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Lock a document

Lock a PDF document in UWP

If you plan on sharing a single PDF document instance across multiple threads, then you need to lock the document. Not locking a PDF document instance shared across threads can result in undefined behaviour, including crashing.

A PDFTron SDK lock is based on two principles:

Reentrant mutex (also known as recursive lock)
Recursive mutex may be locked multiple times by the same process/thread without causing a deadlock.

Readers-writer (RW) lock (also known as shared-exclusive lock)
An RW lock allows concurrent access for read-only operations, while write operations require exclusive access.

Write to a document

To write to a PDF document in a multithreaded environment.

Only one thread can hold a write lock at any given time.
PDFDoc doc = new PDFDoc(filename);

// Lock the document

// ... perform some document processing using write operations

// Now we release the lock

Multithreaded PDF reading
Full source code which illustrates how to use PDFDoc locking mechanisms to access the document concurrently. PDFDoc uses a recursive shared lock model.

Read and write to a document

To read and write to a PDF document in a multithreaded environment.

A thread cannot acquire a write lock while holding a read lock.
PDFDoc doc = new PDFDoc(filename);

// Lock the document, since we are going to write to this thread.

// Optionally note that locking a second time does not cause a deadlock

// You can acquire a read lock while holding a write lock:

// ... perform some document processing using read operations


// ... perform some document processing using write operations

// Releasing the lock decrements our lock count,
// but the thread still holds a write lock on the document

// Now we release the write lock, and
// other tasks can begin execution

Multithreaded PDF reading
Full source code which illustrates how to use PDFDoc locking mechanisms to access the document concurrently. PDFDoc uses a recursive shared lock model.

About locking a document

As computing devices become more parallel in nature, PDFTron is evolving to allow developers to leverage this power in new and exciting ways. PDFTron version 6.0 introduces new locking semantics which allow for concurrent access of a PDFDoc instance. This was done to improve performance during interactive viewing (simultaneous rendering, text extraction, etc.), as well as to open up the possibility for new use cases (parallel rendering). This article introduces the locking system, and will get you on your way to developing parallel applications with PDFTron SDK.

PDFDoc lock

PDFTron uses a recursive read/write locking system. Multiple threads can hold a read lock on the document, but only one thread can hold a write lock at any given time. A thread can acquire an equivalent or weaker lock as many times as it likes without causing a deadlock. In other words, the following is valid:

PDFDoc d = new PDFDoc("foo.pdf");

However, a thread cannot acquire a write lock while holding a read lock. Because one can only acquire a write lock when no read locks are held, this situation would inevitably lead to a deadlock. To avoid this scenario, PDFTron will throw a runtime exception whenever the situation occurs.

Locking in the PDFTron API

Some of our API calls internally acquire a write lock on the document. As a result, these calls can also throw a runtime exception if they are invoked while holding a read lock. This is noted in each method's documentation. Additionally, you will find a complete list of those methods at the end of this article.

In general, the parts of our library that manage the UI will maintain document locks. For the lower level calls which actually modify the document, you are responsible for maintaining document locks.

Client side locking

PDFTron provides the following APIs for locking the document:

void PDFDoc.Lock();
bool PDFDoc.TryLock();
bool PDFDoc.TimedLock(int milliseconds);
void PDFDoc.Unlock();
void PDFDoc.LockRead();
bool PDFDoc.TryLockRead();
bool PDFDoc.TimedLockRead(int milliseconds);
void PDFDoc.UnlockRead();

Migration of earlier code

The new locking system is backwards compatible, meaning previous calls to PDFDoc.Lock now acquire a write lock. If you would like to take advantage of the ability to read a PDF concurrently, it is your responsibility to review document locks and determine whether it is safe to downgrade them to a read lock.

Opting out

Conversely, if you are happy with the existing 'one document, one thread' model previously used in PDFTron 5.9., you can continue to work with this system. No change is required on your behalf.

View interrupt with DocLock

For convenience, PDFView or PDFViewCtrl exposes similar methods, which will be applied to the currently open document or associated document. Additionally, the PDFView.DocLock or PDFViewCtrl.DocLock method takes a cancel_rendering parameter, which will interrupt all worker threads currently accessing the document. This allows you to acquire the write lock as fast as possible:

void PDFViewCtrl.DocLock(bool cancel_rendering);
bool PDFViewCtrl.DocTryLock(int milliseconds);
void PDFViewCtrl.DocUnlock();
void PDFViewCtrl.DocLockRead();
bool PDFViewCtrl.DocTryLockRead(int milliseconds);
void PDFViewCtrl.DocUnlockRead();

Input Filters

At the low level, a PDFDoc uses an input filter to access its PDF data. this data could be stored on the file system, in a memory buffer, or over a network. Now that PDFTron supports concurrent access of PDFDoc across many threads; these input filters must also be made thread-safe. StdFile, which was not a thread-safe filter, is no longer available. Instead, you should now use the new MappedFile filter, which provides thread-safe and efficient read access on a file. Custom user filters are still supported, although they are now wrapped in an internal filter that guarantees thread safety.

API calls which can acquire a write lock

void PDFViewCtrl.CloseDoc();
void PDFViewCtrl.SetDoc(PDFDoc);
PDFDoc.SaveAsync(String, SDFDocSaveOptions)
PDFDoc.SaveAsync(IStorageFile, SDFDocSaveOptions)
PDFDoc.SaveAsync(IOutputStream, SDFDocSaveOptions)
PDFDoc.SaveAsync(IRandomAccessStream, SDFDocSaveOptions)
PDFDoc.SaveAsync(IFilter, SDFDocSaveOptions)
PDFDoc.SaveAsync(IRandomAccessStream, SDFDocSaveOptions, String)
PDFDoc.SaveToNewLocationAsync(IStorageFile, SDFDocSaveOptions)
PDFDoc.SaveToNewLocationAsync(IRandomAccessStream, SDFDocSaveOptions, String)

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