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The main workaround to address this issue is to use prefix '\\?\' before pathes.

In a DOS shell the following command works :

mkdir \\?\c:\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678

whereas the following command leads to the error 'File path or extension too long.'

mkdir c:\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678\a12345678

Moreover, regarding http://bugs.python.org/issue18199, some python functions do not support unc pathes with \\?\ prefix

The following pyaims script fails to create image in a directory with long path :

from soma import aims
i = aims.AimsData_RGB(1,1,1)
i.fill(aims.AimsRGB(0,0,0))
aims.write(i, u'\\\\?\\c:\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\test.ima')

Informations about the Windows long path limitation are available here http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx

An important point about long path names under Windows:

Because you cannot use the "\\?\" prefix with a relative path, relative
paths are always limited to a total of MAX_PATH characters.

Yann Cointepas Tel: +33 1 69 08 78 31
CEA - Neurospin Fax: +33 1 69 08 79 80
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91191 Gif-sur-Yvette cedex, France

On Fri, Nov 29, 2013 at 12:39 PM, wrote:

La demande #8806 a été mise à jour par Souedet, Nicolas.

Informations about the Windows long path limitation are available here
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
------------------------------
*Task #8806: Windows : add support for long pathes (i.e. : > 255
characters) *

- Auteur: Souedet, Nicolas
- Statut: New
- Priorité: Normal
- Assigné à:
- Catégorie:
- Version cible: development-4.5

On windows OS [at least XP and 7, may be 8 (to be checked)], there is
limitation about path length (which must not exceed 255 characters).
This leads to annoying consequences :
1°) It is not possible to package brainvisa with bv_installer
2°) It is not possible to unzip and run packs in a long directory (we
highly recommend to unzip brainvisa pack in c:)
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The only way I found to create long file pathes using mingw compiler under Windows is to use non standard C function _wfopen() with unicode filenames ...

#include <iostream>
#include <string>
#include <stdio.h>

int main(const int argc, const char **argv) {
  std::wstring fname = L"\\\\?\\c:\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\toto.txt"; 
  FILE *pf = _wfopen(fname.c_str(), L"w");
  fprintf( pf, "The file 'crt_fopen.c' was opened\n" );
  fclose( pf );
}

Standard C++ version does not work (it does not raises exception but simply does not create the file) :

#include <fstream>
#include <iostream>
#include <string>

int main(const int argc, const char **argv) {
  std::string fname = "\\\\?\\c:\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\a12345678\\iostream_cpp.txt";
  std::ofstream ofs(fname.c_str());
  ofs << "The file was opened" << std::endl;
  ofs.close();
}

An interesting Python library to deal with path issues on multiple OS : https://pathlib.readthedocs.org/en/latest/ (I am not sure it deals with the Windows long path issue)

It can have an interest at building time to mount long path directories to drives.
A first way is to use the subst command.

subst F: C:\bv\build-bug_fix-Windows-7-i686-release

REGEDIT4
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices] 
"F:"="\\??\\C:\\bv\\build-bug_fix-Windows-7-i686-release" 
"G:"="\\??\\C:\\bv\\build-trunk-Windows-7-i686-release"