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# freopen, freopen_s

From cppreference.com
< c‎ | io

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 all bounds-checked functions, `freopen_s` is only guaranteed to be available of __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>`.

### [edit] 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 effect only on Windows systems. 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.
When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.
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)

### [edit] 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).

### [edit] 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.

### [edit] Example

The following code redirects `stdout` to a file.

Run this code

    #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

### [edit] 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

### [edit] See also

fopen fopen_s (C11) | opens a file (function) [edit]
fclose | closes a file (function) [edit]
C++ documentation for freopen

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