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

freopen, freopen_s - cppreference.com

(1) 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 ); (2) (since C11) 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).

2)

Same as

(1)

, except that

mode

is treated as in

fopen_s

and that the pointer to the file stream is written to

newstreamptr

and the following errors are detected at runtime and call the currently installed

constraint handler

function:

As with all bounds-checked functions, freopen_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdio.h>.
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) 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 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 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. The behavior is undefined if the mode is not one of the strings listed above. Some implementations define additional supported modes (e.g. Windows). In update mode ('+'), both input and output may be performed, but output cannot be followed by input without an intervening call to fflush, fseek, fsetpos or rewind, and input cannot be followed by output without an intervening call to fseek, fsetpos or rewind, unless the input operation encountered end of file. In update mode, implementations are permitted to use binary mode even when text mode is specified. 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. (C11) When using fopen_s or freopen_s, file access permissions for any file created with "w" or "a" prevents other users from accessing it. File access mode flag "u" can optionally be prepended to any specifier that begins with "w" or "a", to enable the default fopen permissions. (C11) Return value

1) A copy of the value of stream on success, null pointer on failure.

2) zero on success (and a copy of the value of stream is written to *newstreamptr, non-zero on error (and null pointer is written to *newstreamptr unless newstreamptr is itself a null pointer).

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

The following code redirects stdout to a file.

#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;
}

Output:

stdout is printed to console
References
See also

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