Skip to main content

Handling Terminal Output with Termios

As I explained in the previous article, Unix-like operating systems, for instance, OS X and Linux, use LF (line feed, 0x0A, \n) as the newline character which moves the cursor to the beginning of the next line. However, the standard-defined behavior of LF only moves the cursor down to the next line, not to the beginning of the line. This difference is acceptable if files are always accessed through operating system-dependent applications. However, Unix-like systems have no distinction between files and input/output; this difference can be problematic when file and process input/output interact.

To handle this difference, a terminal emulator post-processes the output appropriately. The c_oflag in the termios structure defined by the POSIX.1 standard controls this. The c_oflag is a flag for what post-processing the terminal should perform before displaying the received characters.

The most important flag in c_oflag is OPOST. This flag determines whether or not to post-process the input; if the OPOST flag is turned off, the terminal displays the received string as-is, regardless of other flags. In other words, the output stream enters raw mode. Turning this flag off is rare. However, it is a good idea to turn it off when using the terminal for purposes other than displaying text, such as transmitting binary data.

The flag that allows terminals to operate as desired in Unix-like operating systems is ONLCR. When ONLCR is on, the terminal interprets NL as CRNL during output. In other words, even in Unix-like systems, if ONLCR is off, when encountering an LF, the cursor moves to the next line in the current position instead of the beginning of the next line. In Unix-like operating systems, when you need to display a file created in Windows, you can easily display it by turning off the ONLCR flag without converting CRNL to NL.

In addition, there are various flags, such as OCRNL which makes the terminal behave like an old Mac OS, and XTABS which converts a tab character (0x09, \t) into the appropriate number of spaces. Through these flags, developers can have fine control over terminal output and unify the behavior when developing terminal-based applications, ignoring the differences between operating systems.

Labels:

Comments

Post a Comment

Popular posts from this blog

[C++] Handling Exceptions in Constructors

When you use RAII idiom, there are often situations where constructors have to do complex tasks. These complex tasks can sometimes fail, resulting in throwing exceptions. This raises a concern: Is it okay to throw exceptions in constructors? The first concern is memory leaks. Fortunately, memory leaks do not occur. Variables created on the stack are released through stack unwinding, and if an exception occurs during heap allocation with the new operator, the new operator automatically deallocates the memory and returns nullptr . The next concern is whether the destructor of the member variables will be called correctly. However, this is also not a problem. When an exception occurs, member variables can be divided into three categories: fully initialized member variables, member variables being initialized, and uninitialized member variables. Fully initialized member variables have had their constructors called and memory allocations completed successfully. In the example code, t...
Post a Comment
Read more

Iterator Adapters in Rust

An Iterator that takes another iterator and returns a new one is called an iterator adapter . The name "adapter" comes from one of the GoF's design patterns, the adapter pattern . However, in reality, it corresponds more to the decorator pattern , so if you pay too much attention to the name, you might get confused about its purpose. So it's better not to worry too much about the name. Enough complaining about the name, what does an iterator adapter do? An iterator adapter adds a task to be performed when the iterator iterates. This will be easier to understand when you see an example. The map function is one of the famous adapters. The iterator returned by the map function for those who have used functional languages iterates over new values transformed from the original values. Besides, various adapters are already implemented in the standard library. Among them, the most frequently used are those that are convenient to use with loops. Examples include the ...
Post a Comment
Read more

Clear Screen with CSI Sequence

Today, following my previous post , I will explain how to clear the screen using CSI Sequences. There are two sequences in the CSI Sequence for clearing. The first one is the Erase in Line sequence, called EL . It is composed of CSI # K ; it is used to erase lines, as the name suggests. If the # is not provided, the default value is 0, and if a value is provided, it must be one of the three: 0, 1, or 2. The terminal will ignore the sequence if any other value is provided. For example, if you print 0x311b5b334b32 (or 1^[3K2 ), the terminal ignores ^[3K , and the screen displays only 12 . The behavior of 0, 1, and 2 can be summarized as follows. 0 Erases from the cursor to the end of the line. 1 Erases from the beginning of the line to the cursor. 2 Erases the entire line, regardless of the cursor's position. Remember that the EL sequence does not move the cursor's position. Therefore, if you want to erase the current line and write a new line on the c...
Post a Comment
Read more