Tracing API calls on Windows

This post was written by eli on July 5, 2010
Posted Under: Microsoft,Software

Linux has ltrace. Windows has…? I was looking for applications to trace DLL calls, so I could tell why a certain application goes wrong. The classic way is to get hints from library calls. Or system calls. Or both.

In the beginning, I was turned down by they idea, that most trackers only support those basic system DLLs (kerner32 and friends), but I was soon to find out that one gets loads of information about what the application is at only through them.

I found a discussion about such tools in a forum leading my to the WinAPIOverride32 application (“the tracker” henceforth), which is released under GPL. It doesn’t come with a fancy download installation, so you have to run the WinAPIOverride32.exe yourself and read the help file at WinAPIOverride.chm. Having looked at a handful of commercial and non-commercial alternatives, I have to say that it’s excellent. The only reason I looked further, is because it didn’t run on Windows 2000, which is an understandable drawback, and still, a problem for me.

The documentation is pretty straightforward about what to do, but here’s a short summary anyhow:

You can pick your target application by running it from the tracker, or hook on a live process by picking its process number, or even better, drag an icon over the target process’ window. Then click the green “play” button to the upper left. If you chose to run your process ad hoc, you’ll set up the modules immediately after the launch, and then resume operation by confirming a dialog box. As a side feature, this allows starting a process just to halt it immediately, and let a debugger hook on it, and then resume. A bit ugly, but effective (with Microsoft Studio, for example).

Module filters sets what calls are included or excluded, depending on the modules calling them. One problem with including modules such as kernel32 is that several calls are made while the hooks are made (by the tracker itself), so the log explodes with calls while the target application is paused anyhow. This is solved by using the default exclusion list (NotHookedModuleList.txt). Only be sure to have the “Use list” checked, and have the Apply for Monitoring and Apply for Overriding set. Or hell breaks loose.

At this point, the idea is to select which API calls are monitored. Click the torch. There’s a list of monitor files, basically contain the names of the DLLs to be hooked upon, and the function prototypes. One can pinpoint which functions to monitor or not, but the general idea is that the monitor files are sorted according to subjects which the calls cover (registry, I/O, etc).

Choosing kernel32 will give a huge amount of data, which reveals more or less what the target application was doing. Monitoring “reg” is also useful, as it reveals registry access. Specific other DLLs can be helpful as well.

When huge amounts of data comes out, the log will keep running even after monitoring has been stopped. If this goes on for a while, a dialog box opens, saying that unhooking seems to take too much time, and if it should wait. Answering “no” will cut the log right there, possibly causing the target application to crash. Whether to chop it right there or not is an open question, since the critical event is already over, yes, but we’re not sure whether it has been logged.

To make things easier, just minimize the tracker’s window, since it’s mainly the display of the log which slows things down. Quit the target application, wait for the popup telling about the unload of the process, and then look.

A very nice thing is that it’s possible to create monitor files automatically with the MonitoringFileBuilder.exe application, also in the bundle. Pick an application and create a monitor file for all its special DLLs, or pick a DLL to monitor its calls. The only problem with these files is that since the information about the function prototypes is missing, parsing the arguments is impossible.

It’s possible to write the call logs to an XML file or just a simple text file, of course. The only annoying thing is that the output format it 16-bit unicode. Notepad takes this easily, but simple text editors don’t.

In short, it’s a real work horse. And it just happened to help me solve a problem I had. This is the classic case where free software which was written by the person who uses it takes first prize, over several applications which were written to be sold.

I should also mention the Dumper.exe, which allows connection to any process, and not only dump and modify the process’ memory on the fly, but also see what DLL is mapped where in memory (which is useful when reading in-memory raw assembly with a debugger). Also, it allows displaying the call stack for each thread, which is sometimes more instructive than Microsoft’s debugger (is that really a surprise?).

But since I had a brief look on other things, I wrote my impressions. They may not be so accurate.

Other things I had a look at

There’s SpyStudio API monitor, which I had a look on (it isn’t free software, anyhow. That is, free as in free beer, but not as in freedom). Its main drawback is that it logs only specific functions, and doesn’t appear to allow an easy appliance of hooks to a massive amount of functions. In other words, one needs to know what the relevant functions are, which isn’t useful when one wants to know what an application is doing.

I also had a look on API monitor, which was completely useless to me, since it doesn’t allow adding command line arguments when launching a new process. Not to mention that their trial version completely stinks (buy me! buy me!). Everything useful was blocked in the trial version. I wonder if the real version is better. Was that an application I gladly uninstalled.

API sniffers with names such as Kerberos and KaKeeware Application Monitor seem to include trojan horses, according to AVG. Didn’t take the risk.

Rohitab API Monitor v1.5 (which I picked, since v2 is marked Alpha) wouldn’t let me start a new process, and since I was stupid enough to monitor all calls on all processes, this brought a nasty computer crash (when I killed the tracker). After a correspondence with the author, it turns out that the version is 10 years old, and it is possible to start a process with arguments. Then I tried v2. I would summarize it like this: It indeed looks very pretty, and seems to have a zillion features, but somehow I didn’t manage to get the simplest things done with it. Since I don’t want to rely on contacting the author for clarifications all the time, I don’t see it as an option.

Auto Debug appears pretty promising. It’s not free in any way, though, and even though it caught all kernel32 calls, and has neat dissection capabilities, I couldn’t see how to create a simple text output of the whole log. Maybe because I used the application in trial mode.

The Generic Tracker looks very neat, and it’s a command line application, which makes me like it even more. I didn’t try it though, because it allows tracking only four functions (as it’s based upon break points). But it looks useful for several other things as well.

Reader Comments

API Monitor has been completely rewritten and has tons of new features including 64-bit support, approx 10,000 API definitions, API call tree, monitoring of NT services, custom DLL monitoring. See a list of complete features and screenshots at

Written By Rohitab Batra on July 13th, 2010 @ 17:18

strace.exe in cygwin?

Written By Dennis Yurichev on July 13th, 2010 @ 18:06

Hi Eli, Nektra’s co-founder (SpyStudio) here.

Indeed SpyStudio has been improved a lot since 2010 and it’s used by companies such as VMWare and Symantec. If you want to hook more functions you can use our hook engine Deviare that is the foundation for SpyStudio. Deviare includes a console code where you can hook every function you like or add new ones.

With Deviare you can even hook functions that are available only in PDBs. For example, you can take a look at this article, with code, where we intercept Microsoft SQL Server:

Written By Sebastian Wain on October 18th, 2013 @ 18:13

i tried this now.
wow 8yrs ago
None of them usefull for me because
i want to hook local func calls
these sotwars can only list external calls (api )

i will make softwear for hooking local calls soon.
utilising debugger api is os dependent.
with root permission reading process memmory is platform depended
how these softwears works .i think they are using win32 debugger api.
but anti debugging tecniques will defend them well.
debugger must make himself transperant

Written By ironboykabeesh on March 15th, 2018 @ 09:48


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