Microsoft KB Archive/151071

= FIX: Spawn Hangs When Inheriting Pipe Handle =

Article ID: 151071

Article Last Modified on 10/17/2003

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APPLIES TO

 The C Run-Time (CRT), when used with:  Microsoft Visual C++ 4.0 Standard Edition

 Microsoft Visual C++ 4.1 Subscription 

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This article was previously published under Q151071



SYMPTOMS
When asynchronously spawning a child process that is inheriting a pipe handle, the parent process hangs before returning from the spawn call.



CAUSE
When you starting Visual C++ version 4.0, the startup code attempts to validate the inherited handle values by calling GetFileType for each handle. With Windows NT, if two write handles for the same object are passed to the child process, the GetFileType hangs when called for the second one.



RESOLUTION
NOTE: In order to use the _pipe function to communicate between a parent and child process, each process must have only one handle open on the pipe, and each process must be of opposite sense (the parent has a read handle open, the child has a write handle open). To implement this behavior, combine the _O_NOINHERIT flag using the bit-wise-OR operator (|) into the third argument of the _pipe call. Then, use _dup or _dup2 to create an inheritable copy of the pipe handle you want to pass to the child. Close the original handle and spawn the child process. Immediately upon returning from the spawn call in the parent process, close the duplicate handle.

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STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 32-bit Edition version 4.2.

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Sample Code
// Beeper.Cpp /* Compile options needed: None */ #include  <stdio.h>   #include   <string.h>

int   main {     int i;

for (i=0; i<100; ++i) {         printf("\nThis is speaker beep number %d... \n\7", i+1); }

return 0; }

// BeepFilter.Cpp /* Compile options needed: Execute as: BeepFilter.exe Beeper.exe */   #include   <windows.h>   #include   <process.h>   #include   <memory.h>   #include   <string.h>   #include   <stdio.h>   #include   <fcntl.h>   #include   <io.h>

#define   OUT_BUFF_SIZE   512 #define   READ_HANDLE       0 #define   WRITE_HANDLE      1 #define   BEEP_CHAR         7

char  szBuffer[OUT_BUFF_SIZE];

int   Filter(char* szBuff, ULONG nSize, int nChar) {     char*   szPos = szBuff + nSize -1; char*  szEnd = szPos; int    nRet = nSize;

while  (szPos > szBuff) {         if  (*szPos == nChar) {             memmove(szPos, szPos+1, szEnd - szPos); --nRet; }

--szPos; }

return nRet; }

int   main(int argc, char** argv) {     int nExitCode = STILL_ACTIVE;

if (argc >= 2) {         HANDLE  hProcess; int    hStdOut; int    hStdOutPipe[2];

// Create the pipe if(_pipe(hStdOutPipe, 512, O_BINARY | O_NOINHERIT) == -1) return 1;

// Duplicate stdout handle (next line will close original) hStdOut = _dup(_fileno(stdout));

// Duplicate write end of pipe to stdout handle if (_dup2(hStdOutPipe[WRITE_HANDLE], _fileno(stdout)) != 0) return 2;

// Close original write end of pipe close(hStdOutPipe[WRITE_HANDLE]);

// Spawn process hProcess = (HANDLE)spawnvp(P_NOWAIT, argv[1],                     (const char* const*)&argv[1]);

// Duplicate copy of original stdout back into stdout if (_dup2(hStdOut, _fileno(stdout)) != 0) return 3;

// Close duplicate copy of original stdout close(hStdOut);

if (hProcess) {             int     nOutRead;

while  (nExitCode == STILL_ACTIVE) {                 nOutRead = read(hStdOutPipe[READ_HANDLE],                                  szBuffer, OUT_BUFF_SIZE); if (nOutRead) {                     nOutRead = Filter(szBuffer, nOutRead, BEEP_CHAR); fwrite(szBuffer, 1, nOutRead, stdout); }

if (!GetExitCodeProcess(hProcess, (unsigned long*)&nExitCode)) return 4; }         }      }

printf("\nPress \'ENTER\' key to continue... "); getchar; return nExitCode; }

Additional query words: kbVC400bug 4.00 4.10 4.20 vcfixlist420 CRTIss

Keywords: kbbug kbfix kbcrt KB151071

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