Input/output with files

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Common Lisp provides a rich library of functionality for dealing with files. In this chapter I'll focus on a few basic file-related tasks: Common Lisp provides a stream abstraction for reading and writing data and an abstraction, called pathnamesfor manipulating filenames in an operating system-independent way.

Additionally, Common Lisp provides other bits of functionality unique to Lisp such as the ability to read and write s-expressions. You obtain a stream from which you can read a file's contents with the OPEN function. By default OPEN returns a character-based input stream you can pass to a variety of functions that read one or more characters of text: The only required argument to OPEN is the name of the file to read.

As you'll see in the section "Filenames," Common Lisp provides a couple of ways to represent a filename, but the simplest is to use a string containing the name in the local file-naming syntax. You can use the object returned as the first argument to any of the read functions. Of course, a number of things can go wrong while trying to open and read from a file. The file may not exist. Or you may unexpectedly hit the end of the file while reading. In Chapter 19, I'll discuss how to recover from such errors.

For now, however, there's a lighter-weight solution: If you want to open a possibly nonexistent file without OPEN signaling an error, you can use the keyword argument: The three possible values are: Thus, you can change the previous example to deal with the possibility that the file may not exist.

Thus, you could print all the lines in a file like this:. Each time it's called, it reads a single s-expression, skipping whitespace and comments, and returns the Lisp object denoted by the s-expression.

In other words, it contains four s-expressions: You can read those expressions like this:. They even--as the previous example demonstrated--give you comments for free. And because s-expressions were designed to be human editable, it's also a fine format for things like configuration files. By default OPEN returns character streams, which translate the underlying bytes to characters according to a particular character-encoding scheme.

READ-BYTElike the character-reading functions, also accepts optional arguments to specify whether it should signal an error if called at the end of the file and what value to return if not. You pass it a sequence typically a vector and a stream, and it attempts to fill the sequence with data from the stream.

It returns the index of the first element of the sequence that wasn't filled or the length of the sequence if it was able to completely fill it. You can also pass: The sequence argument must be a type that can hold elements of the stream's element type.

To write data to a file, you need an output stream, which you obtain by calling OPEN with a: When opening a file for output, OPEN assumes the file shouldn't already exist and will signal an error if it does. However, you can change that behavior with the: Re absolute fastest way to save and read data from text file or binary file typical use of OPEN for output looks like this:.

Common Lisp also provides several functions for writing data: Two different functions can print just a newline: For example, suppose you have one function that generates output that should always be followed by a line break and another that should start on a new line. But assume that if the functions are called one after the other, you don't want a blank line between the two bits of output.

If you use FRESH-LINE at the beginning of the prolific health options and trading incorporated website hosts function, its output will always start on a new line, but if it's called right after the first, it won't emit an extra line break.

Several functions output Lisp data as s-expressions: PRINT prints an s-expression preceded by an end-of-line and followed by a space.

PRIN1 prints just the s-expression. However, not all objects can be printed in a form that READ will understand. When it's NILthese functions will print the object in a special syntax that's guaranteed to cause READ to signal an error if it tries to read it; otherwise they will signal an error rather than print the object.

I'll discuss some of the more important details of FORMATwhich essentially defines a mini-language for emitting formatted output, in Chapter To write binary data to a file, you have to OPEN the file with the same: As anyone who has written code that deals with lots of files knows, it's important to re absolute fastest way to save and read data from text file or binary file files when you're done with them, because file handles tend to be a scarce resource.

If you open files and don't close them, you'll soon discover you can't open any more files. For instance, you could always structure your file using code like this:. However, this approach suffers from two problems.

One is simply that it's error prone--if you forget the CLOSEthe code will leak a file handle every time it runs. Or, as you'll see in Chapter 19, if any of the code before the CLOSE signals an error, control may jump out of the LET to an error handler and never come back to close the stream. Common Lisp provides a general solution to the problem of how to ensure that certain code always runs: This is the basic form:.

The forms in body-forms are evaluated with stream-var bound to a file stream opened by a call to OPEN with open-arguments as its arguments.

Thus, you can write this to read a line from a file:. In that case, you must take care to eventually close the stream yourself, or you'll leak file descriptors and may eventually end up unable to open any more files. So far you've used strings to represent filenames. However, using strings as filenames ties your code to a particular operating system and file system.

To avoid this kind of nonportability, Common Lisp provides another representation of filenames: Pathnames represent filenames in re absolute fastest way to save and read data from text file or binary file structured way that makes them easy to manipulate without tying them to a particular filename syntax. And the burden of translating back and forth between strings in the local syntax--called namestrings --and pathnames is placed on the Lisp implementation.

Unfortunately, as with many abstractions designed to hide the details of fundamentally different underlying systems, the pathname abstraction introduces its own complications. When pathnames were designed, the set of file systems in general use was quite a bit more variegated than those in common use today. Consequently, some nooks and crannies of the pathname abstraction make little sense if all you're concerned about is representing Unix or Windows filenames. However, once you understand which parts of the pathname abstraction you can ignore as artifacts of pathnames' evolutionary history, they do provide a convenient way to manipulate filenames.

Most places a filename is called for, you can use either a namestring or a pathname. Which to use depends mostly on where the name originated. Filenames provided by the user--for example, as arguments or as values in configuration files--will typically be namestrings, since the user knows what operating system they're running on and shouldn't be expected to re absolute fastest way to save and read data from text file or binary file about the details of how Lisp represents filenames.

But programmatically generated filenames will be pathnames because you can create them portably. A stream returned by OPEN also represents a filename, namely, the filename that was originally used to open the stream. Together these three types are collectively referred to as pathname designators.

All the built-in functions that expect a filename argument accept all three types of pathname designator. For instance, all the places in the previous section where you used a string to represent a filename, you could also have passed a pathname object or a stream.

The historical diversity of file systems in existence during the 70s and 80s can be easy to forget. Kent Pitman, one of the principal technical editors of the Common Lisp standard, described the situation once in comp. If you look at the pathname abstraction from the point of view of any single file system, it seems baroque. However, if you take even two such similar file systems as Windows and Unix, you can already begin to see differences the pathname system can help abstract away--Windows filenames contain a drive letter, for instance, while Unix re absolute fastest way to save and read data from text file or binary file don't.

The other advantage of having the pathname abstraction designed to handle the wide variety of file systems that existed in the past is that it's more likely to be able to handle file systems that may exist re absolute fastest way to save and read data from text file or binary file the future. If, say, versioning file systems come back into vogue, Common Lisp will be ready.

A pathname is a structured object that represents a filename using six components: Most of these components take on atomic values, usually strings; only the directory component is further structured, containing a list of directory names as strings prefaced with the keyword: However, not all pathname components are needed on all platforms--this is one of the reasons pathnames strike many new Lispers as gratuitously complex.

On the other hand, you don't really need to worry about which components may or may not be used to represent names on a particular file system unless you need to create a new pathname object from scratch, which you'll almost never need to do. Instead, you'll usually get hold of pathname objects either by letting the implementation parse a file system-specific namestring into a pathname object or by creating a new pathname that takes most of its components from an existing pathname.

It takes a pathname designator and returns an equivalent pathname object. When the designator is already a pathname, it's simply returned. When it's a stream, the original filename is extracted and returned. When the designator is a namestring, however, it's parsed according to the local filename syntax. The language standard, as a platform-neutral document, doesn't specify any particular mapping from namestring to pathname, but most implementations follow the same conventions on a given operating system.

On Unix file systems, only the directory, name, and type components are typically used. On Windows, one more component--usually the re absolute fastest way to save and read data from text file or binary file or host--holds the drive letter.

On these platforms, a namestring is parsed by first splitting it into elements on the path separator--a slash on Unix and a slash or backslash on Windows. The drive letter on Windows will be placed into either the device or the host component. All but the last of the other name elements are placed in a list starting with: This list becomes the directory component of the pathname.

The last element is then split on the rightmost dot, if any, and the two parts put into the name and type components of the pathname. Like many other built-in objects, pathnames have their own read syntax, p followed by a double-quoted string. This allows you to print and read back s-expressions containing pathname objects, but because the syntax depends on the namestring parsing algorithm, such data isn't necessarily portable between operating systems.

It takes one keyword argument for each pathname component and returns a pathname with any supplied components filled in and the rest NIL.

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IO classes to work with files and directories in iOS that you would use in any. However, despite the familiar classes and methods, iOS implements some restrictions on the files that can be created or accessed and also provides special features for certain directories.

This article outlines these restrictions and features, and demonstrates how file access works in a Xamarin. You can use Xamarin.

IO classes in the. The File class lets you create, delete, and read files, and the Directory class allows you to create, delete, or enumerate the contents of directories. You can also use Stream subclasses, which can provide a greater degree of control over file operations such as compression or position search within a file.

This article discusses the features and restrictions of the iOS file system in detail, and includes a sample application that demonstrates how to use Xamarin. IO classes for file system operations on iOS. The following code snippets illustrate some common file operations.

This code enumerates the subdirectories in the current directory specified by the ". Your output will be a list of all the files and folders that are deployed with your application displayed in the console window while you are debugging. To read a text file, you only need a single line of code. This example will display the contents of a text file in the Application Output window. Although working with the complete System. Xml namespace is beyond the scope of this article, you can easily deserialize an XML document from the file system by using a StreamReader like this:.

Xml namespace for more information about serialization. You should also review the Xamarin. This sample shows how to use the Environment class to access the Documents folder where we can create files and directories.

For more information about the System. Working with Json data in a Xamarin. Simply add the NuGet package to your application's project:. Finally, create an instance of the Account class, serialize it to json data and write it to a file:. NET's documentation for more information about working with json data in a.

Despite the similarities between Xamarin. NET in some important ways. In order to include a file in the assembly, you must mark it with a special build action, called Content.

This could be confusing for. This means if your filename casing differs between the file itself and the references to it in code, your code might still work in the simulator but that it would fail on a real device. Combine method, which adjusts for the current platform, rather than hardcode a particular path separator. This is a simple step that makes your code more portable to other platforms. This restriction is known as the Application Sandbox.

In terms of the file system, your application is limited to creating and deleting files and directories in its home directory. The home directory is a unique location in the file system where your application and all its data are stored. You cannot choose or change the location of the home directory for your application; however iOS and Xamarin. The Application Bundle is the folder that contains your application.

It is distinguished from other folders by having the. Your Application Bundle contains your executable file and all the content files, images, etc. When you browse to your Application Bundle in Mac OS, it appears with a different icon than you see in other directories and the. Right-click on this icon and choose View Package Contents to browse the contents of the Application Bundle directory. The contents appear just like the contents of a regular directory, as shown here:.

When your application is installed on a device, the operating system creates its home directory and places your Application Bundle inside. Your code can access the Application Bundle to read data, but nothing should be written to that root directory, as it is signed and any modifications will invalidate your application and prevent it from launching. So, although nothing should be written to the root directory, in iOS 7 and earlier creates a number of directories within the application root directory that are available for use.

The earlier directory and file examples accessed the Documents directory. To write to another directory you must construct a path using the ".. These files can be accessed in iTunes when the device is connected and the user chooses the Apps tab.

For example, the following screenshot shows the files in selected app shared via iTunes:. Users can only access the top-level items in this directory via iTunes. They cannot see the contents of any subdirectories although they can copy them to their computer or delete them.

Your application should take this into consideration and be resilient to destructive updates of the Documents folder. The sample code for this article creates both a file and a folder in the Documents folder in SampleCode.

This screenshot shows how these appear in iTunes:. Refer to the Working with Images article for information about how to set icons for the application and for any custom document types you create. If the UIFileSharingEnabled key is false or not present, then file sharing is, by default, disabled and users will not be able to interact with your Documentsdirectory. Backing up a large amount of data can take a long time.

If you decide you need to back up any particular document or data, your application should only use the Documents and Library folders for this. For transient data or files that can be easily retrieved from the network, use either the Caches or the tmp directory.

Apple introduced iCloud Backup functionality with iOS 5. This feature provides the user with a complete backup in case their device is lost, stolen or damaged. To comply with the iOS Data Storage Guidelines you should limit the amount of data that gets backed up by adhering to the following items:. When SetSkipBackupAttribute is true the file will not be backed-up, regardless of the directory it is stored in even the Documents directory. You can query the attribute using the GetSkipBackupAttribute method, and you can reset it by calling the SetSkipBackupAttribute method with false , like this:.

Since App Extensions run as part of a host application as opposed to their containing app , the sharing of data isn't automatic included so extra work is required.

App Groups are the mechanism iOS uses to allow different apps to share data. If the applications have been properly configured with the correct entitlements and provisioning, they can access a shared directory outside of their normal iOS sandbox. This value must also be referenced in each project's Entitlements. The iOS app and the extension can also share files using a common file path given they have been properly configured with the correct entitlements and provisioning:.

If the Group Path returned is null , check the configuration of the entitlements and the provisioning profile and make sure that they are correct. When a new version of your application is downloaded, iOS creates a new home directory and stores the new Application Bundle in it. This article showed that file system operations are as simple with Xamarin. It also introduced the Application Sandbox and examined the security implications that it causes.

Next, it explored the concept of an Application Bundle. Finally, it enumerated the specialized directories available to your application and explained their roles during application upgrades and backups. Our new feedback system is built on GitHub Issues. For more information on this change, please read our blog post. General File Access Xamarin. Working with directories This code enumerates the subdirectories in the current directory specified by the ". Xml namespace is beyond the scope of this article, you can easily deserialize an XML document from the file system by using a StreamReader like this: Creating Files and Directories This sample shows how to use the Environment class to access the Documents folder where we can create files and directories.

WriteAllText filename, "Write this text into a file" ; Creating a directory is a very similar process: Combine documents, "NewDirectory" ; Directory. CreateDirectory directoryname ; For more information about the System. Serializing Json Working with Json data in a Xamarin. Simply add the NuGet package to your application's project: WriteAllText filename, json ; Refer to Json.

Special Considerations Despite the similarities between Xamarin. The contents appear just like the contents of a regular directory, as shown here: Application Directories When your application is installed on a device, the operating system creates its home directory and places your Application Bundle inside. These directories and their purposes are listed below: If you need to access the content files inside your Application Bundle, the path to this directory is available via the NSBundle.

The contents of this directory can be made available to the user through iTunes file sharing although this is disabled by default. As long as sensitive files remain hidden, these files will not be exposed and potentially moved, modified, or deleted by iTunes if file sharing is enabled in a future version.

You can use the Environment. MyDocuments method to get the path to the Documents directory for your application. The contents of this directory are backed up by iTunes. The contents of this directory are never exposed to the user via iTunes.

You can create your own subdirectories in Library; however, there are already some system-created directories here that you should be aware of, including Preferences and Caches. The contents of this directory except for the Caches subdirectory are backed up by iTunes.