如何在CPP中使用“ GetExitCodeProcess”，“ CreateProcess”和“ WaitForSingleObject”函数？

Question

I saw this three connected function in the "Windows.h" lib, and I wanted to know how do I exactly use them and why should I use them? I tried to search for information in google or in the MSDN documentation, but I found nothing!

If someone could please explain to me how should I (and why should I...) use these three connected functions, it will be great for me, and for the other 1000 people who also stand in front of the same problem and will visit here to solve it.

So, how in a simple sample of code should I use every function?

Answer 1

Possibly the question is too broad, but since the technical details are covered by the documentation , readers may benefit from a higher level view, that includes C++-specific aspects.

GetExitCodeProcess .

GetExitCodeProcess is used to check the exit code of a process. By convention in Windows, Unix and in C++, exit code 0 (eg as returned from main ) denotes success. In C++ this value is available as the macro symbol EXIT_SUCCESS from <stdlib.h> . On other systems EXIT_SUCCESS needs not be 0, but in C++ exit code 0 always denotes success, possibly in addition to the value of EXIT_SUCCESS .

The Windows and Unix-land convention is that any other exit code denotes failure, but the only portable failure value in C++ is the one denoted by EXIT_FAILURE . In Windows this is conventionally the value 1. Unfortunately that conflicts with the value of a Windows error code, about “incorrect function” (as reported by errlook.exe ). For this reason one may choose to use Windows' own general failure value E_FAIL to indicate a general failure, instead of using the portable but imperfect EXIT_FAILURE .

Two relevant differences between Windows and Unix-land:

• In Windows a non-zero exit code is often an error code that tells you something about the cause of failure. It can be passed to a tool such as Microsoft's errlook to get a textual description, the same as via the API function FormatMessage . In Unix-land process exit codes used as failure cause indications are much less common.

• In Unix-land tools can generally be relied on to indicate failure or success via the exit code. This is not so in Windows. Even many of the built-in and standard commands fail to do so.

Especially the latter point is important for use of GetExitCodeProcess . You need to know that the process you're checking, is one that produces a meaningful exit-code.

Also note well that the code STILL_ACTIVE is defined as a low number, as I recall around 270 or thereabouts, and can be produced by a process, so that it is not a reliable indicator of whether a process is still active or not. Ie, don't use GetExitCodeProcess to poll for a process to finish. Depending on the process & circumstances, such code might hang…

WaitForSingleObject .

Instead of waiting for a process to end by polling its exit code with GetExitCodeProcess , which might hang as explained above, use eg WaitForSingleObject , or a family member.

CreateProcess .

CreateProcess used to be the fundamental way to create a new process, used internally by higher level functions such as ShellExecute .

With the advent of User Access Control security, UAC, one needs to use ShellExecute (or family member) to run a process with elevated access.

Where the process to be started is a console subsystem process, it can often be more convenient to use the C++ standard library's system function, which uses the cmd.exe command interpreter to run a specified command, waits for that to end, and returns the process exit code.