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Defined in header <stdio.h> | ||
| (1) | ||
| (until C99) | ||
| (since C99) | ||
| (2) | (since C11) | |
stream, ignoring any errors. Then, iffilename is not null, attempts to open the file specified byfilename usingmode as if byfopen, and associates that file with the file stream pointed to bystream. Iffilename is a null pointer, then the function attempts to reopen the file that is already associated withstream (it is implementation defined which mode changes are allowed in this case).mode is treated as infopen_s and that the pointer to the file stream is written tonewstreamptr and the following errors are detected at runtime and call the currently installedconstraint handler function:newstreamptr is a null pointerstream is a null pointermode is a null pointerfreopen_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 constant1 before including<stdio.h>.Contents |
| filename | - | file name to associate the file stream to |
| mode | - | null-terminated character string determining newfile access mode |
| stream | - | the file stream to modify |
| newstreamptr | - | pointer to a pointer where the function stores the result (an out-parameter) |
| 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 tofflush,fseek,fsetpos orrewind, and input cannot be followed by output without an intervening call tofseek,fsetpos orrewind, 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 usingfopen_s orfreopen_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 defaultfopen permissions.(C11) | ||||
stream on success, null pointer on failure.stream is written to*newstreamptr, non-zero on error (and null pointer is written to*newstreamptr unlessnewstreamptr is itself a null pointer).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 byfwide.
Microsoft CRT version offreopen does not support any mode changes whenfilename is a null pointer and treats this as an error (seedocumentation). A possible workaround is the non-standard function_setmode().
The following code redirectsstdout 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");returnEXIT_FAILURE;}puts("stdout is redirected to a file");// this is written to redir.txtfclose(stdout);returnEXIT_SUCCESS;}
Output:
stdout is printed to console
(C11) | opens a file (function)[edit] |
| closes a file (function)[edit] | |
C++ documentation forfreopen | |
