The recursion of the WinHttpReadData function is bounded, which means that although the WinHttpReadData is acting synchronously and it tends to recurse, it will only recurse for a specified maximum number of times after which it will continue the completions on another thread. This other thread can also have recursion up to the amount specified.
The number of times WinHttpReadData will recurse depends on the operating system you are running your WinHttp application on. The information is as follows:
| Operating System | Maximum number of recursions |
Windows XP SP2 and above
Windows 2003 and the service packs
Windows Vista, 2008 and the service packs | 64 |
| Windows 7, Windows 2008 R2 | 2 |
WinHttpReadData can behave synchronously if there is enough data buffered at the local socket, where no further wait is needed and the data is available immediately.
This recursive behavior is usually seen for a large file transfer over the localhost server or a high speed network connection.
Using WinHttp API logging, you will see the below behavior which explains the recursion:
HH:MM:SS:ZZZ ::*000000n* :: WinHttpQueryDataAvailable(0xdb0000, 0x0, 0x124fa7c)
HH:MM:SS:ZZZ ::*000000n* :: WinHttpReadData(0xdb0000, 0xc75e20, 455, 0x0)
HH:MM:SS:ZZZ ::*000000n* :: WinHttpQueryDataAvailable(0xdb0000, 0x0, 0x124eac8)
………
………
HH:MM:SS:ZZZ ::*000000n* :: WinHttpReadData(0xdb0000, 0xc76018, 8192, 0x0)
HH:MM:SS:ZZZ ::*000000n* :: WinHttpQueryDataAvailable(0xdb0000, 0x0, 0x124db14)
HH:MM:SS:ZZZ ::*000000n* :: WinHttpQueryDataAvailable() returning TRUE
HH:MM:SS:ZZZ ::*000000n* :: WinHttpReadData() returning TRUE
HH:MM:SS:ZZZ ::*000000n* :: WinHttpQueryDataAvailable() returning TRUE
HH:MM:SS:ZZZ ::*000000n* :: WinHttpReadData() returning TRUE
HH:MM:SS:ZZZ ::*000000n* :: WinHttpQueryDataAvailable() returning TRUE
If you control the stack size of each thread and set the maximum thread stack size to a very low value, you will experience a stack overflow due to the 64 levels of recursive behavior on operating systems earlier than Windows 7 and Windows 2008 R2. Therefore care should be taken when designing applications and prevent allocating too little of the default thread stack size available for each application thread.
Please reference:
http://msdn.microsoft.com/en-us/library/ms686774.aspx for more information on thread stack size and how to control it.