std::freopen

 

C++

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Defined in header <cstdio>
std::FILE* freopen( const char* filename, const char* mode, std::FILE* stream );

Reassigns an existing file stream stream to a different file identified by filename using specified mode. mode is used to determine the new file access mode.

Parameters

filename	-	file name to associate the file stream to
mode	-	null-terminated character string determining new file access mode File access mode string Meaning Explanation Action if file already exists Action if file does not exist "r" read Open a file for reading read from start failure to open "w" write Create a file for writing destroy contents create new "a" append Append to a file write to end create new "r+" read extended Open a file for read/write read from start error "w+" write extended Create a file for read/write destroy contents create new "a+" append extended Open a file for read/write write to end create new 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.
stream	-	the file stream to modify

Return value

stream on success, NULL on failure

Example

The following code redirects stdout to a file

Run this code

#include <cstdio>
 
int main()
{
    std::printf("stdout is printed to console");
    std::freopen("redir.txt", "w", stdout);
    std::printf("stdout is redirected to a file");
    std::fclose(stdout);
}

Output:

stdout is printed to console

See also

fopen	opens a file (function)
fclose	closes a file (function)
C documentation for freopen