std::freopen(3)

NAME
std::freopen - std::freopen

Synopsis
   Defined in header <cstdio>
   std::FILE* freopen( const char* filename, const char* mode, std::FILE* stream );
   First, attempts to close the file associated with stream, ignoring any errors. Then,
   if filename is not null, attempts to open the file specified by filename using mode
   as if by std::fopen, and associates that file with the file stream pointed to by
   stream. If filename is a null pointer, then the function attempts to reopen the file
   that is already associated with stream (it is implementation defined which mode
   changes are allowed in this case).

Parameters
   filename - file name to associate the file stream to
   mode - null-terminated character string determining new file access mode
   stream - the file stream to modify
  File access flags
    File access Meaning Explanation Action if file Action if file
    mode string already exists does not exist
   "r" read Open a file for read from start return NULL and
                        reading set error
   "w" write Create a file for destroy contents create new
                        writing
   "a" append Append to a file write to end create new
   "r+" read extended Open a file for read from start return NULL and
                        read/write set error
   "w+" write extended Create a file for destroy contents create new
                        read/write
   "a+" append extended Open a file for write to end create new
                        read/write
   File access mode flag "b" can optionally be specified to open a file in binary mode.
   This flag has no effect on POSIX systems, but on Windows, for example, it disables
   special handling of '\n' and '\x1A'.
   On the append file access modes, data is written to the end of the file regardless
   of the current position of the file position indicator.
   File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This
   flag forces the function to fail if the file exists, instead of overwriting it.
   (C++17)
   The behavior is undefined if the mode is not one of the strings listed above. Some
   implementations define additional supported modes (e.g. Windows).

Return value
   stream on success, a null pointer on failure

Notes
   std::freopen is the only way to change the narrow/wide orientation of a stream once
   it has been established by an I/O operation or by std::fwide.
   Microsoft CRT version of std::freopen does not support any mode changes when
   filename is a null pointer and treats this as an error (see documentation). A
   possible workaround is the non-standard function _setmode().

Example
   The following code redirects stdout to a file // Run this code
 #include <cstdio>
 int main()
 {
     std::printf("stdout is printed to console\n");
     if(std::freopen("redir.txt", "w", stdout)) {
         std::printf("stdout is redirected to a file\n"); // this is written to redir.txt
         std::fclose(stdout);
     }
 }

Output:
 stdout is printed to console

See also
   fopen opens a file
          (function)
   fclose closes a file
          (function)