Temporary Files

• Sometimes it is nice to be able to make a temporary file.
  • A common task for a compiler.
  • The problem is pragmatically generating a unique name.
  • This is not impossible, but care needs to be taken.
• mkstemp

  • #include <stdlib.h>
    int mkstemp(char *template);
    int mkostemp(char *template, int flags);
    int mkstemps(char *template, int suffixlen);
    int mkostemps(char *template, int suffixlen, int flags);
           

  • mkstempuses a template and makes a file.
    • The last six characters of the template must be XXXXXX
    • The template will be modified, so it must not be a constant.
    • The man page suggests a character array.
    • The file is created with rw for user.
    • Opened with the flag O_RDWR | O_CREAT | OEXCL
    • A file descriptor is returned.
  • mkostemp is the same but you can pass a flag constructed from
    • O_APPEND, O_CLOEXEC, O_SYNC.
  • mkstemps allows the template to be in the form prefixXXXXXXsuffix.
    • but the length of the suffix must be given.
  • All return a file descriptor or -1 on failure.
• If you just want to build a temp name

  • #include <stdlib.h>
    char *mktemp(char *template);
    	

  • Same as above, but does not open a file
  • Returns an empty string on failure.
• Unlinking a file

  • #include 
    int unlink(const char * pathname);
           

  • Deletes a name from the file system.
  • If that was the last link to the name, the file is removed.
  • If a process has that file open, the fill will still exist until the last file descriptor is closed.
  • If the name is a symbolic link, the link is removed.
  • Return 0 on success, -1 on error
• So for temporary files the standard is to
  • Create the file with mkstemp
  • unlink the file
  • use the file, it will continue to exist until the file descriptor is closed.
  • close the file descriptor when done with the temporary file.