How to generate a good random seed to pass to srand()?

Question

I am writing a C++ program which needs to create a temporary file for its internal usage. I would like to allow concurrent executions of the program by running multiple processes, so the temporary file name needs to be randomized, that way each spawned process will generate a unique temporary file name for its own use.

I am using rand() to generate random characters for part of the file name, so i need to initialize the random number generator's seed using srand().

What options are there for passing a good argument to srand() such that two processes will not be initialized with the same seed value? My code needs to work both on Windows and on Linux.

Answer 1

The question is actually asking how to create a uniquely-named temporary file.

The operating system probably provides an API for this, which means you do not have to generate your own name.

On Windows, its called GetTempFileName() and GetTempPath().

On Unix, use tmpfile().

(Windows supports tmpfile() too; however, I've heard reports that from others that, whilst it works nicely on XP, it fails on Vista if you're on the C: drive and you are not an administrator; best to use the GetTempFileName() method with a custom, safe path)

Answer 2

If you are trully just needing a temp file you can use:

FILE* tmpfile();        // Generate an unnamed temporary file.
char* tmpnam(char* s);  // Generate a file name in the director specified by TMPDIR

Use time:

srand(time(NULL));

Also note that rand() probably is not thread safe.
So be careful how you use it. Two threads entering rand at the same time are going to cause problems.

Answer 3

I've never used it myself, but you might be able to use tmpnam() to generate your temporary file name. A quick search indicated support on both Windows and Linux.