How can I use wstring(s) in Linux APIs

Introduction

In Linux programming, use of wide character strings is becoming increasingly common. Wide character strings, or wstrings, are a sequence of wide characters, each of which can represent a single Unicode character. This makes them particularly useful when working with text that includes characters from a wide range of languages, as well as text that includes special characters such as accents, symbols, and emojis. In this article, we will explore how to use wstrings in Linux APIs, with examples and explanations.

What are wstrings and why use them?

A wide character string, or wstring, is a sequence of wide characters, where each character is represented by a larger number of bytes than a regular character. In C++, wide characters are represented using wchar_t data type. Wide characters are used to represent characters from languages other than English, such as Arabic, Chinese, or Russian, as well as special characters such as accents, symbols, and emojis.

Using wstrings in Linux APIs can be beneficial in several ways. First, it allows programmers to work with text that includes a wide range of characters, which may not be supported by regular character strings. Second, it can help to avoid errors and bugs caused by incorrect character encoding. Finally, it can make code more readable and maintainable, as use of wstrings can provide clarity and consistency.

Example 

Example of using wstrings in Linux APIs − To illustrate use of wstrings in Linux APIs, let's consider an example of a program that reads a file containing a list of names in various languages and displays them on screen. Here is code for reading file using wstrings −

#include <iostream>
#include <fstream>
#include <locale>

int main() {
   std::wifstream inputFile("names.txt");
   inputFile.imbue(std::locale(""));

      if (inputFile) {
         std::wstring name;
         while (std::getline(inputFile, name)) {
         std::wcout << name << std::endl;
      }
   } else {
      std::wcerr << "Error: unable to open input file." << std::endl;
      return 1;
   }
   return 0;
}

In this example, we use wifstream class to read file, and we set locale to system default using imbue. This ensures that correct character encoding is used when reading file. We then read each line of file using std::getline function and store it in a wstring variable. Finally, we display name on screen using std::wcout.

Converting between wstrings and regular strings − In some cases, it may be necessary to convert between wstrings and regular strings. This can be done using various functions and classes provided by standard library.

Example 

To convert a regular string to a wstring, you can use std::wstring_convert class, which is available in <locale> header. Here is an example −

#include <iostream>
#include <locale>
#include <codecvt>

int main() {
   std::string name = "John Smith";
   std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
   std::wstring wname = converter.from_bytes(name);
   std::wcout << wname << std::endl;
   return 0;
}

In this example, we use std::wstring_convert class to convert a regular string to a wstring. We specify encoding of original string using codecvt_utf8 class, and then use from_bytes function to perform conversion. Finally, we display wstring on screen using std::wcout.

Example 

Conversely, to convert a wstring to a regular string, you can use same std::wstring_convert class, but this time you would use to_bytes function. Here is an example −

#include <iostream>
#include <locale>
#include <codecvt>

int main() {
   std::wstring wname = L"John Smith";
   std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
   std::string name = converter.to_bytes(wname);
   std::cout << name << std::endl;
   return 0;
}

In this example, we use same std::wstring_convert class to convert a wstring to a regular string. We specify encoding of target string as UTF-8, and then use to_bytes function to perform conversion. Finally, we display regular string on screen using std::cout.

Using wstrings in system calls − In addition to using wstrings in higher-level APIs, it is also possible to use them in system calls. However, this requires a slightly different approach, as system calls usually expect arguments in form of regular strings, not wstrings.

To pass a wstring to a system call, you must first convert it to a regular string using to_bytes function. You can then pass regular string as an argument to system call.

Example 

Here is an example of using open system call with a wstring:

#include <fcntl.h>
#include <iostream>
#include <locale>
#include <codecvt>

int main() {
   std::wstring filename = L"test.txt";
   std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
   std::string name = converter.to_bytes(filename);
   int fileDescriptor = open(name.c_str(), O_RDONLY);
   if (fileDescriptor == -1) {
      std::cerr << "Error: unable to open file." << std::endl;
      return 1;
   }
   // do something with file
   close(fileDescriptor);
   return 0;
}

In this example, we use open system call to open a file specified by a wstring. We first convert wstring to a regular string using to_bytes function, and then pass regular string to open function. If call is successful, we obtain a file descriptor, which we can then use to read or write to file.

Using wstrings in file I/O

In addition to using wstrings in system calls, it is also possible to use them in file I/O operations. To do this, you must first convert wstring to a regular string, as we did in previous example. You can then use regular string as filename when opening, reading, or writing to a file.

Example 

Here is an example of using a wstring to write text to a file −

#include <fstream>
#include <iostream>
#include <locale>
#include <codecvt>

int main() {
   std::wstring filename = L"test.txt";
   std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
   std::string name = converter.to_bytes(filename);
   std::ofstream file(name);
   if (!file.is_open()) {
      std::cerr << "Error: unable to open file." << std::endl;
      return 1;
   }
   std::wstring text = L"Hello, world!";
   file << converter.to_bytes(text) << std::endl;
   file.close();
   return 0;
}

In this example, we use std::ofstream class to write text to a file specified by a wstring. We first convert wstring to a regular string using to_bytes function, and then use regular string as filename when opening file. We then write some text to file, also converting it to a regular string using to_bytes. Finally, we close file.

Using wstrings in command-line arguments

Another way to use wstrings in Linux APIs is to pass them as command-line arguments. To do this, you must first convert wstrings to regular strings, and then pass them to your program as command-line arguments.

Example 

Here is an example of using a wstring as a command-line argument −

#include <iostream>
#include <locale>
#include <codecvt>

int main(int argc, char** argv) {
   if (argc < 2) {
      std::cerr << "Error: no argument provided." << std::endl;
      return 1;
   }
   std::wstring_convert<std::codecvt_utf8<wchar_t>> converter;
   std::wstring arg = converter.from_bytes(argv[1]);
   std::wcout << arg << std::endl;
   return 0;
}

In this example, we use main function to receive a command-line argument specified by a regular string. We then convert regular string to a wstring using from_bytes function, and display resulting wstring on screen using std::wcout.

Conclusion

In this article, we have explored how to use wstrings in Linux APIs, with examples and explanations. We have seen that wstrings can be useful when working with text that includes characters from a wide range of languages, as well as text that includes special characters such as accents, symbols, and emojis. We have also seen how to convert between wstrings and regular strings, and how to use wstrings in system calls. By using wstrings in our Linux programs, we can ensure that our code is more robust, readable, and maintainable, and that it can handle text in a wide range of languages and character encodings.