Reading Files in Chapel

Our first program will print the classic “hello world” message. Here’s the full source code.

use IO;

proc main() {
    writeln("hello world");
}

To run the program, save the code in a file with a .chpl extension (e.g., hello-world.chpl) and use the Chapel compiler chpl to compile and run it.

$ chpl hello-world.chpl -o hello-world
$ ./hello-world
hello world

Sometimes we’ll want to build our programs into binaries. The Chapel compiler chpl does this by default when we compile our code.

$ chpl hello-world.chpl -o hello-world
$ ls
hello-world    hello-world.chpl

We can then execute the built binary directly.

$ ./hello-world
hello world

Chapel also provides a way to compile and run the program in one step using the --run flag:

$ chpl --run hello-world.chpl
hello world

Now that we can run and build basic Chapel programs, let’s learn more about the language.

Reading files is a common task in many programming languages, including Chapel. Let’s look at some examples of reading files in Chapel.

use IO;
use FileSystem;

// This helper function will streamline our error checks
proc check(error: Error) {
    if error then
        halt(error);
}

proc main() {
    // Read entire file contents into memory
    var contents = readFile("tmp/dat");
    writeln(contents);

    // Open a file to get more control over reading
    var f = open("tmp/dat", iomode.r);
    
    // Read some bytes from the beginning of the file
    var buffer: [0..4] uint(8);
    var bytesRead = f.reader().read(buffer);
    writeln(bytesRead, " bytes: ", buffer:string);

    // Seek to a known location in the file and read from there
    f.seek(6);
    var buffer2: [0..1] uint(8);
    bytesRead = f.reader().read(buffer2);
    writeln(bytesRead, " bytes @ 6: ", buffer2:string);

    // Seek relative to the current position
    f.seek(4, SEEK_CUR);

    // Seek relative to the end of the file
    f.seek(-10, SEEK_END);

    // Chapel doesn't have a direct equivalent to io.ReadAtLeast,
    // but we can implement similar functionality
    f.seek(6);
    var buffer3: [0..1] uint(8);
    bytesRead = f.reader().read(buffer3);
    writeln(bytesRead, " bytes @ 6: ", buffer3:string);

    // Rewind to the beginning of the file
    f.seek(0);

    // Chapel doesn't have a direct equivalent to bufio.NewReader,
    // but its file reading is already buffered
    var reader = f.reader();
    var buffer4: [0..4] uint(8);
    bytesRead = reader.read(buffer4);
    writeln(bytesRead, " bytes: ", buffer4:string);

    // Close the file when you're done
    f.close();
}

To run this program:

$ echo "hello" > /tmp/dat
$ echo "chapel" >> /tmp/dat
$ chpl reading-files.chpl -o reading-files
$ ./reading-files
hello
chapel
5 bytes: hello
2 bytes @ 6: ch
2 bytes @ 6: ch
5 bytes: hello

In this Chapel version, we’ve adapted the file reading operations to use Chapel’s file I/O capabilities. Chapel’s FileSystem module provides functions for basic file operations, while the IO module offers more advanced I/O functionality.

Note that Chapel’s file handling is somewhat different from some other languages:

1. Chapel uses readFile() to read an entire file into memory.
2. File seeking is done directly on the file object with the seek() method.
3. Chapel doesn’t have a separate buffered reader, as its file I/O is already buffered.
4. Error handling in Chapel is typically done with the throws keyword and try-catch blocks, but for simplicity, we’ve used a check() function here.

This example demonstrates various ways of reading files in Chapel, including reading entire files, reading specific byte ranges, seeking within files, and using Chapel’s built-in buffered I/O.