`_cwait`

Artikel
12/02/2022

Waits until another process terminates.

Important

This API cannot be used in applications that execute in the Windows Runtime. For more information, see CRT functions not supported in Universal Windows Platform apps.

Syntax

intptr_t _cwait(
   int *termstat,
   intptr_t procHandle,
   int action
);

Parameters

termstat
Pointer to a buffer where the result code of the specified process will be stored, or NULL.

procHandle
The handle to the process to wait on (that is, the process that has to terminate before _cwait can return).

action
NULL: Ignored by Windows operating system applications; for other applications: action code to perform on procHandle.

Return value

When the specified process has successfully completed, returns the handle of the specified process and sets termstat to the result code that's returned by the specified process. Otherwise, returns -1 and sets errno as follows.

`errno` value	Description
`ECHILD`	No specified process exists, `procHandle` is invalid, or the call to the `GetExitCodeProcess` or `WaitForSingleObject` API failed.
`EINVAL`	`action` is invalid.

For more information about these and other return codes, see errno, _doserrno, _sys_errlist, and _sys_nerr.

Remarks

The _cwait function waits for the termination of the process ID of the specified process that's provided by procHandle. The value of procHandle that's passed to _cwait should be the value that's returned by the call to the _spawn function that created the specified process. If the process ID terminates before _cwait is called, _cwait returns immediately. _cwait can be used by any process to wait for any other known process for which a valid handle (procHandle) exists.

termstat points to a buffer where the return code of the specified process will be stored. The value of termstat indicates whether the specified process terminated normally by calling the Windows ExitProcess API. ExitProcess is called internally if the specified process calls exit or _exit, returns from main, or reaches the end of main. For more information about the value that's passed back through termstat, see GetExitCodeProcess. If _cwait is called by using a NULL value for termstat, the return code of the specified process isn't stored.

The action parameter is ignored by the Windows operating system because parent-child relationships aren't implemented in these environments.

Unless procHandle is -1 or -2 (handles to the current process or thread), the handle will be closed. In this situation, don't use the returned handle.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Requirements

Routine	Required header	Optional header
`_cwait`	<process.h>	<errno.h>

For more compatibility information, see Compatibility.

Example

// crt_cwait.c
// compile with: /c
// This program launches several processes and waits
// for a specified process to finish.

#define _CRT_RAND_S

#include <windows.h>
#include <process.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>

// Macro to get a random integer within a specified range
#define getrandom( min, max ) (( (rand_s (&number), number) % (int)((( max ) + 1 ) - ( min ))) + ( min ))

struct PROCESS
{
    intptr_t hProcess;
    char    name[40];
} process[4] = { { 0, "Ann" }, { 0, "Beth" }, { 0, "Carl" }, { 0, "Dave" } };

int main(int argc, char* argv[])
{
    int termstat, c;
    unsigned int number;

    srand((unsigned)time(NULL));    // Seed randomizer

    // If no arguments, this is the calling process
    if (argc == 1)
    {
        // Spawn processes in numeric order
        for (c = 0; c < 4; c++) {
            _flushall();
            process[c].hProcess = _spawnl(_P_NOWAIT, argv[0], argv[0],
                process[c].name, NULL);
        }

        // Wait for randomly specified process, and respond when done
        c = getrandom(0, 3);
        printf("Come here, %s.\n", process[c].name);
        _cwait(&termstat, process[c].hProcess, _WAIT_CHILD);
        printf("Thank you, %s.\n", process[c].name);

    }
    // If there are arguments, this must be a spawned process
    else
    {
        // Delay for a period that's determined by process number
        Sleep((argv[1][0] - 'A' + 1) * 1000L);
        printf("Hi, Dad. It's %s.\n", argv[1]);
    }
}

The order of the output varies from run to run.

Hi, Dad. It's Ann.
Come here, Ann.
Thank you, Ann.
Hi, Dad. It's Beth.
Hi, Dad. It's Carl.
Hi, Dad. It's Dave.

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