C++ Cookbook

10.9. Creating a Temporary Filename and File

Problem

You have to store some stuff on disk temporarily, and you don’t want to have to write a routine that generates a unique name yourself.

Solution

Use either the tmpfile or tmpnam functions, declared in <cstdio>. tmpfile returns a FILE* that is already opened for writing, and tmpnam generates a unique filename that you can open yourself. Example 10-13 shows how to use tmpfile.

Example 10-13. Creating a temporary file

#include <iostream>
#include <cstdio>

int main() {

   FILE* pf = NULL;
   char buf[256];

   pf = tmpfile();  // Create and open a temp file

   if (pf) {
      fputs("This is a temp file", pf);  // Write some data to it
   }

   fseek(pf, 5, SEEK_SET);  // Reset the file position
   fgets(buf, 255, pf);     // Read a string from it
   fclose(pf);

   std::cout << buf << '\n';
}

Discussion

There are two ways to create a temporary file; Example 10-13 shows the first way. The function tmpfile is declared in <cstdio>, takes no parameters, and returns a FILE* if successful, NULL if not. The FILE* is the same type you can use with the C input/output functions fread, fwrite, fgets, fputs, etc. tmpfile opens the temporary file in “wb+” mode, which means you can write to it or read from it in binary mode (i.e., the characters are not interpreted as they are read). When your program terminates normally, the temporary file created by tmpfile is automatically deleted.

This may or may not work for you depending on your requirements. You will notice that tmpfile does not give ...