Reopen a stream with a different file and mode.
  freopen first tries to close any file already open associated with the given stream.
  Then, whether the stream was closed or not, freopen opens the file which name is pointed by filename string and associates it with the specified stream. The operations allowed to the file returned are defined by the mode parameter as in fopen function.
  This function is specially useful for redirecting system defined streams stdin, stdout and stderr to other files.

Parameters.

filename

name of the file to be opened. This paramenter must follow operating system's specifications and can include a path if the system supports it.

mode

type of access requested. It can be:

"r"	Open a file for reading. The file must exist.
"w"	Create an empty file for writing. If a file with the same name already exists its content is erased.
"a"	Append to a file. Writing operations append data at the end of the file. The file is created if it doesn't exist.
"r+"	Open a file for reading and writing. The file must exist.
"w+"	Create an empty file for reading and writing. If a file with the same name already exists its content is erased before it is opened.
"a+"	Open a file for reading and appending. All writing operations are done at the end of the file protecting the previous content to be overwritten. You can reposition (fseek, rewind) the pointer to anywhere in the file for reading, but writing operations will move back to the end of file. The file is created if it doesn't exist.

The mode parameter serves also to specify whether we want to open the file as text or binary, adding t or b characters to this access mode string.

t	Text mode. In text mode the end of file is assumed to be at first Ctrl-Z character. Some conversions can occur reading and writing with End Of Line / Feedback characters depending on your compiler and your Operating System.
b	Binary mode. End of file is reached at last byte of the file. No conversions.

This additional t or b characters can be appended to the read/write mode at the end of the string (like "rb", "wt", "r+t", "w+b"...) or can be inserted between the letter and the + character (like "rt+", "wb+").
  If t nor b are given, it is used the default method (commonly t). In most compilers this default method is specified by the global variable _fmode defined in stdio.h.

stream

pointer to open file that has to be reopened.

Return Value.
  If the file has been succesfully reopened the function returns a pointer to the file. Otherwise a NULL pointer is returned.

Portability.
  Defined in ANSI-C.

Example.

/* freopen example: redirecting stdout */
#include <stdio.h>

main ()
{
  freopen ("myfile.txt","w",stdout);
  printf ("This sentence is redirected to a file.");
  fclose (stdout);
  return 0;
}

See also.
  fopen, fclose