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freopen, freopen_s

Defined in header `<stdio.h>`
	(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:

newstreamptr is a null pointer
stream is a null pointer
mode is a null pointer

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

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)

stream - the file stream to modify

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

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.

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);
}

Output:

stdout is printed to console

References

C11 standard (ISO/IEC 9899:2011):

7.21.5.4 The freopen function (p: 307)
K.3.5.2.2 The freopen_s function (p: 590)

C99 standard (ISO/IEC 9899:1999):

7.19.5.4 The freopen function (p: 272-273)

C89/C90 standard (ISO/IEC 9899:1990):

4.9.5.4 The freopen function