.
  - FD_SET: add a file descriptor to a set.
  - FD_CLR: remove a file descriptor from a set.
  - FD_ISSET: determine which set a file descriptor belongs to, can be used after select returns, as the set has been modified.
- The value of nfds is the maximum number of file descriptors in the three sets plus one.
  - The index of file descriptors starts from 0.
- struct timeval timeout
  - Specifies the time interval for select to block while waiting for each file descriptor to be ready.
    - Here, blocking means purely blocking, not blocking I/O.
    - [Personal understanding] The synchronization in synchronous I/O multiplexing is reflected here.
  - Three conditions for stopping blocking:
    - A file descriptor is ready.
    - Signal interruption [kill].
    - Timeout.
  - The time interval is not precise, it is difficult to achieve true precision.
    - System clock granularity, kernel scheduling delay.
  - The timeval structure has two members: seconds, microseconds.
    - If both are 0, it will return immediately, can be used for polling.
    - If NULL, it will wait indefinitely.
  - The timeout update function only works on Linux.
  - For compatibility, try not to use it, and use more common functions.
Return Value
- Returns the number of file descriptors [ready] in all sets at that time.
  - Based on the number, use the macro FD_ISSET to inquire about the status change of all file descriptors.
  - 0: Time is up, and no interesting events occurred.
  - -1: error, and sets errno; at this time, the set will not change, and timeout becomes undefined.

[PS]

select → poll → epoll, increasingly advanced.
- man poll
- man epoll
- All perform similar tasks to select.

Code Demonstration#

Implementing Interfaces to Make Files Non-blocking and Blocking#

common.h
- Two interfaces.
head.h
- The order of header files matters, generally placing your own project's header files at the end.
- Reference The Path and Order of #include——Google C++ Programming Style.
common.c
- When setting flags, do not change the original flags.
- 👉 First get, then add/remove flags using bitwise OR / AND.

Non-blocking File Descriptor 0#

Common files 0, 1, 2 do not need to be opened manually.
- These three files are automatically opened when creating a process.
- They are inherited.
Compile command: gcc 1.test.c ../common/common.c -I ../common/
- Remember to add the common.c file during compilation.
After setting file descriptor 0 to non-blocking, the key change is that reading and writing of file descriptor 0 becomes non-blocking.
- ① No sleep.
  - scanf reads file descriptor 0.
  - But there is no data, it proceeds directly without blocking to wait for data in file descriptor 0.
- ② With sleep.
  - During the 5 seconds of sleep, input data in the terminal [+enter, line buffering].
    - The process pauses, not occupying CPU.
    - But the standard input stream remains open, and the data entered by the user in the terminal will be passed to file descriptor 0 by the kernel.
      - The file is maintained by the kernel, and during the sleep process, the kernel can write data to the file.
  - After sleep ends, scanf reads the data from file descriptor 0.
- 【Clarification】 Non-blocking refers to a certain object, such as file descriptor 0; not to a certain function or operation.
[PS]
- Blocking: may waste time.
- Non-blocking: may waste resources.
  - Generally used in large programs and high-concurrency servers.

Select on File Descriptor 0#

Copied from the man manual — EXAMPLE in man select.
select can perceive the arrival of I/O.
Running effect.
- Input ls in the terminal, and after the program ends, ls will still execute.
- Because the content of the buffer has not been taken away [like scanf], it is ultimately taken by zsh.
select was blocking during the monitored time interval.
- In all user-level programs, so-called blocking means sleeping [sleep()].
- Blocking termination conditions: ready, signal interruption, timeout.
[Extended Application]
- Set a timer for blocking places, use default values on timeout.
  - Avoid blocking for too long due to exceptional situations [SSH host unreachable...].
  - More user-friendly.

Additional Knowledge Points#

There are many ways to perceive I/O, among which the kernel fully knows the arrival of IO.

Points to Consider#

When using scanf, when entering a carriage return in the terminal, is the data placed in the buffer, or is the buffer cleared and given to scanf?
- An article to help you understand C/C++ language input and output streams and buffers
- It is the former.

Tips#

Course Preview: Multiprocessing [fork...].
Consider the implementation of cp.
- Examine file read and write operations.
- Must be blocking.