This small document attempts to serve as a guide to the programmer who wants to use the library. It's not a replacement for the man page or reading the code, but is a good starting point for everyone who wants to get involved with it.
The library is not complex to use at all, and the interfaces were designed to be as intuitive as possible, so the text is structured as a guide to present the reader all the common structures and functions the way they're normally used.
This is a library which provides a transaction-oriented I/O API.
We say this is a transaction-oriented API because we make transactions the center of our operations, and journaled because we use a journal (which takes the form of a directory with files on it) to guarantee coherency even after a crash at any point.
In this document, we think of a transaction as a list of (buffer, length, offset) to be written to a file. That triplet is called an operation, so we can say that a transaction represents an ordered group of operations on the same file.
The act of committing a transaction means writing all the elements of that list; and rolling back means to undo a previous commit, and leave the data just as it was before doing the commit.
The library provides several guarantees, the most relevant and useful being that at any point of time, even if the machine crash horribly, a transaction will be either fully applied or not applied at all.
To achieve this, the library uses what is called a journal, a very vague and fashionable term we use to describe a set of auxiliary files that get created to store temporary data at several stages. The proper definition and how we use them is outside the scope of this document, and you as a programmer shouldn't need to deal with it. In case you're curious, it's described in a bit more detail in another text which talks about how the library works internally.
libjio has two basic opaque types which have a very strong relationship, and represent the essential objects it deals with. Note that you don't manipulate them directly, but use them through the API.
The first is jfs_t, usually called the file structure, and it represents an open file, just like a regular file descriptor or a FILE *.
Then second is jtrans_t, usually called the transaction structure, which represents a single transaction.
First of all, as with regular I/O, you need to open your files. This is done with jopen(), which looks a lot like open() but returns a file structure instead of a file descriptor (this will be very common among all the functions), and adds a new parameter jflags that can be used to modify some library behaviour we'll see later, and is normally not used.
Now that you have opened a file, the next thing to do would be to create a transaction. This is what jtrans_new() is for: it takes a file structure and returns a new transaction structure.
To add a write operation to the transaction, use jtrans_add_w(). You can add as many operations as you want. Operations within a transaction may overlap, and will be applied in order.
Finally, to apply our transaction to the file, use jtrans_commit().
When you're done using the file, call jclose().
Let's put it all together and code a nice "hello world" program (return values are ignored for simplicity):
char buf[] = "Hello world!";
jfs_t *file;
jtrans_t *trans;
file = jopen("filename", O_RDWR | O_CREAT, 0600, 0);
trans = jtrans_new(file, 0);
jtrans_add_w(trans, buf, strlen(buf), 0);
jtrans_commit(trans);
jtrans_free(trans);
jclose(file);
As we've seen, you open the file and initialize the structure with jopen() (with the parameter jflags being the last 0), create a new transaction with jtrans_new(), then add an operation with jtrans_add_w() (the last 0 is the offset, in this case the beginning of the file), commit the transaction with jtrans_commit(), free it with jtrans_free(), and finally close the file with jclose().
Reading is much easier: the library provides three functions, jread(), jpread() and jreadv(), that behave exactly like read(), pread() and readv(), except that they play safe with libjio's writing code. You should use these to read from files when using libjio.
You can also add read operations to a transaction using jtrans_add_r(), and the data will be read atomically at commit time.
An essential part of the library is taking care of recovering from crashes and be able to assure a file is consistent. When you're working with the file, this is taking care of; but what about when you first open it? To answer that question, the library provides you with a function named jfsck(), which checks the integrity of a file and makes sure that everything is consistent.
It must be called "offline", that is when you are not actively committing and rollbacking; it is normally done before calling jopen() and is very, very important.
You can also do this manually with an utility named jiofsck, which can be used from the shell to perform the checking.
There is a very nice and important feature in transactions, that allows them to be "undone", which means that you can undo a transaction and leave the file just as it was the moment before applying it. The action of undoing it is called rollback, and the function is called jtrans_rollback(), which takes the transaction as the only parameter.
Be aware that rollbacking a transaction can be dangerous if you're not careful and cause you a lot of troubles. For instance, consider you have two transactions (let's call them 1 and 2, and assume they were applied in that order) that modify the same offset, and you rollback transaction 1; then 2 would be lost. It is not an dangerous operation itself, but its use requires care and thought.
There is a set of functions that emulate the UNIX API (read(), write(), and so on) which make each operation a transaction. This can be useful if you don't need to have the full power of the transactions but only to provide guarantees between the different functions. They are a lot like the normal UNIX functions, but instead of getting a file descriptor as their first parameter they get a file structure. You can check out the manual page to see the details, but they work just like their UNIX version, only that they preserve atomicity and thread-safety within each call.
In particular, the group of functions related to reading (which was described above in Basic operation) are extremely useful because they take care of the locking needed for the library proper behaviour. You should use them instead of the regular calls.
The full function list is available on the man page and I won't reproduce it here; however the naming is quite simple: just prepend a 'j' to all the names: jread(), jwrite(), etc.
The library is completely safe to use in multi-process and/or multi-thread applications, as long as you abide by the following rules:
- Within a process, a file must not be held open at the same time more than once, due to fcntl() locking limitations. Opening, closing and then opening again is safe.
- jclose() must only be called when there are no other I/O operations in progress.
- jfsck() must only be called when the file is known not to be open by any process.
- jmove_journal() must only be called when the file is known not to be open by any other processes.
All other operations (committing a transaction, rolling it back, adding operations, etc.) and all the wrappers are safe and don't require any special considerations.
If you need to increase performance, you can use lingering transactions. In this mode, transactions take up more disk space but allows you to do the synchronous write only once, making commits much faster. To use them, just add J_LINGER to the jflags parameter in jopen(). You should call jsync() frequently to avoid using up too much space, or start an asynchronous thread that calls jsync() automatically using jfs_autosync_start(). Note that files opened with this mode must not be opened by more than one process at the same time.
The library creates a single directory for each file opened, named after it. So if we open a file output, a directory named .output.jio will be created. We call it the journal directory, and it's used internally by the library to save temporary data; you shouldn't modify any of the files that are inside it, nor move it while it's in use.
It doesn't grow much (it only uses space for transactions that are in the process of committing) and gets automatically cleaned while working with it so you can (and should) ignore it. Besides that, the file you work with has no special modification and is just like any other file, all the internal stuff is kept isolated on the journal directory.
Besides the UNIX-alike API you can find an ANSI C alike API, which emulates the traditional fread(), fwrite(), etc. It's still in development and has not been tested carefully, so I won't spend time documenting them. Let me know if you need them.
If you have pkg-config in your build environment, then you can get the build flags you need to use when building and linking against the library by running:
pkg-config --cflags --libs libjio
If pkg-config is not available, you have to make sure your application uses the Large File Support ("LFS" from now on), to be able to handle large files properly. This means that you will have to pass some special standard flags to the compiler, so your C library uses the same data types as the library. For instance, on 32-bit platforms (like x86), when using LFS, offsets are usually 64 bits, as opposed to the usual 32.
The library is always built with LFS; however, linking it against an application without LFS support could lead to serious problems because this kind of size differences and ABI compatibility.
The Single Unix Specification standard proposes a simple and practical way to get the flags you need to pass your C compiler to tell you want to compile your application with LFS: use a program called "getconf" which should be called like "getconf LFS_CFLAGS", and it outputs the appropiate parameters.
In the end, the command line would be something like:
gcc `getconf LFS_CFLAGS` app.c -ljio -o app
If you want more detailed information or examples, you can check out how the library and sample applications get built.
If you're still interested in learning more, you can find some small and clean samples are in the samples directory (full.c is a simple and complete one), other more advanced examples can be found in the web page, as well as modifications to well known software to make use of the library. For more information about the inner workings of the library, you can read the "libjio" document, the internal API reference, and the source code.