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This chapter is from the book

Process Creation

The fundamental Windows process management function is CreateProcess, which creates a process with a single thread. It is necessary to specify the name of an executable program file as part of the CreateProcess call.

It is common to speak of parent and child processes, but these relationships are not actually maintained by Windows. It is simply convenient to refer to the process that creates a child process as the parent.

CreateProcess has ten parameters to support its flexibility and power. Initially, it is simple to use default values. Just as with CreateFile, it is appropriate to explain all the CreateProcess parameters. Related functions then become easier to understand.

Note first that the function does not return a HANDLE; rather, two separate handles, one each for the process and the thread, are returned in a structure specified in the call. CreateProcess creates a new process with a primary thread. The example programs are always very careful to close both of these handles when they are no longer needed in order to avoid resource leaks; a common defect is to neglect to close the thread handle. Closing a thread handle, for instance, does not terminate the thread; the CloseHandle function only deletes the reference to the thread within the process that called CreateProcess.

BOOL CreateProcess (
   LPCTSTR lpApplicationName,
   LPTSTR lpCommandLine,
   LPSECURITY_ATTRIBUTES lpsaProcess,
   LPSECURITY_ATTRIBUTES lpsaThread,
   BOOL bInheritHandles,
   DWORD dwCreationFlags,
   LPVOID lpEnvironment,
   LPCTSTR lpCurDir,
   LPSTARTUPINFO lpStartupInfo,
   LPPROCESS_INFORMATION lpProcInfo)

Return: TRUE only if the process and thread are successfully created.

Parameters

Some parameters require extensive explanations in the following sections, and many are illustrated in the program examples.

lpApplicationName and lpCommandLine (this is an LPTSTR and not an LPCTSTR) together specify the executable program and the command line arguments, as explained in the next section.

lpsaProcess and lpsaThread point to the process and thread security attribute structures. NULL values imply default security and will be used until Chapter 15, which covers Windows security.

bInheritHandles indicates whether the new process inherits copies of the calling process's inheritable open handles (files, mappings, and so on). Inherited handles have the same attributes as the originals and are discussed in detail in a later section.

dwCreationFlags combines several flags, including the following.

  • CREATE_SUSPENDED indicates that the primary thread is in a suspended state and will run only when ResumeThread is called.

  • DETACHED_PROCESS and CREATE_NEW_CONSOLE are mutually exclusive; don't set both. The first flag creates a process without a console, and the second flag gives the new process a console of its own. If neither flag is set, the process inherits the parent's console.

  • CREATE_NEW_PROCESS_GROUP specifies that the new process is the root of a new process group. All processes in a group receive a console control signal (Ctrl-c or Ctrl-break) if they all share the same console. Console control handlers were described in Chapter 4 and illustrated in Program 4-5. These process groups have similarities to UNIX process groups and are described later in this chapter.

Several of the flags control the priority of the new process's threads. The possible values are explained in more detail at the end of Chapter 7. For now, just use the parent's priority (specify nothing) or NORMAL_PRIORITY_CLASS.

lpEnvironment points to an environment block for the new process. If NULL, the process uses the parent's environment. The environment block contains name and value strings, such as the search path.

lpCurDir specifies the drive and directory for the new process. If NULL, the parent's working directory is used.

lpStartupInfo specifies the main window appearance and standard device handles for the new process. Use the parent's information, which is obtained from GetStartupInfo. Alternatively, zero out the associated STARTUPINFO structure before calling CreateProcess. To specify the standard input, output, and error handles, set the standard handler fields (hStdInput, hStdOutput, and hStdError) in the STARTUPINFO structure. For this to be effective, also set another STARTUPINFO member, dwFlags, to STARTF_USESTDHANDLES, and set all the handles that the child process will require. Be certain that the handles are inheritable and that the CreateProcess bInheritHandles flag is set. The Inheritable Handles subsection gives more information and an example.

lpProcInfo specifies the structure for containing the returned process, thread handles, and identification. The PROCESS_INFORMATION structure is as follows:

typedef struct PROCESS_INFORMATION {
   HANDLE hProcess;
   HANDLE hThread;
   DWORD dwProcessId;
   DWORD dwThreadId;
} PROCESS_INFORMATION;

Why do processes and threads need handles in addition to IDs? The ID is unique to the object for its entire lifetime and in all processes, whereas a given process may have several handles, each having distinct attributes, such as security access. For this reason, some process management functions require IDs, and others require handles. Furthermore, process handles are required for the general-purpose, handle-based functions. Examples include the wait functions discussed later in this chapter, which allow waiting on handles for several different object types, including processes. Just as with file handles, process and thread handles should be closed when no longer required.

Note: The new process obtains environment, working directory, and other information from the CreateProcess call. Once this call is complete, any changes in the parent will not be reflected in the child process. For example, the parent might change its working directory after the CreateProcess call, but the child process working directory will not be affected, unless the child changes its own working directory. The two processes are entirely independent.

The UNIX and Windows process models are considerably different. First, Windows has no equivalent to the UNIX fork function, which makes a copy of the parent, including the parent's data space, heap, and stack. fork is difficult to emulate exactly in Windows, and, while this may seem to be a limitation, fork is also difficult to use in a multithreaded UNIX system because there are numerous problems with creating an exact replica of a multithreaded system with exact copies of all threads and synchronization objects, especially on an SMP system. Therefore, fork, by itself, is not really appropriate in any multithreaded system.

CreateProcess is, however, similar to the common UNIX sequence of successive calls to fork and execl (or one of five other exec functions). In contrast to Windows, the search directories in UNIX are determined entirely by the PATH environment variable.

As previously mentioned, Windows does not maintain parent-child relationships among processes. Thus, a child process will continue to run after the creating parent process terminates. Furthermore, there are no process groups in Windows. There is, however, a limited form of process group that specifies all the processes to receive a console control event.

Windows processes are identified both by handles and by process IDs, whereas UNIX has no process handles.

Specifying the Executable Image and the Command Line

Either lpApplicationName or lpCommandLine specifies the executable image name. The rules are as follows.

  • lpApplicationName, if not NULL, is the name of the executable. Quotation marks can be used if the image name contains spaces. More detailed rules are described below.

  • Otherwise, the executable is the first token in lpCommandLine.

Usually, only lpCommandLine is specified, with lpApplicationName being NULL. Nonetheless, there are detailed rules for lpApplicationName.

  • If lpApplicationName is not NULL, it specifies the executable module. Specify the full path and file name, or use a partial name and the current drive and directory will be used; there is no additional searching. Include the file extension, such as .EXE or .BAT, in the name.

  • If the lpApplicationName string is NULL, the first white-space-delimited token in lpCommandLine is the program name. If the name does not contain a full directory path, the search sequence is as follows:

    1. The directory of the current process's image

    2. The current directory

    3. The Windows system directory, which can be retrieved with GetSystemDirectory

    4. The Windows directory, which is retrievable with GetWindowsDirectory

    5. The directories as specified in the environment variable PATH

The new process can obtain the command line using the usual argv mechanism, or it can invoke GetCommandLine to obtain the command line as a single string.

Notice that the command line is not a constant string. This is consistent with the fact that the argv parameters to the main program are not constant. A program could modify its arguments, although it is advisable to make any changes in a copy of the argument string.

The new process is not required to be built with the same UNICODE definition as that of the parent process. All combinations are possible. Using _tmain as discussed in Chapter 2 is helpful in developing code for either UNICODE or ASCII operation.

Inheritable Handles

Frequently, a child process requires access to an object referenced by a handle in the parent; if this handle is inheritable, the child can receive a copy of the parent's open handle. The standard input and output handles are frequently shared with the child in this way. To make a handle inheritable so that a child receives and can use a copy requires several steps.

The bInheritHandles flag on the CreateProcess call determines whether the child process will inherit copies of the inheritable handles of open files, processes, and so on. The flag can be regarded as a master switch applying to all handles.

It is also necessary to make an individual handle inheritable; it is not done by default. To create an inheritable handle, use a SECURITY_ATTRIBUTES structure at creation time or duplicate an existing handle.

The SECURITY_ATTRIBUTES structure has a flag, bInheritHandle, that should be set to TRUE. Also, recall that nLength should be set to sizeof (SECURITY_ATTRIBUTES).

The following code segment shows how an inheritable file or other handle is typically created. In this example, the security descriptor within the security attributes structure is NULL; Chapter 15 shows how to include a security descriptor.

HANDLE h1, h2, h3;
SECURITY_ATTRIBUTES sa =
    {sizeof(SECURITY_ATTRIBUTES), NULL, TRUE };
...
h1 = CreateFile (..., &sa, ... ); /* Inheritable. */
h2 = CreateFile (..., NULL, ... ); /* Not inheritable. */
h3 = CreateFile (..., &sa, ...);
   /* Inheritable. sa can be reused. */

A child process still needs to know the value of an inheritable handle, so the parent needs to communicate handle values to the child using an interprocess communication (IPC) mechanism or by assigning the handle to standard I/O in the STARTUPINFO structure, as is done in the first example of this chapter (Program 6-1) and in several additional examples throughout the book. This is generally the preferred technique because it allows I/O redirection in a standard way and no changes are needed in the child program.

Alternatively, nonfile handles and handles that are not used to redirect standard I/O can be converted to text and placed in a command line or in an environment variable. This approach is valid if the handle is inheritable because both parent and child processes identify the handle with the same handle value. Exercise 6–2 suggests how to demonstrate this, and a solution is presented on the book's Web site.

The inherited handles are distinct copies. Therefore, a parent and child might be accessing the same file using different file pointers. Furthermore, each of the two processes can and should close its own handle.

Figure 6-2 shows how two processes can have distinct handle tables with two distinct handles associated with the same file or other object. Process 1 is the parent, and Process 2 is the child. The handles will have identical values in both processes if the child's handle has been inherited, as is the case with Handles 1 and 3.

06fig02.gifFigure 6-2 Process Handle Tables

On the other hand, the handle values may be distinct. For example, there are two handles for File D, where Process 2 obtained a handle by calling CreateFile rather than by inheritance. Finally, as is the case with Files B and E, one process may have a handle to an object while the other does not. This would be the case when the child process creates the handle or when a handle is duplicated from one process to another, as described in the upcoming Duplicating Handles section.

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