Member Avatar for Ancient Dragon

_tempnam() always returns a filename in the "c:\\temp" directory, regardless of the parameters. This example was extracted from MSDN. I need to generate a unique filename in a directory other than c:\temp -- any ideas how to do that other than writing my own function? maybe a c++ solution?

Note this is one of may examples where Microsoft uses void main() instead of int main() :evil: Not that I mind -- but can be confusing to newbes.

Thanks

#include <stdio.h>

void main( void )
{
   char  *name2;


   /* Create a temporary filename in temporary directory with the
    * prefix "stq". The actual destination directory may vary
    * depending on the state of the TMP environment variable and
    * the global variable P_tmpdir.
    */
   
   if( ( name2 = _tempnam( "c:\\h", "stq" ) ) != NULL )
   {
      printf( "%s is safe to use as a temporary file.\n", name2 );
	  FILE*fp = fopen(name2,"wt");
	  fprintf(fp,"Hello World\n");
	  fclose(fp);
   }
   else
      printf( "Cannot create a unique filename\n" );
}
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Short summary and practical fix for the observed behavior (as discussed by , and ): the CRT helpers will prefer the system temp environment (TMP/TEMP), which is why _tempnam kept returning a path under C:\Temp until those vars were changed. For reliably getting a unique name in a specific directory, prefer the Win32 APIs or the safer CRT/C++ helpers rather than relying on _tempnam/tmpnam/_mktemp.

A simple, robust Windows approach is to call GetTempFileName (give it the desired directory in lpPathName). When called with uUnique == 0 it will produce a unique name and create an empty file (avoiding the classic time-of-check/time-of-use race). See the API notes and the “creating and using temporary files” guidance for details.
GetTempFileName (and GetTempPath/GetTempPath2 to find temp dirs). (learn.microsoft.com)

Security and best-practice notes: the old functions tmpnam/_mktemp/_tempnam are error-prone and considered unsafe for new code; prefer their “_s” secure variants if sticking with CRT, or use CreateFile with CREATE_NEW (atomic create) on a GUID-based name to guarantee exclusive creation. Also, if creating many files, consider GUIDs or appending the process id to reduce collisions. See Microsoft’s secure-CRT guidance and CreateFile docs for the correct flags.
Security-enhanced CRT functions. (learn.microsoft.com)

If a C++-style solution was intended (as asked), C++17’s std::filesystem provides temp_directory_path() to locate the system temp directory; combine that with a GUID or the Win32 CreateFile CREATE_NEW pattern to make a safe unique file in any directory. For quick troubleshooting: inspect TMP/TEMP (or P_tmpdir) in the environment if a CRT helper keeps returning the “wrong” folder — that’s what caused the original surprise in this thread. (docs4dev.com)

Code hint (illustrative): use GetTempFileName with the directory wanted, then open the returned path (it already exists when uUnique==0), or generate a GUID and call CreateFile with CREATE_NEW for an atomic create.

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Member Avatar for Narue

>I need to generate a unique filename in a directory other than c:\temp
Is this a directory that you specify or just any directory other than C:\temp? If it's the latter then tmpnam will create the file in the local directory. If it's the former then you're SOL and need to do it manually.

Member Avatar for Ancient Dragon

Is this a directory that you specify.

A directory I specify, as shown in the example program I posted. This might be another famous example where MSDN is wrong.

I decided to create unique filenames another way by using the current system time from GetLocalTime().

Thanks anyway for your reply.

Member Avatar for Ancient Dragon

Ah! Ha! I overlooked that -- I have a temp and tmp environment variables.

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