freopen, freopen_s
From cppreference.com
Defined in header <stdio.h>
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||
(1) | ||
(until C99) | ||
(since C99) | ||
(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 includingstdio.h
.
Parameters
filename | - | file name to associate the file stream to | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
mode | - | null-terminated character string determining new file access mode
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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.
Run this code
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
See also
(C11) |
opens a file (function) |
closes a file (function) |