CODEWITHSUNDEEP
c++winapifstreamchild-processfilehandle
Mercalli
Mercalli

Reputation: 709

Controlling inheritability of file handles created by C++ std::fstream in Windows

In Windows, when creating a process with CreateProcess, one can pass true as the bInheritHandles argument.

CreateProcess( , , , , bInheritHandles, , , , )

This means that all file handles marked to be inheritable will be, indeed, inherited by the child process.

How can we control whether the underlying file handle created by C++ std::fstream class is inheritable or not?

Upvotes: 4

Views: 980

Answers (2)

Pouria P
Pouria P

Reputation: 595

If you're using fopen to open the file you can specify the Windows-specific "N" mode in the fopen parameters to make the handles not inheritable.

Example:

#include <stdio.h>
#include <stdlib.h>

int main(void) {
  FILE *fp = fopen("SomeFile.txt", "rwN");
  if (!fp) {
    return -1;
  }

  system("SomeProcess.exe");

  fclose(fp);
  return 0;
}

Sources:

https://wiki.sei.cmu.edu/confluence/display/c/WIN03-C.+Understand+HANDLE+inheritance

https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/fopen-wfopen?view=vs-2019

Upvotes: 2

HrW
HrW

Reputation: 256

The C runtime creates inheritable handles by default.

ofstream outFile("filename.txt") ;
CreateProcess("program.exe", ..., true, ...) ; //program.exe will inherit the above file handle

So, if you want a handle to be inherited, you don't need to do anything.

If you DO NOT want a handle to be inherited, you have to set the handle's HANDLE_FLAG_INHERIT flag yourself using WinAPI function SetHandleInformation, like this:

FILE* filePtr = fopen("filename.txt", "w") ;
SetHandleInformation( (HANDLE)_get_osfhandle(_fileno(filePtr)), HANDLE_FLAG_INHERIT, 0) ;
ofstream outFile(filePtr) ;

In the third line, above, the constructor ofstream(FILE*) is an extension to the standard that exists in Visual Studio (I don't know about other compilers).

After that constructor, filePtr is now owned by outFile, so calling outFile.close() closes filePtr as well. You can completely forget about the filePtr variable.

Documentation: fopen, _fileno, _get_osfhandle, SetHandleInformation

Upvotes: 3

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