The WriteProcessMemory function writes memory in a specified process. The entire area to be written to must be accessible, or the operation fails.

BOOL WriteProcessMemory(

    HANDLE hProcess,   // handle to process whose memory is written to 
    LPVOID lpBaseAddress,   // address to start writing to
    LPVOID lpBuffer,   // pointer to buffer to write data to
    DWORD nSize,   // number of bytes to write
    LPDWORD lpNumberOfBytesWritten    // actual number of bytes written
   );   
 

Parameters

hProcess

Identifies an open handle to a process whose memory is to be written to. The handle must have PROCESS_VM_WRITE and PROCESS_VM_OPERATION access to the process.

lpBaseAddress

Points to the base address in the specified process to be written to. Before any data transfer occurs, the system verifies that all data in the base address and memory of the specified size is accessible for write access. If this is the case, the function proceeds; otherwise, the function fails.

lpBuffer

Points to the buffer that supplies data to be written into the address space of the specified process.

nSize

Specifies the requested number of bytes to write into the specified process.

lpNumberOfBytesWritten

Points to the actual number of bytes transferred into the specified process. This parameter is optional. If lpNumberOfBytesWritten is NULL, the parameter is ignored.

 

Return Values

If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError. The function will fail if the requested write operation crosses into an area of the process that is inaccessible.

Remarks

WriteProcessMemory copies the data from the specified buffer in the current process to the address range of the specified process. Any process that has a handle with PROCESS_VM_WRITE and PROCESS_VM_OPERATION access to the process to be written to can call the function. The process whose address space is being written to is typically, but not necessarily, being debugged.
The entire area to be written to must be accessible. If it is not, the function fails as noted previously.