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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../io/c/freopen.html below:

std::freopen - cppreference.com

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).

[edit] 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 [edit] File access flags 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 return NULL and set error "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 return NULL and set 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. 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). [edit] Return value

stream on success, a null pointer on failure.

[edit] 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().

[edit] Example

The following code redirects stdout to a file.

#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

[edit] See also

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