std::freopen

Header: <cstdio>

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).

# Declarations

std::FILE* freopen( const char* filename, const char* mode, std::FILE* stream );

# 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

# 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

#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);
    }
}