The Storage Kit: BResources

Derived from: none

Declared in: be/storage/Resources.h

Library: libbe.so

Overview

You may not want to be here...The BResources class was designed for a specific purpose: To provide a means to bundle application "resources" (icons, in particular) within the application executable itself. If you want to add new resources to your own application (resources that you want to have "stick" to the executable), then you've come to the right place. But you shouldn't use BResources to add data to a regular data file--use attributes instead.

The data that a file contains is either "flat," or it's "structured." To read a flat file, you simply open it (through a BFile object) and start Read()'ing. Structured data requires that you understand the structure. Typically, an application understands the structure either because it's a well-known format, or because the application itself wrote the file in the first place.

The BResources class defines a simple design for storing structured data. The structure is a series of "resources," where each resource is key/value pair. A single "resource file" can hold an unlimited number of resources; a single resource within a resource file can contain an unlimited amount of data.

Resources are sort of like attributes in that they store chunks of data that are looked up through the use of a key. But note these differences:

Resources are stored in the file itself, such that if you copy the file, you copy the resources, as well.
Resources can't be queried.
Only plain files can have resources. (In other words, directories and symbolic links can't have resources.)

Initializing a BResources Object

The BResources class provides the means for reading and writing a file's resources, but it doesn't let you access the file directly. Instead, you must initialize the BResources object by passing it a valid BFile object, either in the constructor or the SetTo() function. Note the following:

The BFile that you pass in is copied by the BResources object. Thus, initializing a BResources object opens a new file descriptor into the file. You can delete the "original" BFile immediately after you use it to initialize the BResources object.
If the BFile that you pass in is open with write permission, the file is automatically locked by the BResources object (the object Lock()'s its copy of the BFile that you passed in). It's unlocked when you re-initialize or delete the BResources.
If you want to write resources, the BFile must not be locked when you pass it in. The BResources needs to be able to lock its copy of your object.
The BFile must be open for reading (at least).
Unfortunately, BResources lacks an InitCheck() function. If you want to check initialization errors, you should always initialize through SetTo(), rather than through the constructor.

Identifying and Creating Resource Files

You can't use just any old file as a BResources initializer: The file must be an actual resource file. Simply initializing a BResources object with an existing non-resource file will not transform the file into a resource file--unless you tell the initializer to clobber the existing file.

For example, this initialization fails:

   /* "fido" exists, but isn't a resource file. */
   BFile file("/boot/home/fido", B_READ_WRITE);
   BResources res;
   status_t err;
   
   if ((err = res.SetTo(&file)) != B_NO_ERROR) 
   ...

And this one succeeds...

   /* The second arg to SetTo() is the "clobber?" flag. */
   if ((err = res.SetTo(&file, true)) != B_NO_ERROR) 
   ...

...but at a price: fido's existing data is destroyed (truncated to 0 bytes), and a new "resource header" is written to the file. Having gained a resource header, fido can thereafter be used to initialize a BResources object.

Clobber-setting a resource file is possible, but, as mentioned at the top of this class description, you'll probably never create resource files directly yourself

So where do resource files come from if you don't create them yourself? Step right up...

Executables as Resource Files

The only files that are naturally resource-ful are application executables. For example, here we initialize a BResources object with the IconWorld executable:

   BPath path;
   BFile file;
   BResources res;
   
   find_directory(B_APPS_DIRECTORY, &path);
   path.Append("IconWorld");
   file.SetTo(&path, B_READ_ONLY);
   
   if (res.SetTo(&file) != B_NO_ERROR) 
      ...

The BResources object is now primed to look at IconWorld's resources. But be aware that an application's "app-like" resources (its icons, signature, app flags) should be accessed through the BAppFileInfo class.

Resource Data

After you've initialized your BResources object, you use the FiddleResource() functions to examine and manipulate the file's resources:

Generative Functions

AddResource() adds a new resource to the file.
RemoveResource() removes an existing resource from the file.

Data Functions

WriteResource() writes some amount of new data into an existing resource.
ReadResource() reads a range of data from a resource and gives you a copy.
FindResource() returns a pointer to a resource's data.

Info Functions

HasResource() tells you if the file contains a specified resource.
GetResourceInfo() returns information about a resource.

As mentioned earlier, the BFile that you use to initialize a BResources object must be open for reading. If you also want to modify the resources (by adding, removing, or writing) the BFile must also be open for writing.

Identifying a Resource within a Resource File

A single resource within a resource file is tagged with a data type, an ID, and a name:

The data type is one of the type_code types (B_INT32_TYPE, B_STRING_TYPE, and so on) that characterize different types of data. The data type that you assign to a resource doesn't restrict the type of data that the resource can contain, it simply serves as a way to label the type of data that you're putting into the resource so you'll know how to cast it when you retrieve it.
The ID is an arbitrary integer that you invent yourself. It need only be meaningful to the application that uses the resource file.
The name is optional, but can be useful: You can look up a resource by its name, if it has one.

Taken singly, none of these tags needs to be unique: Any number of resources (within the same file) can have the same data type, ID, or name. It's the combination of the data type constant and the ID that uniquely identifies a resource within a file. The name, on the other hand, is more of a convenience; it never needs to be unique when combined with the data type or with the ID.

Data Format

All resource data is assumed to be "raw": If you want to store a NULL-terminated string in a resource, for example, you have to write the NULL as part of the string data, or the application that reads the resource from the resource must apply the NULL itself. Put more generally, the data in a resource doesn't assume any particular structure or format, it's simply a vector of bytes.

Data Ownership

Resource data that you retrieve from a BResources object belongs to your application. Specifically, the data that's returned by FindResource() is malloc()'d by the object for you--it's your responsibility to free() the data when your finished with it (and your right to realloc() the returned pointer if you want).

Individual changes that you make to the resource file are visible to other BResources (that are open on the same file) as soon as they are made. You can't, for example, bundle up a bunch of changes and then "commit" them all at the same time.

Reading and Writing a Resource File as a Plain File

Just because a file is a resource file, that doesn't mean that you're prevented from reading and writing it as a plain file (through the BFile object). For example, it's possible to create a resource file, add some resources to it, and then use a BFile object to seek to the end of the file and write some flat data. But you have to keep track of the "data map" yourself--if you go back and add more resources to the file (or extend the size of the existing ones), your flat data will be overwritten: The BResources object doesn't preserve non-resource data that lives in the file that it's operating on.

Constructor and Destructor

BResources()

      BResources(void)
      BResources(BFile *file, bool clobber = false)

Creates a new BResources object. You can initialize the object by passing a pointer to a valid BFile; without the argument, the object won't refer to a file until SetTo() is called.

If clobber is true, the file that's referred to by BFile is truncated (it's data is erased), and a new resource file header is written to the file. If clobber is false and the file doesn't otherwise doesn't have a resource header, the initialization fails.

BResources copies the BFile argument; after the constructor returns, you can, for example, delete the BFile that you passed in.

~BResources()

      virtual ~BResources()

Destroys the BResources object.

Member Functions

AddResource()

      status_t AddResource(type_code type,
         int32 id,
         const void *data,
         size_t length,
         const char *name = NULL)

Adds a new resource to the file. For this function to have an effect, the file must be open for writing. The arguments are:

type is one of the type_code constants defined in be/support/TypeConstants.h.
id is the ID number that you want to assign to the resource. The value of the ID has no meaning other than that which your application gives it; the only restriction on the ID is that the combination of it and the data type constant must be unique across all resources in this resource file.
data is a pointer to the data that you want the resource to hold.
length is the length of the data buffer, in bytes.
name is optional, and needn't be unique. Or even interesting.

Ownership of the data pointer isn't assigned to the BResources object by this function; after AddResource() returns, your application can free or otherwise manipulate the buffer that data points to without affecting the data that was written to the file.

RETURN CODES

B_NO_ERROR. The resource was successfully added.
B_ERROR. The file isn't open for writing, the resources already exists, or the resource couldn't otherwise be written.

Currently, AddResource() will write over an existing resource. In this case, the function returns a positive integer (specifically, it returns the number of bytes that it just wrote), but it doesn't change the name of the resource. To work around this bug, you should call RemoveResource() just before calling AddResource().

FindResource()

      void *FindResource(type_code type,
         int32 id,
         size_t *length)

      void *FindResource(type_code type,
         const char *name,
         size_t *length)

Finds the resource identified by the first two arguments, and returns a pointer to a copy of the resource's data. The size of the data, in bytes, is returned by reference in *length.

The data that's returned by FindResource() is allocated (by the object) through malloc(); it's the caller's responsibility to free() the data (and the caller's right to realloc() it).

If the first two arguments don't identify an existing resource, NULL is returned.

GetResourceInfo()

      bool GetResourceInfo(int32 byIndex,
         type_code *typeFound,
         int32 *idFound,
         const char **nameFound,
         size_t *lengthFound)

      bool GetResourceInfo(type_code byType,
         int32 andIndex,
         int32 *idFound,
         const char **nameFound,
         size_t *lengthFound)

      bool GetResourceInfo(type_code byType,
         int32 andId,
         const char **nameFound,
         size_t *lengthFound)

      bool GetResourceInfo(type_code byType,
         const char *andName,
         int32 *idFound,
         size_t *lengthFound)

These functions return information about a specific resource, as identified by the first one or two arguments:

The first version (byIndex) searches for the byIndex'th resource in the file.
The second (byType/andIndex) searches for the byIndex'th resource that has the given type.
The third (byType/andId) looks for the resource with the unique combination of type and ID.
The third (byType/andName) looks for the first resource that has the given type and name.

The other arguments return the other statistics about the resource (if found).

The pointer that's returned in *foundName belongs to the BResources. Don't free it.

The functions return true if a resource was found, and false otherwise.

HasResource()

      bool HasResource(type_code type, int32 id)

      bool HasResource(type_code type, const char *name)

Returns true if the resource file contains a resource as identified by the arguments, otherwise it returns false.

Keep in mind that there may be more than one resource in the file with the same name and type combination. The type and id combo, on the other hand, is unique. See "Identifying a Resource within a Resource File."

ReadResource()

      status_t ReadResource(type_code type,
         int32 id,
         void *data,
         off_t offset,
         size_t length)

Reads data from an existing resource (identified by type and id) and copies it into the data buffer. offset gives the location (measured in bytes from the start of the resource data) from which the read commences, and length is the number of bytes you want to read. The data buffer must already be allocated and should be at least length bytes long.

You can ask for more data than the resource contains; in this case, the buffer is filled with as much resource data as exists (or from offset to the end of the resource). However, note that the function doesn't tell you how much data it actually read.

RETURN CODES

B_NO_ERROR. The resource was found and read.
B_ERROR. The resource wasn't found.

RemoveResource()

      status_t RemoveResource(type_code type, int32 id)

Removes the resource identified by the arguments. See "Identifying a Resource within a Resource File."

RETURN CODES

B_NO_ERROR. The resource was removed.
B_ERROR. The resource wasn't found, or the file isn't open for writing.

SetTo()

      status_t SetTo(BFile *file, bool clobber = false)

Unlocks and closes the object's previous BFile, and re-initializes it to refer to a copy of the argument. If the new BFile is open for writing, the BResources' copy of the BFile is locked.

RETURN CODES

B_NO_ERROR. The resource was removed.
B_BAD_VALUE. The argument BFile is invalid (uninitialized).
B_ERROR. The BResources couldn't be initialized (for whatever reason).

WriteResource()

      status_t WriteResource(type_code type,
         int32 id,
         const void *data,
         off_t offset,
         size_t length)

Writes data into an existing resource, possibly overwriting the data that the resource currently contains.

The type and id arguments identify the target resource; this resource must already be present in the file--WriteResource() doesn't create a new resource if the type/id combination fails to identify a winner. See "Identifying a Resource within a Resource File."
data is a pointer to the new data that you want to place in the resource.
length is the length of the data buffer.
offset gives the location at which you want the new data to be written; the offset is taken as the number of bytes from the beginning of the existing resource data.

If the new data is placed such that it exceeds the size of the current resource data, the resource grows to accommodate the new data.

You can't use this function to "shrink" a resource. To remove a portion of data from a resource, you have to remove the resource and then re-add it.

RETURN CODES

B_NO_ERROR. The resource was written.
B_ERROR. The resource wasn't found, or the file isn't open for writing.

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Last modified March 26, 1998.

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The Storage Kit: BResources