freopen, freopen_s

Header: <stdio.h>

1. 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 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

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

(until C99)

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

(since C99)

errno_t freopen_s( FILE *restrict *restrict newstreamptr,
const char *restrict filename, const char *restrict mode,
FILE *restrict stream );

(since C11)

# 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
• newstreamptr: pointer to a pointer where the function stores the result (an out-parameter)

# Notes

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 fwide.

Microsoft CRT version of 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 <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    puts("stdout is printed to console");
    if (freopen("redir.txt", "w", stdout) == NULL)
    {
       perror("freopen() failed");
       return EXIT_FAILURE;
    }
    puts("stdout is redirected to a file"); // this is written to redir.txt
    fclose(stdout);
    return EXIT_SUCCESS;
}

# See also

• fopenfopen_s
• fclose
• C++ documentation