Intercepting System API Calls

by Seung-Woo Kim


There are many cases where it is necessary for software developers or testers to intercept system function calls in order to instrument code or to extend operating-system functionality. There are a few packages available that provide this functionality, such as the Detours* library from Microsoft or OK Thinking Software's Syringe*. On the other hand, developers may wish to implement this functionality themselves, without implementing third-party software.

This article describes various ways of function interception and presents a generic method to achieve this task without relying on commercial software packages or being bound to GNU* licensing. All materials in this paper were either developed by Intel or modified from MSDN* sample code.

Two Basic Techniques for Intercepting System Function Calls

Most methods of intercepting arbitrary function calls work by preparing a DLL that replaces the target function to be intercepted and then injecting the DLL to the target process; upon attaching to the target process, the DLL hooks itself to the target function. This technique is suitable, because the source code for the target application is not available most of the time, and it is relatively simple to write a DLL that contains the replacement function, separating it from the rest of the software.

Two intercepting methods have been studied and analyzed. Syringe works by modifying the function import entries (thunking table). On the other hand, the Detours library directly modifies the target function (in the target process space) to make an unconditional jump to the replacement function. Optionally, it provides a trampoline function that can call the original function.

The Detours technique follows this latter method because Syringe has trouble finding the thunks in many cases, and it does not provide trampoline capability to call the original function. Injecting the DLL works the same way in both cases.

The overall workflow to intercept system function calls is as follows:

DLL Injection - First, the main software opens the target process and forces it to load the DLL that contains the replacement functions.

Target Function Modification - When the DLL attaches to the process, it modifies the target function in the target process space so that it directly jumps to the replacement function in the DLL. Optionally, a trampoline function can call the original function.

Target Function Intercepted - When the target function is called, it directly jumps to the replacement function in the DLL. If the developer wishes to invoke the original functionality, he or she calls the trampoline function.


DLL Injection

This section is entirely based on the MSDN article, "Escape from DLL Hell with Custom Debugging and Instrumentation Tools and Utilities*," which includes downloadable source code. Inject.cpp and Inject.h are available as an appendix to this article. They are customized for easy integration – just include them in a project and call InjectLib. The algorithm to force the target process to load the DLL works as follows:

Open the target process by calling OpenProcess.

Allocate memory in the target process by calling VirtualAllocEx. Write to the allocated memory the name of the DLL to be injected using WriteProcessMemory.

Get the address of LoadLibrary by calling GetProcAddress(GetModuleHandle(TEXT("Kernel32")), "LoadLibraryW");

Call CreateRemoteThread, specifying the entry point of LoadLibrary and the name of the DLL (in step 2) as its argument. The target process will load the DLL.

Free the allocat ed memory using VirtualFreeEx. It is not needed anymore.


Inject.cpp incorporates a great deal more functionality, including substantial security features, but the preceding steps are sufficient to illustrate core concepts.

Target-Function Modification

Target-function modification is self-modifying code that is well documented on MSDN*, although there are a few pitfalls in injecting jmp into the process memory. This section shows almost complete sample code to avoid confusion.

The two aspects of target-function modification are replacement and trampoline functions. The following code snippet is an example DLL to intercept the GetSystemPowerStatus API:

The first thing this code does upon attaching is to call InterceptAPI. It requires the name of the module containing the target function, the name of the target function, and the address of the replacement function. GetSystemPowerStatus is in kernel32.dll. Other basic Win32* APIs, such as MessageBox and PeekMessage, are available in user32.dll. MSDN specifies the module to which each API belongs; a future enhancement could automatically find the correct module for a given API.

InterceptAPI overwrites the first five bytes of the target function to an unconditional jump (opcode 0xE9), followed by the displacement to the replacement function as a signed integer (four bytes). The displacement starts at the next instruction; hence, pbReplaced - (pbTargetCode +4) is required. Two cautions are necessary to make this code work:

Change the protection mode of the region overwritten by VirtualProtect. Otherwise, an access-violation error occurs.

FlushInstructionCache is necessary to support those cases where the instructions are already in cache. Otherwise, old code will run from cache, even though the instructions have been changed in memory.


Now, when the GetSystemPowerStatus function is called, all it does is to jump to our replacement function, and it returns directly to the caller, successfully intercepting the call.

Trampoline Function

In many cases, the replacement function needs to call the original target function in addition to its own code, in order to extend the capability of the API, rather than replacing the whole thing. A trampoline function provides this functionality. The theory behind trampoline functions is as follows:

Prepare a dummy function that has the same declaration that will be used as the trampoline. Make sure the dummy function is more than 10 bytes long.

Before overwriting the first five bytes of the target function, copy them to the beginning of the trampoline function.

Overwrite from the sixth byte of the trampoline with an unconditional jump to the sixth byte of the target function

Overwrite the target function as before.

When a trampoline function is called (from the replacement function or anywhere else), it executes the first five bytes of the copied original code, and then jumps to the sixth byte of the real original code. The control returns to the caller of the trampoline. After optionally completing additional tasks, control returns to the caller of the API.


One additional complication exists, in that the sixth byte of the original code may be part of the previous instruction. In that case, the function overwrites part of the previous instruction and then crashes. In the case of GetSystemPowerStatus, the beginning of a new instruction after the first five bytes is the seventh byte. Thus, for this scheme to work, six bytes need to be copied to the trampoline, and the code must adjust this offset accordingly.

The number of bytes that the code needs to copy depends upon the API. It is necessary to look at the original target code (using a debugger or a disassembler) and to cou nt the number of bytes to copy. A future enhancement could automatically detect the correct offset. Assuming that we know the correct offset, the following code shows the extended InterceptAPI function that sets up the trampoline function as well:

BOOL InterceptAPI(HMODULE hLocalModule, const char* c_szDllName, const char* c_szApiName, DWORD dwReplaced, DWORD dwTrampoline, int offset)


int i;

DWORD dwOldProtect;

DWORD dwAddressToIntercept = (DWORD)GetProcAddress(

GetModuleHandle((char*)c_szDllName), (char*)c_szApiName);

BYTE *pbTargetCode = (BYTE *) dwAddressToIntercept;

BYTE *pbReplaced = (BYTE *) dwReplaced;

BYTE *pbTrampoline = (BYTE *) dwTrampoline;

// Change the protection of the trampoline region

// so that we can overwrite the first 5 + offset bytes.

VirtualProtect((void *) dwTrampoline, 5+offset, PAGE_WRITECOPY, &dwOldProtect);

for (i=0;i<offset;i++)

*pbTrampoline++ = *pbTargetCode++;

pbTargetCode = (BYTE *) dwAddressToIntercept;

// Insert unconditional jump in the trampoline.

*pbTrampoline++ = 0xE9;        // jump rel32

*((signed int *)(pbTrampoline)) = (pbTargetCode+offset) - (pbTrampoline + 4);

VirtualProtect((void *) dwTrampoline, 5+offset, PAGE_EXECUTE, &dwOldProtect);

// Overwrite the first 5 bytes of the target function

VirtualProtect((void *) dwAddressToIntercept, 5, PAGE_WRITECOPY, &dwOldProtect);

*pbTargetCode++ = 0xE9;        // jump rel32

*((signed int *)(pbTargetCode)) = pbReplaced - (pbTargetCode +4);

VirtualProtect((void *) dwAddressToIntercept, 5, PAGE_EXECUTE, &dwOldProtect);

// Flush the instruction cache to make sure 

// the modified code is executed.

FlushInstructionCache(GetCurrentProcess(), NULL, NULL);

return TRUE;




This article describes a generic method to intercept system function calls, as well as providing trampoline functions to retain the original functionality. Because this paper is a summary of methods, rather than a complete package, some details are not implemented:

Automatic detection of the module containing the target API.

Automatic detection of the offset for the trampoline function.
Removing replacement functions and ejecting the DLL. (For now, the only way to clean up is to close the application.)


Nevertheless, the techniques, explanations, and source code in this article should be sufficient for developers to implement software that can intercept any system function calls without relying on third-party software packages.

About the Author

Seung-Woo Kim received his Ph.D in Computer Science at University of Minnesota and is currently working as a Senior Application Engineer at Intel. He specializes in the performance optimization for technical and commercial software. He can be reached at this email address.

Additional Resources



Intel Software Forums


Download the PDF (130KB)


#include "stdafx.h"

#include "Inject.h"

#include "tchar.h"

#include "malloc.h"    // For alloca 

#include "pi.h" 

#ifdef UNICODE

#define InjectLib InjectLibW


#define InjectLib InjectLibA

#endif   // !UNICODE

BOOL AdjustDacl(HANDLE h, DWORD DesiredAccess)


// the WORLD Sid is trivial to form programmatically (S-1-1-0)














ACL* pdacl = 0;

DWORD err = SetEntriesInAcl(1, &ea, 0, &pdacl);

if (err == ERROR_SUCCESS)


err = SetSecurityInfo(h, SE_KERNEL_OBJECT, DACL_SECURITY_INFORMATION, 0, 0, pdacl, 0);


return(err == ERROR_SUCCESS);





// Useful helper function for enabling a single privilege

BOOL EnableTokenPrivilege(HANDLE htok, LPCTSTR szPrivilege, TOKEN_PRIVILEGES& tpOld)



tp.PrivilegeCount = 1;

tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;

if (LookupPrivilegeValue(0, szPrivilege, &tp.Privileges[0].Luid))


// htok must have been opened with the following permissions:

// TOKEN_QUERY (to get the old priv setting)

// TOKEN_ADJUST_PRIVILEGES (to adjust the priv)

DWORD cbOld = sizeof tpOld;

if (AdjustTokenPrivileges(htok, FALSE, &tp, cbOld, &tpOld, &cbOld))

// Note that AdjustTokenPrivileges may succeed, and yet

// some privileges weren't actually adjusted.

// You've got to check GetLastError() to be sure!

return(ERROR_NOT_ALL_ASSIGNED != GetLastError());







// Corresponding restoration helper function

BOOL RestoreTokenPrivilege(HANDLE htok, const TOKEN_PRIVILEGES& tpOld)


return(AdjustTokenPrivileges(htok, FALSE, const_cast<TOKEN_PRIVILEGES*>(&tpOld), 0, 0, 0));


HANDLE GetProcessHandleWithEnoughRights(DWORD PID, DWORD AccessRights)


HANDLE hProcess = ::OpenProcess(AccessRights, FALSE, PID);

if (hProcess == NULL)



if (hpWriteDAC == NULL)


// hmm, we don't have permissions to modify the DACL...

// time to take ownership...

HANDLE htok;

if (!OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES, &htok))




if (EnableTokenPrivilege(htok, SE_TAKE_OWNERSHIP_NAME, tpOld))


// SeTakeOwnershipPrivilege allows us to open objects with

// WRITE_OWNER, but that's about it, so we'll update the owner,

// and dup the handle so we can get WRITE_DAC permissions.

HANDLE hpWriteOwner = OpenProcess(WRITE_OWNER, FALSE, PID);

if (hpWriteOwner != NULL)


BYTE buf[512]; // this should always be big enough

DWORD cb = sizeof buf;

if (GetTokenInformation(htok, TokenUser, buf, cb, &cb))


DWORD err = 






0, 0, 0 


if (err == ERROR_SUCCESS)


// now that we're the owner, we've implicitly got WRITE_DAC

// permissions, so ask the system to reevaluate our request,

// giving us a handle with WRITE_DAC permissions

if (









hpWriteDAC = NULL;



// don't forget to close handle



// not truly necessary in this app,

// but included for completeness

RestoreTokenPrivilege(htok, tpOld);


// don't forget to close the token handle



if (hpWriteDAC)


// we've now got a handle that allows us WRITE_DAC permission

AdjustDacl(hpWriteDAC, AccessRights);

// now that we've granted ourselves permission to access 

// the process, ask the system to reevaluate our request,

// giving us a handle with right permissions

if (











hProcess = NULL;






BOOL WINAPI InjectLibW(DWORD dwProcessId, PCWSTR pszLibFile) 


BOOL fOk = FALSE; // Assume that the function fails

HANDLE hProcess = NULL, hThread = NULL;

PWSTR pszLibFileRemote = NULL;

// Get a handle for the target process.

hProcess = 



PROCESS_QUERY_INFORMATION |   // Required by Alpha

PROCESS_CREATE_THREAD     |   // For CreateRemoteThread

PROCESS_VM_OPERATION      |   // For VirtualAllocEx/VirtualFreeEx

PROCESS_VM_WRITE              // For WriteProcessMemory


if (hProcess == NULL)


// Calculate the number of bytes needed for the DLL's pathname

int cch = 1 + lstrlenW(pszLibFile);

int cb  = cch * sizeof(WCHAR);

// Allocate space in the remote process for the pathname

pszLibFileRemote = 

(PWSTR) VirtualAllocEx(hProcess, NULL, cb, MEM_COMMIT, PAGE_READWRITE);

if (pszLibFileRemote != NULL)


// Copy the DLL's pathname to the remote process's address space

if (WriteProcessMemory(hProcess, pszLibFileRemote, 

(PVOID) pszLibFile, cb, NULL))


// Get the real address of LoadLibraryW in Kernel32.dll


GetProcAddress(GetModuleHandle(TEXT("Kernel32")), "LoadLibraryW");

if (pfnThreadRtn != NULL)


// Create a remote thread that calls LoadLibraryW(DLLPathname)

hThread = CreateRemoteThread(hProcess, NULL, 0, 

pfnThreadRtn, pszLibFileRemote, 0, NULL);

if (hThread != NULL)


// Wait for the remote thread to terminate

WaitForSingleObject(hThread, INFINITE);

fOk = TRUE; // Everything executed successfully





// Free the remote memory that contained the DLL's pathname

VirtualFreeEx(hProcess, pszLibFileRemote, 0, MEM_RELEASE);





BOOL WINAPI InjectLibA(DWORD dwProcessId, PCSTR pszLibFile) {

// Allocate a (stack) buffer for the Unicode version of the pathname

PWSTR pszLibFileW = (PWSTR) 

_alloca((lstrlenA(pszLibFile) + 1) * sizeof(WCHAR));

// Convert the ANSI pathname to its Unicode equivalent

wsprintfW(pszLibFileW, L"%S", pszLibFile);

// Call the Unicode version of the function to actually do the work.

return(InjectLibW(dwProcessId, pszLibFileW));



For more complete information about compiler optimizations, see our Optimization Notice.
PDF icon 216313-216313.pdf325.45 KB


Buddhika C.'s picture

 Can you make a tutorial about that ,"How to count FPS of Direct3D or OpenGL program using Win32::SetWindowsHookEx Function",which means using simple DLL injection with API Hooking.Please can you provide full tutorial in C/C++? 
I have read from internet sources that ,in Directx/Direct3D we have to hook with Present() and EndScene() functions in order to calculate FPS of external(Game) Program
MSDN links : -

Software call fraps is doing this ,But I want to learn mechanism how it's doing that?
Thank you.

CodeMaker C.'s picture

Thank you very much!!
I really want to learn more about this and other tools!
Good luck!

Seung-Woo Kim (Intel)'s picture

Thanks, CodeMaker. These days, there are other public tools such as pin tools (probe mode does pretty much the same thing.) I believe it's open source and you might want to try it. And of course you can use the code here. Just ensure to add it to the reference.

Seung-Woo Kim (Intel)'s picture

Thanks, CodeMaker. These days, there are other public tools such as pin tools (probe mode does pretty much the same thing.) I believe it's open source and you might want to try it. And of course you can use the code here. Just ensure to add it to the reference.

CodeMaker C.'s picture

Can I use your article for my report the university?
Strangely, I still thought that only those tools like Microsoft Detours and BoxedApp are capable to do it. Great job.
best wishes

anonymous's picture

perfect codes , cheers man

asmfan's picture

The only safe trampoline methods are: to use double trampoline 2bytes (rel8) + 5bytes (rel32) jumps in place of {mov edi, edi} or using only 5bytes (rel32) trampoline that's not spanning underlying instructions to avoid potential crash - this one is available using length disassembly (or checking instructions pointer of all threads running under IRQL (=dispatch) on all processors).

This example by intel will finally badly chrash if one thread will execute wrong byte - part of trampoline if interception will interrupt the execution process of e.g. {mov edi, edi} instruction with the next byte now 3rd byte of trampoline or 2nd byte of rel32 offset - which is unknown for rescheduled/resumed previously interrupted thread.

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