The Storage Kit: BFile

The Storage Kit Table of Contents | The Storage Kit Index

# BFile

Derived from: BNode, BPositionIO

Declared in: be/storage/File.h

Library: libbe.so

###### Summary

A BFile lets you read and write the data portion of a file. It does this by implementing the `Read()` /`Write()` and `ReadAt()`/`WriteAt()` functions that are declared by the BPositionIO class.

### Initializing and Opening

When you construct (or otherwise initialize) a BFile, the file is automatically opened. The file is closed when you re-initialize or destroy the object.

At each initialization, you're asked to supply an "open mode" value. this is a combination of flags that tells the object whether you want to read and/or write the file, create it if it doesn't exist, truncate it, and so on.

You can also initialize a BFile, and create a new file at the same time, through BDirectory's `CreateFile()` function. In this case, you don't have to supply an open mode—the BFile that's returned to you will automatically be open for reading and writing. (You are asked if you want the creation to fail if the named file already exists.)

### Access to Directories and Symbolic Links

Although BFiles are meant to be used to access regular files, you aren't prevented from opening and reading a directory (you won't be able to write the directory, however). This isn't exactly a feature—there's not much reason to access a directory this way—you should simply be aware that it's not an error.

Symbolic links, however, can't be opened by a BFile—not because it's illegal, but because if you ask to open a symbolic link, the link is automatically traversed. The node that the BFile ends up opening will be the file or directory that the link points to.

This is a feature; very few applications should ever need to look at a symbolic link. (If yours is one of the few that does want to, you should go visit the BSymLink class.)

## Constructor and Destructor

### BFile()

`BFile(`void`) `
`BFile(`const BFile &file`) `
`BFile(`const entry_ref *ref, uint32 openMode`) `
`BFile(`const BEntry *entry, uint32 openMode`) `
`BFile(`const char *path, uint32 openMode`) `
`BFile(` BDirectory *dir, const char *path, uint32 openMode`) `

Creates a new BFile object, initializes it according to the arguments, and sets `InitCheck()` to return the status of the initialization.

The default constructor does nothing and sets `InitCheck()` to `B_NO_INIT`. To initialize the object, call `SetTo()`.

The copy constructor creates a new BFile that's open on the same file as that of the argument. Note that the two objects maintain separate data pointers into the same file:

- Separate pointers: Reading and writing through one object does not affect the position of the data pointer in the other object.

- Same file: If one object writes to the file, the other object will see the written data.

For information on the other constructors, see the analogous `SetTo()` functions.

### ~BFile()

virtual ~`BFile() `

Closes the object's file, frees its file descriptor, and destroys the object.

## Member Functions

### GetSize() , SetSize()

virtual status_t `GetSize(`off_t *size`)` const
virtual status_t `SetSize(`off_t size`) `

These functions get and set the size, in bytes, of the object's file. ``

GetSize() returns the size of the file's data portion in the size argument; the measurement doesn't include attributes.

`SetSize()` sets the size of the data portion to the size given by the argument:

- Enlarging a file adds (uninitialized) bytes to its end.

- Shrinking a file removes bytes from the end.

RETURN CODES

- `B_OK`. The file's size was successfully gotten or set.

- `B_NOT_ALLOWED`. (`SetSize()`) The file lives on a read-only volume.

- `B_DEVICE_FULL`. (`SetSize()`) No more room on the file's device.

### InitCheck()

status_t `InitCheck(`void`)` const

Returns the status of the most recent initialization.

RETURN CODES

- `B_OK`. The object is initialized.

- `B_NO_INIT`. The object is uninitialized.

- See `SetTo()` for other errors.

### IsReadable() , IsWritable()

bool `IsReadable(`void`)` const
bool `IsWritable(`void`)` const

These functions tell you whether the BFile was initialized to read or write its file. If the object isn't (properly) initialized, they both return `false`.

Note that these functions don't query the actual file to check permissions, they only tell you what the access request was when the BFile object was initialized.

### Position() see Seek()

### Read() , ReadAt() , Write() , WriteAt()

virtual ssize_t `Read(`void *buffer, size_t size`) `
virtual ssize_t `ReadAt(`off_t location, void *buffer, size_t size`) `
virtual ssize_t `Write(`const void *buffer, size_t size`) `
virtual ssize_t `WriteAt(`off_t location, const void *buffer, size_t size`) `

These functions, which are inherited from BPositionIO, read and write the file's data; note that they don't touch the file's attributes.

The `Read()` and `ReadAt()` functions read size bytes of data from the file and place this data in buffer. The buffer that buffer points to must already be allocated, and must be large enough to accommodate the read data. Note that the read-into buffer is not null-terminated by the reading functions.

The two functions differ in that...

- `Read()` reads the data starting at the current location of the file's data pointer, and increments the file pointer as it reads.

- `ReadAt()` reads the data from the location specified by the location argument, which is taken as a measure in bytes from the beginning of the file. `ReadAt()` does not bump the file's data pointer.

`Write()` and `WriteAt()` write size bytes of data into the file; the data is taken from the buffer argument. The two functions differ in their use (or non-use) of the file's data pointer in the same manner as `Read()` and `ReadAt()`.

All four functions return the number of bytes that were actually read or written; negative return values indicate an error.

Reading fewer-than-size bytes isn't uncommon—consider the case where the file is smaller than the size of your buffer. If you want your buffer to be `NULL`-terminated, you can use the return value to set the `NULL`:

       char buf[1024];
       ssize_t amt_read;

       if ((amt_read = file.Read((void *)buf, 1024)) < 0)
          /* handle errors first */
       else
          /* otherwise set null */
          buf[amt_read] = >0>;

A successful `Write()` or `WriteAt()`, on the other hand, will always write exactly the number of bytes you requested. In other words, `Write()` returns either the size value that you passed to it, or else it returns a negative (error) value.

| Error codes returned by these functions can vary depending on the file system handling the operation. For this reason, specific error codes aren't listed here.

### Seek() , Position()

virtual off_t `Seek(`off_t offset, int32 seekMode`) `
virtual off_t `Position(`void`)` const

`Seek()` sets the location of the file's data pointer. The new location is reckoned as offset bytes from the position given by the seekMode constant:

Constant | Meaning
`SEEK_SET` | Seek from the beginning of the file.
`SEEK_CUR` | Seek from the pointer's current position.
`SEEK_END` | Seek from the end of the file.

If you `Seek()` to a position that's past the end of the file and then do a `Write()`, the file will be extended (padded with garbage) from the old end of file to the `Seek()`'d position. If you don't follow the `Seek()` with a `Write()`, the file isn't extended.

`Seek()` returns the new position as measured (in bytes) from the beginning of the file.

`Position()` returns the current position as measured (in bytes) from the beginning of the file. It doesn't move the pointer.

RETURN CODES

- `B_ERROR`. Attempted to `Seek()` "before" the beginning of the file, or you called `Position()` after such a `Seek()`. You also get `B_ERROR` if you call `Seek()` on an uninitialized file.

- `B_BAD_FILE`. `Position()` called on an uninitialized file.

| If you do a "before the beginning" seek, subsequent `Read()` and `Write()` calls do not fail. But they almost certainly aren't doing what you want (you shouldn't be "before the file," anyway). The moral: Always check your `Seek()` return.

### SetSize() see GetSize()

### SetTo() , Unset()

status_t `SetTo(`const entry_ref *ref, uint32 openMode`) `
status_t `SetTo(`const BEntry *entry, uint32 openMode`) `
status_t `SetTo(`const char *path, uint32 openMode`) `
status_t `SetTo(`const BDirectory *dir, const char *path, uint32 openMode`) `
void `Unset(`void`) `

Closes the BFile's current file (if any), and opens the file specified by the arguments. If the specified file is a symbolic link, the link is automatically traversed (recursively, if necessary). Note that you're not prevented from opening a directory as a BFile, but you are prevented from writing it.

- In the path function, path can be absolute or relative, and can contain "." and ".." elements. If path is relative, it's reckoned off of the current working directory.

- In the dir/path function, path must be relative and is reckoned off of dir.

openMode is a combination of flags that determines how the file is opened and what this object can do with it once it is open. There are two sets of flags; you must pass one (and only one) of the following "read/write" constants:

Constant | Meaning
--- | ---
`B_READ_ONLY` | This object can read, but not write, the file.
`B_WRITE_ONLY` | This object can write, but not read, the file.
`B_READ_WRITE` | This object can read and write the file.

You can also pass any number of the following (these are optional):

Constant | Meaning
--- | ---
`B_CREATE_FILE` | Create the file if it doesn't already exist.
`B_FAIL_IF_EXISTS` | If the file already exists, the initialization (of the BFile object) fails.
`B_ERASE_FILE` | If the file already exists, erase all its data and attributes.
`B_OPEN_AT_END` | Sets the data pointer to point to the end of the file.

To open a file for reading and writing, for example, you simply pass:

       file.SetTo(entry, B_READ_WRITE);

Here we create a new file or erase its data if it already exists:

       file.SetTo(entry, B_READ_WRITE | B_CREATE_FILE | B_ERASE_FILE);

And here we create a new file, but only if it doesn't already exist:

       file.SetTo(entry, B_READ_WRITE | B_CREATE_FILE | B_FAIL_IF_EXISTS);

`Unset()` closes the object's file and sets its`InitCheck()` value to `B_NO_INIT`.

RETURN CODES

- `B_OK`. The file was successfully opened.

- `B_BAD_VALUE`. `NULL` path in dir/path, or some other argument is uninitialized.

- `B_ENTRY_NOT_FOUND`. File not found, or couldn't create the file.

- `B_FILE_EXISTS`. File exists (and you set `B_FAIL_IF_EXISTS`).

- `B_PERMISSION_DENIED`. Read or write permission request denied.

- `B_NO_MEMORY`. Couldn't allocate necessary memory to complete the operation.

### Write() see Read()

### WriteAt() see Read()

## Operators

### = (assignment)

BFile& operator`=(`const BFile &File`) `

In the expression

       BFile a = b;

BFile a is initialized to refer to the same file as b. To gauge the success of the assignment, you should call `InitCheck()` immediately afterwards. You can't assign a BFile to itself (`B_BAD_VALUE`).

Assigning to an uninitialized BFile is "successful": The assigned-to BFile will also be uninitialized (`B_NO_INIT`).

The Storage Kit Table of Contents | The Storage Kit Index

The Be Book,
...in lovely HTML...
for BeOS Release 5.

Copyright © 2000 Be, Inc. All rights reserved..