freopen

 

C

Language

Standard Library

Type support

Dynamic memory management

Error handling

Program utilities

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

 

File input/output

Functions
File access
fopen freopen fclose fflush	fwide setbuf setvbuf
Direct input/output
fread
fwrite
Unformatted input/output
fgetc fgets fputc fputs getchar gets putchar puts ungetc	fgetwc fgetws fputwc fputws getwchar putwchar ungetwc
Formatted input/output
scanffscanfsscanf vscanfvfscanfvsscanf (C99)(C99)(C99) printffprintfsprintfsnprintf (C99) vprintfvfprintfvsprintfvsnprintf (C99)	wscanffwscanfswscanf vwscanfvfwscanfvswscanf (C99)(C99)(C99) wprintffwprintfswprintf vwprintfvfwprintfvswprintf
File positioning
ftell
fgetpos
fseek
fsetpos
rewind
Error handling
clearerr
feof
ferror
perror
Operations on files
remove
rename
tmpfile
tmpnam

 

Defined in header <stdio.h>
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)

Reassigns an existing file stream stream to a different file identified by filename using specified mode. mode is used to determine the new file access mode.

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. 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)
stream	-	the file stream to modify

Return value

stream on success, NULL on failure

Example

freopen with error checking. The following code redirects stdout to a file

Run this code

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    printf("stdout is printed to console");
    if (freopen("redir.txt", "w", stdout) == NULL)
    {
       perror("freopen()");
       fprintf(stderr,"freopen() failed in file %s at line # %d\n", __FILE__,__LINE__-3);
       exit(EXIT_FAILURE);
    }
    printf("stdout is redirected to a file");
    fclose(stdout);
    return EXIT_SUCCESS;
}

Output:

stdout is printed to console

See also

fopen	opens a file (function)
fclose	closes a file (function)
C++ documentation for freopen