std:: freopen
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Defined in header
<cstdio>
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||
|
std::
FILE
*
freopen
(
const
char
*
filename,
const
char
*
mode,
std::
FILE
*
stream
)
;
|
||
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).
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 |
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 ). | ||||
Return value
stream on success, a null pointer on failure.
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()
.
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
See also
|
opens a file
(function) |
|
|
closes a file
(function) |
|
|
C documentation
for
freopen
|
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